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P E R G A M O N
Microelectronics Reliability 38 (I 998) 1277-1286
MICROELECTRONICS
RELI BILITY
Electronic systems p ackag ing future re l iabi l ity chal lenges
J B a r r e t t
National Microelectronics Resear ch Centre University College Cork Ireland
A i m r a e t
P ackag ing has a dominan t e f f ec t on e l ec t ron ic sy s tem cost , pe r fo rmance , w e igh t , s i ze and long t e rm
rel iab il ity. Recen t yea r s ha ve the r e fo re s een r ap id deve lopmen ts in packag ing to me e t the cha l l enges o f
min ia tu r i sa t i en and cos t r educ t ion w h i l e de l ive r ing inc r eas ed e l ec t ri ca l pe~ o rma nce and r e l iab il ity. M ee t ing the
re l iab i l i ty dem and s posed by these deve lopm ents a l ready presents ma jor cha l lenges to th e re l iab i l i ty pract it ioner .
H ow ever , t he t echno logy roadm aps fo r in t eg ra ted c i r cu it s and e l ec t ron ic s y stems imp ly a con t inuous inc r eas e in
the s ca le o f ex i s t ing cha l l enges w h i l e emerg ing componen t and s ys tem techno log ies , s uch a s op t i ca l
in te rconnec t ions and mic ros ys tems , w i l l p r e s en t comple te ly new cha l l enges. Thes e emerg ing cha l l enges w i l l
me an tha t the r e l iab i l ity p r ac t i tione r w i l l i nc r easing ly be invo lved f rom the ve ry f i r s t phas es o f coml~ m en t o r
sys tem conce pt ion and des ign a l l the way throu gh to product ion an d green d isposal. Fur ther, to ensm'e that
re l iab i l i ty targets are real is t ica l ly se t and m et , the re l iab i l i ty pract i t ioner wi l l increa s ingly funct ion as p er t o f a
mu l t id i s c ip lina ry t eam, ma ny o f w hos e m emb er s m ay no t be long to the t r ad i tiona l e l ec tron ics d is c ip lines . By
exam in ing c tw rent r e li ab i li ty cha l lenges and by the u s e o f t echno logy readm aps , th i s pape r t r i e s to fo recast f u t~ e
rel iab i l i ty chal len ges in e lect ronic packaging . In the context of th is paper the term Packaging is used to
encompas s the va r ious a s s embly and in te rconnec t ion t echno log ies and t echn iques w h ich a r e u s ed to b u i ld an
e lec t ron ic com ponen t o r s y stem. ©
1998 Elsevier Science Ltd. All rights reserved.
1 . l a t r o d a e t i e a
The e lect ronic sys tem packaging re l iab i l i ty
chal len ge covers a very broad spectrum: a t one
extrem e there are short - time- to-prof it , shor t product
l i fe cycle , cos t dr iven volume products , such as
mob i l e phones and pe r s ona l compu te r s , w h ich
operate in re la t ively ben ign environments ; a t the
other ex trem e are cr i tica l sys tems , such as those for
av ion ics app li ca tions , w h ich m ay be m any yea r s in
ope ra t ion in ha r s h env i ronmen ts and a r e
cha rac te r i s ed by longe r deve lopmen t cyc le s ,
s ma l l e r p roduc t ion vo lumes and h ighe r cos ts . A t
each ex t r eme and ac ros s the s pec t rum as a w ho le
the p acka ging re l iab i l i ty practi tioner has a ro le in
ens u r ing a l eve l o f s y s tem r e l iab i l ity w h ich m ee t s
the r equ i r emen t s o f the end u s e r and the end u s e
environment . Given the d ivers i ty of mater ia ls and
componen t s w h ich e r e u s ed in e l ec t ron ic s y s tem
m a n u f a c t u r e a n d t h e c o m p l e x i t y o f m a n y o f t h e
indiv idual sys tem components used , par t icu lar ly
in tegrated c ircu its , there are po tent ia l ly m any w ays
in w h ich an e l ec t ron ic s y s tem may f a i l . The
packag ing r e l i ab i li ty p r ac ti t ione r has by neces s i ty
been , o r has had to become , an e l ec t ron ic j azk -o f -
a l l - l rades requir ing expert ise in mater ia ls , ICs ,
pass ive components , product engineer ing ,
manufac tu r ing and the env i ronmen t a s w e l l a s in
0026-2714/98/ - see fron t matter. © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0026-2714(98)00129-2
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278 J. Barrett/Microelectronics Reliability 38 1998) 1277-128 6
t he m ore obv i ous d i s c i p l i nes o f r e l i ab i l i t y phys i c s
and s ta t is ti cs. Increas ing ly , pack agin g re l iab i l i ty
has becom e a t eam , r a t he r t han an i nd i v i dua l ,
d i s c i p l i ne and i t is a d i s c i p l i ne wh i ch wi l l becom e
i nc reas i ng l y cha l l eng i ng , d r i ven by deve l opm en t s
s uch as :
• T h e g r o w t h i n I C c o m p l e x i t y
• Por tabi l i ty
• Har s h env i ron m e n t app l i ca t i ons
• Op t o -packag i ng and i n t e rconnec t i on
• Green e l ec t ron i cs
• M i c r o s y s te m s a n d m i c r o m e c h a n i c a l s y s te m s
5000
4 0 0 0
3000
2000
1000
0
ie a r e a m m 2 )
' 7 8 ' 8 8 ' 9 8 ' 0 8
T h e s e d e v e l o p m e n t s a n d t h e i r i m p l i c a ti o n s f o r
fu t u re s y s t em s packag i ng r e l i ab i l i t y a re d i s cus s ed
i n t he r em a i nder o f t h i s pape r .
2 . G r o w t h i n I C c o m p l e x i t y
M o o r e ' s L a w , w h i c h p r e d i c t s a d o u b l i n g o f I C
p r o c e s s i n g p o w e r e v e r y 1 8 m o n t h s h a s p r o v e d t o b e
m ore o r l e s s va l i d s i nce i t was f i r s t p ropos ed by
G o r d o n M o o r e , o n e o f t h e f o u n d e r s o f I n t e l , in
1965 . Th i s has been ach i eved t h rough IC fea t u re
s i ze r educ t i ons and t h rough i nc reas ed f ab r i ca t i on
y i e l d s wh i ch a l l ow l a rge r d i e s i zes . Inev i t ab l y t h i s
h a s s e t e q u a ll y c o m p l e x c h a l le n g e s f o r p a c k a g i n g
eng i nee r s a s l ead i ng edge IC a rea , ope ra t i ng
f requenc y and powe r d i s s ipa t i on have i nc reas ed , a s
s hown i n F i gu res l . ( a ) - . ( c ) [1 ] . Thes e evo l u t i ons
have been pa r t i cu l a r l y r ap i d i n t he l a s t f i ve yea r s
a n d t h e p a c e s h o w s n o s i g n o f s lo w i n g i n t h e n e x t
t e n y e a r s. A t t h e s a m e t i m e , t h e p r e d i c te d c o s t p e r
I / O o f h i g h p in c o u n t p a c k a g e s r e q u i re s a 5 0 c o s t
r educ t i on i n t he nex t 10 yea r s [1 ], a t r end w h i ch i s
t yp i ca l o f cos t evo l u t i on i n t he pas t decade . Th e
p a c k a g i n g e n g i n e e r h a s h a d a n d w i l l h a v e t h e t a s k
o f p r o v i d i n g m o r e c o m p l e x p a c k a g i n g s o l u ti o n s a t
r educe d cos t . Th e t echn i ca l cha l l enges f ac i ng t he
re l i ab i l i t y p rac t i t i one r have g rown and wi l l
c o n t i n u e t o g r o w a c c o r d in g l y . A r e v i e w o f t h e s e
cha l l enges i n t he e l ec t r i ca l , t he rm a l and
t h e r m o m e c h a n i c a l d o m a i n s f o ll o w s .
6000
5 0 0 0
4000
3 0 0 0
2000
1000
0
m P ower supply (mV)
m F r e q u e n c y ( M H z )
- |
l m m
m m m
78 88 98 08
200
150
100
50
0
I P o w e r ( W ) |
i
|
_ I I
| I
78 88 98 08
F i gu res 1 .( a )- (c ) . Evo l u t i on o f IC a rea and I / 0
c o u n t ; o p e r a t i n g f r e q u e n c y a n d p o w e r s u p p l y
vo l t age ; and pow er d i s s ipa t i on v s . t i m e .
2 1 E l e c t r i c a l r e l i a b i li t y
El ec t r ica l r e l i ab il i ty i nvo l ves t he i n t eg r it y o f
s i gna l p ropaga t i on i n a s y s t em under a l l ope ra t i ng
cond i t i ons . F ac t o r s wh i ch a re m ak i n g t he r e l i ab il i ty
cha l l enge m ore d i f f icu l t inc l ude :
• h i ghe r ope ra t i ng f r equenc i es
• l o w e r p o w e r s u p p l y v o l ta g e s
• m i n i a t u r i s a t i on
• E M C / R F I r e g u l at o r y m e a s u r e s
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J. Barrett/Microelectronics Reliability 38 1998) 12 77-1286 279
H i g h e r o p e r a t i n g f r e q u e n c ie s a n d l o w e r s u p p l y
v o l t a g e s m a k e t h e t a s k o f e n s u r i n g s i g n a l
p ro p ag a t i o n i n t eg r i t y i n c reas i n g l y d i f f i cu l t . As
d i g i t a l f r e q u e n c i e s e n t e r t h e G H z r e g i o n , R F a n d
m i c r o w a v e p a r a m e t e r s , w h i c h c a n l a r g e l y b e
i g n o r e d a t l o w e r f r e q u e n c i e s , m u s t b e t a k e n i n t o
acco u n t an d each i n d i v i d u a l s i g n a l t r ace i n a
p a c k a g e o r c i r c u i t b o a r d m u s t b e d e s i g n e d w i t h
i m p e d a n c e a n d n o i s e c o n t r o l i n m i n d . I n t h i s
co n t ex t , a t r ad i t io n a l w i re -b o n d wi l l n o t b e s u i t ab l e
fo r ch i p - t o -b o a rd i n te rco n n ec t i o n a s i t s i m p ed an ce
can n o t b e t i g h t l y co n t ro l l ed . M i n i a t u r i s a t i o n
s i m p l i f i e s t h i s ch a l l en g e t o a ce r t a i n ex t en t , i n t h a t
s i g n a l t r aces a re o v e ra l l m ad e s h o r t e r , r ed u c i n g
t h e i r p a ras i ti c l o ad i n g an d co u p l i n g len g t h s .
Ho wev er , m i n i a t u r i s a t i o n d o es n o t i n g en e ra l
i m p l y a r e d u c t i o n i n p a c k a g e o r s y s t e m c o m p l e x it y :
t h e p res s u re , l a rg e l y p ro d u ced b y t h e d es i r e fo r
p o r t ab i li t y , co n t i n u o u s l y ex i s t s t o p ac k m e t e
fu n c t i o n a l i t y i n t o s m a l l e r o u t l i n es . Th i s i s u s u a l l y
ach i ev ed b y a co m b i n a t i o n o f g rea t e r i n t eg ra t io n a t
t h e I C l e v e l a n d b y i n c r e a s i n g t h e I C p a c k i n g
d en s i t y a t b o a rd l ev e l . P ack i n g d en s i t i e s ( r a t i o o f
t o t a l a r ea o f a l l IC s t o t h e a rea o f t h e c i r cu i t b o a rd )
o f 1 0 we re r e l a t i v e l y r ecen t l y r eg a rd e d as h i g h ; it
i s n o w n o t u n u s u a l t o s e e p a c k i n g d e n s i t ie s g r e a t e r
t h a n 5 0 i n h i g h d e n s it y m u l t i c h i p m o d u l e s ( s e e
F i g u re 2 ) .
5 :
/ / / / t l l i l i i i i i l I I I I I I I I I I I ~ , ~ \ ~ X \ \ \ \ \ °
sel f - inductm~ce and in e lect r ical coupl ing between
adja cen t l ines resu l t s .
F u r t h e r co m p l i ca t i n g t h e t a s k a re t h e t i g h t e r
r eg u l a t i o n s o n e l ec t ro m ag n e t i c co m p a t i b i l i t y an d
rad i o f r eq u en cy in t e r f e ren ce (EM C / R F I ) . D es i g n
t o o l s fo r E M C / R F I h a v e l a g g e d b e h i n d t h e n e e d s o f
i n c reas i n g l y co m p l ex , m i n i a t u r i s ed s y s t em s
o p e r a t i n g a t h i g h f r e q u e n c i e s . E M C / R F I
p recau t i o n s i n m a n y s y s t em d es i g n s a re s t il l r o o t ed
i n ru l e s -o f - t h u m b an d d es i g n e r ex p er i en ce r a t h e r
t h an i n an a l y s i s o f s o u rces an d e f f ec t s an d t h e i r
im pac t on e lect r ical re li ab il ity . D es ign ers s t i l l hav e
t o w a i t fo r te s t i n g r e s u l t s f ro m a co m p l i an ce
l a b o r a t o r y b e f o r e h a v i n g c o n f i d e n c e i n E M C / R F I
s y s t em p e r fo rm an ce . S o l v i n g t h i s i s s u e wi l l b e o n e
o f t h e m a j o r ch a l l en g es i n d es i g n fo r e l ec t r i ca l
reliability.
C u r ren t a p p ro ach es t o t h e e l ec t ri ca l r e li ab i li t y i s s u e
i n c l u d e :
M i n i m i s i n g ch i p - t o -b o a rd i n t e rcu rm ec t i o n
p aras i t i c s b y a l m o s t co m p l e t e e l i m i n a t i o n o f
i n te r m e d i a t e p a c k a g i n g t h r o u g h t h e u s e o f f li p -
ch i p a s s em b l y . Th i s a l s o eas es t h e t h e rm a l
re l i ab i l i t y t a s k b u t s i g n i f i can t l y i n c reas es t h e
ch a l l en g e o f t h e rm o m ech an i ca l r e l i ab i l i t y ( s ee
sect ion 2.2).
U s e o f n e w l o w e r d i e l e c tr i c c o n s t a n t o rg a n i c
m a t e r i a l s a n d c o n t r o l l e d i m p e d a n c e
i n t e rco rm ec t i o n s i n p ack ag es an d c i r cu i t
b o a rd s . H o we v er , l o n g t e rm i n t ~ Tac i a l s tab i l it y
o f t h es e d i e l ec t r i c m a t e r i a l s u n d er t h e rm a l ,
t h e r m o m e c h a n i c a l a n d e n v i r o n m e n t a l s ~ e s s
t h en en t e r s t h e r e l i ab i li t y ch a l l en g e .
F i g u re 2 . A P en t i u m s i l ico n -o n - s i l ico n m u l t i ch i p
m o d u l e w i t h h i g h p a c k i n g d e n s i ty pho t o
cour t es y Eur opr ac t i ce -MC M/ ET H) .
T h i s l e v el o f p a c k i n g i s a c h i e v e d b y t h e u s e o f
f i n e p i t c h w i r i n g b o a r d s w h e r e t r a c k d i m e n s i o n s
a n d s p a c e s a r e d r i v e n t o t h e m i n i m u m a l lo w e d b y
m a n u f a c t u r i n g te c h n o l o g y . C o n s e q u e n t i n c r e a se s i n
D e v e l o p m e n t o f m o r e s o p h i st ic a t e d i n te g r a te d
d es i g n t o o l s wh i ch can e f f i c i en t l y an a l y s e a l l
aspec t s of e lect r ical re li ab il ity . Th is i s an
i m p o r t an t r eq u i r em en t o n t h e e l ec t r i ca l
r e l iab i l it y ro a d m ap .
In t h e fu t u re , it i s l i k e l y th a t o p t i ca l i n t e rco n n ec t i o n
wi l l n eed t o b e u s ed a t b o a rd l ev e l fo r c r i t ica l s i g n a l
p a t h s ( s ee S ec t i o n 5 )
2 .2 . T her ma l and t her m omecha n i ca l r e l iab i l it y
T h e t r e n d t o w a r d s i n c r e a s i n g d e n s i t y i n
i n te r c o n n e c t io n s is m a t c h e d b y i n c r ea s e d p o w e r
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128 J. Barrett/Micro electronics Reliability 38 1998) 1277 -1286
dens i ty in sys tems as the forces of increased
operating ~eq uenc y and increased packing dens i ty
com bine to increase IC po wer dissipat ion and the
num ber of ICs in a given board area. M eet ing the
chal lenge of heat rem oval is on e the m ost important
aspects of package and sys tem des ign and
increas ingly requires the use of n ew mater ials (e .g.
thermal inter face pads a nd ph ase chan ge mater ials )
and techniques (e.g. integrated micro-channel heat
pipes, m icro-fans and heatsinks). Overall, ho wev er,
the pressure exists to al low higher junct ion
temperatures and consequent exposure of
packaging mater ials to higher cont inuous
temperatures. T his is com bined with the in creased
use of organic mater ials to reduce elect r ical
parasitics, c ost and weight. T he reliabili ty challen ge
therefore becom es that o f ensur ing stabi li ty of the
organ ic ma terials them selves, and the ir interfaces to
other organic and inorganic materials , under
cont inuous exposure to m ore elevated temperatures .
To quote t~om the SIA roadm ap for semiconductors
[1]:
Standard methods and acceptance cr i ter ia for
interfacial adhesion are lacking. Funda me ntal wo rk
is nee ded to establish adhe sion strength and
degrad ation rate versu s environmental factors, all as
a function of interfacial physical and chem ical
proper t ies . New mater ials are being added to an
already complex, poorly understood interfaeial
adh esion reliability issue.
Adding to the thermal s t ress on mater ials and
inter faces is the problem of thermom echanical
stress. Increases in die area, coupled with higher
temperature gradients and transients, and the
additional com plication o f the use of organic
mater ials whose coeff icients of thermal expans ion
(CTE) are usual ly higher than that of s il icon, m ean
that both absolute and cy cl ic thermom echanical
stresses on ICs, components and materials will
continue to increase. This is without even
consider ing the ef fect of cycles in the external
therm al environment. Compressive, tensile and
shear forces exis t at ev ery level of pack aging and
can lead to delaminat ion, fat igue and cracking of
ICs, plastics an d solder joints.
2. 2.1. Flip chip assembly
The problem of thermom echanical st ress is
bec om ing particularly acute in flip-chip assem bly
of ICs. In fl ip-chip, metal bumps are deposited
Figure 3. M icrograph of bump ed IC a nd cross-
section o f fl ip-chip assem bly to circuit board.
direct ly on the bond-pads of the IC and th e IC is
then d irectly reflo w soldered to i ts circuit board, a s
shown in Figure 3. This almost completely
elim inate s the electrical parasities associated w ith
the chip-to-boa rd interconnections. Flip-chip also
al lows an almost unl imited number of I /O
connections as it facilitates area-array distribution
of connections across the entire IC surface.
Howe ver , the IC is now co nnected with less than a
100 lain gap to its substrate. In the ea rly application
o f fl ip-chip, eerarnic substrates we re used by IBM
[2] which had a reasonable CTE match to sil icon
and IC areas w ere cons iderably smal ler than current
and pro jected sizes. E ven tod ay, ce rarnie substrates
are used for the Motorola PowerPC chip [3] .
However , for reasons akeady explained above,
organic substrates are inoreasingly used for flip-
chip interconnection. The C TEs o f even the most
optimised organic substrates remain sufficiently
higher than that of sil icon to cause early fatigu.e
fai lure of the m icro-solder joints (< .001 mm in
volume) induced by cyc l i c thermumechanica l
stress. Th e use o f organic underfil l adhesives [4]
has e nabled this problem to be alleviated for current
generation flip-chip assemblies. However, given
projected IC size, power dissipation and I/O,
considerable work must be done on materials ,
interfaces [5], solder metallurgy and thermo-
mechanical simulation tools if future generation
flip-chip assemblies are to meet their reliability
targets.
2.2 .2 Plas tic packag ing
While fl ip-chip assembly is l ikely to be the
technique of choice for chip-to-board
interconnection o f leading ed ge ICs, conventional
transfer moulded plastic encapsulated packaging
wil l cont inue to dominate the market in terms of
volume. Plastic packaging has made considerable
progress since the early stages of dual-in-lines with
low reliabili ty caused its use to be restricted in
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J. Barrett/Microelectronics Reliability 38 1998) 12 77-1286
1281
many hi-tel applications, a restriction which has
only in recen t years begun to be l if ted. Intervening
year s have seen the emerSance of new body
formats, progression to ever higher pin¢ounts and
improvements in rel iabi l i ty which now al low
osmmercial -off - the-shelf COTS) plas t ic pachtged
ICs to be used even in hi-tel applications. This
rel iabi l ity improvem ent has been brought about
throug h a combination o f ma terials and process
engineer ing to redu ce s tress and m ois ture induced
failures, an d application o f finite eleme nt
simulation tools to the therm om ech anica l s | ress
problem. How ever, the reliabili ty challeng es can be
expected to cont inue growing in the coming years
in topics suc h as:
• Stress and m oisture issues for large die in very
th in packages where there m ay be on ly a f ew
htmdred micron s of plas tic over the IC.
• The dem and for increased power handling.
• The use of plas t ic packages in increas ingly
harsh application enviro nm ents (see Section 4).
• The abi li ty to handle higher FO counts in
sm alle r footprints.
Am ong the tools that wi ll be ne eded to succeed in
meeting these challenges, and those of f l ip-chip,
will be:
The abili ty to quickly develop application
specific encapsulant materials through the use
of t echniques such as molecu lar model l ing and
synthesis
Integrated design and simulation tools th at will
be able to perfo rm virtual prototyping o f
packages through integrat ion o f thermal and
therm om echa nical simulations with reliabili ty
models. Essential here also will be validated
compact models (and the means to obtain
them) for material and interface behaviour in
the intended use environment. While i t
unlik ely that absolute a ccu racy can be obtained
fi 'om such tools, they shou ld at least be a ble to
quic kly assess the comparative reliabili ty
performa nce of di f ferent designs and mater ial
sets
Por tab i l i t y
It is estima ted that in 2000, ov er 50% o f all
electron ic products m anufa ctured will be portable.
This will includ e increa singly sophisticated sma rt
cards as wel l as the com mun icat ion, oomputin8 and
entm ainm ent systems a lr eady in use a nd which a te
conv erging to one integrated portable system. Ne w
applications a re also eme rging in portable chem ical
and biological analysis systems (see Section 7
below). Portability's m ain sy stem c riteria are:
• Light weight and smal l s ize
• Low to modera te cos t
• M axim um functionality per cm 3
• Low power consumpt ion
• Resistance to shock, vibration and wa ter
These are dr iving man y of the issues al ready
discussed in section 3 above:
• Maximum integration on sil icon with
consequent growth in die s ize, I /O count and
individual die po wer consumption
• Reduce d overal l sys tem pow er consumption
• Use of organic mater ials
• Use o f low profile, thin pack aging techniques
• Use of high dens i ty ci rcui t boards
To reduce size and increase reliabili ty there is
active ong oing research in the integration o f passive
components onto and into both inorganic and
organic circuit boards. Doing this provides more
board surface area for ICs (thus increasing the
pack ing density and associated reliability p roblem s)
and also eliminates the so lder joints associated with
discrete components , o f which there m ay be m any
hun dreds even in a relatively sma ll system such as a
mob i le phone. As solder joints are viewed as w eak
links in electronic systems, this should,
theoretically, le ad to increased reliability. H ow eve r,
the ci rcui t board i tsel f wi l l becom e a m ore com plex
ma terials system, incorporating resistive, dielec tric
and/or magnet ic mater ials . New chal lenges in
electrical reliability and in materials/interfacial
reliability will the refore have to be addressed.
Smart cards , which are among the highes t
production volume consumer electronic products,
present unique problems in terms of packaging
reliability. Th ey are in ge neral constrained to credit
card thickness and the protection that can be
afforded to the IC is m inimal . Future smar t cards
will be expected to have increased functionality
while cont inuing to be able to wi ths tand the wear
and tear o f daily use. Considerable ingen uity will
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b e r e q u i r e d t o d e v e l o p p a c k a g i n g w i t h a d e q u a t e
re l iab i l it y an d y e t b e co m p a t i b l e w i t h h i g h v o l u m e ,
v e ry l o w co s t p ro d u c t i o n .
4 H a r s h e n v i r o n m e n t a p p l i c a ti o n s
I t h as g en e ra l l y p ro v en p o s s i b l e t o o b t a i n
ad eq u a t e e l ec t ro n i c p ack ag i n g r e l i ab i l i t y ev en i n
t h e h a r s h e s t a e r o sp a c e a n d m a r i t i m e e n v i r o n m e n t s .
Ho wev er , t h i s r e l i ab i l i t y h as t y p i ca l l y n o t co m e
c h e a p l y a n d p r o d u c t d e v e l o p m e n t t i m e s w e r e
u s u a l l y l o n g an d p ro d u c t i o n v o l u m es r e l a t i v e l y
s m a l l. A n u m b e r o f h a r s h a p p l ic a t io n e n v i r o n m e n t s
a re , h o wev er , i m p o s i n g a r ev e r s a l o f t h i s tr en d .
D o m i n a n t a m o n g t h e s e i s a u t o m o t i v e e l e c t r o n i c s
b u t t h e d e m a n d f o r l o w e r c o s t c o m m u n i c a t i o n s
s a t e ll it e s , co m m erc i a l a i r c r s R c o n t ro l s y s t em s an d
d r i l l -h ead e l ec t ro n i c s y s t em s a re a l s o i n f l u m t i a l .
T h e u n d e r - h o o d a u t o m o t i v e e n v i ro n m e n t i s p e t h a p s
t h e h a r s h e s t c o n v e n t io n a l e n v i r o n m e n t i n w h i c h
an e l ec t ro n i c p ack ag e h as t o f i m c t i o n . Ho w ev er , t h e
a u t o m o t i v e i n d u s t r y d e m m d s h i g h v o l u m e
p ro d u c t i o n , l o w u n i t co s t an d r e l a t i v e l y s h o r t
s y s te m d e v e l o p m e n t t im e s . A s t h e s o p h is t ic a t io n o f
en g i n e m o n i t o r i n g an d co n t ro l s y s t em s i n c reas es ,
e l ec t ro n i cs i s c l a i m i n g a h i g h e r p ro p o r t i o n o f
o v era l l v eh i c l e co s t an d t h e t r en d t o ward s co s t
r e d u c t io n i n p a c k a g i n g c a n t h e r e fo r e b e p r e d i c t e d t o
i n c reas e . R e l i ab i l i t y s p ec i f i ca t i o n s a re h o wev er
l i k e l y t o b e c o m e m o r e s t r i n g e n t a s a u t o m o t i v e
e l ec t ro n i cs i n c reas i n g l y t ak e o v e r s a fe t y c r i t i ca l
fu n c t i o n s . Th e r e l i ab i l i t y ch a l l en g e s e t o u t i s
t h e re fo re t o p ro v i d e h i g h r e l iab i l it y a t l o w co s t i n a
v e r y h a r s h e n v i r o n m e n t . T o m e e t t h i s c h a l l en g e , a l l
l ev e l s o f s y s t em re l i ab i l i t y w i l l h av e t o b e
ad d res s ed b o t h o n an d o f f - ch i p an d s o p h i s t i ca t ed
d es i g n fo r r e l iab i l it y m e t h o d o l o g i es w i l l h av e t o b e
d e v e l o p e d a n d a p p l i e d w h i c h w i l l c o n c t a r e n t ly
a d d r e s s m a n y o f t h e i s su e s w h i c h a r e d i s c u s se d i n
t h e o t h e r s ec t i o n s o f t h i s p ap e r . S u cce s s fu l l y
m e e t i n g t h i s c h a l l e n g e w i l l h o w e v e r p r o v i d e
m at e r i a l s , p ro ces s es an d d es i g n m e t h o d o l o g i es
w h i c h w i l l s u p p o r t l o w e r c o s t a p p l i c a t i o n o f
e l ec t ro n i c s y s tem s i n o t h e r h a r s h en v i ro n m en t s .
5 . O p t o - p a d m O n g a n d I n t e r e o n n e e t i o n
D u e t o i n c r e a s i n g o p e r a t i n g
f r eq u en c i es ,
o p t i ca l i n . c o n n e c t i o n s a l r e a d y i n u s e a t
b a c k p l a n e l e v e l a r e i n c r e a s in g l y l i k e l y t o b e
ap p l i ed a t i n d i v i d u a l b o a rd l ev e l . On e o f t h e f i r s t
ap p l i ca t i o n s wh i ch wi l l d r i v e t h i s d ev e l o p m en t i s
l i k e l y t o b e c l o ck d i s t r i b u t io n b u t i t a l s o wi l l b e
increas ingly requi red for cr i t i ca l s ignal
i n te r c o n n e c ti o n s . T w o d e v e l o p m e n t s ar e n e e d e d t o
en ab l e b o a rd l ev e l o p t i ca l i n t e rco n n ec t i o n s : l o w
c o s t p o l y m e r i c o p t ic a l i n t e rc o n n e c t s a n d a m e a n s o f
a l l o wi n g co n v en t i o n a l s i l i co n IC s t o e f f i c i en t l y
em i t l i g h t .
R & D i s a l r e a d y u n d e r w a y o n o p t i c a l
i n t e rco n n ec t s t h a t can b e i n t eg ra t ed in t o
m a i n s t r eam b o ard m an u fac t u r i n g p ro ces s es [6 ]. I t is
t h e re fo re l i k e l y t h a t t h e i n t eg ra t ed o p t i ca l
i n t e rco n n ec t i o n s wi l l a t l eas t b e av a i l ab le a l t h o u g h ,
i n a s i m i l a r m an n er t o i n t eg ra t ed p as s i v es , n ew
re l i ab i l i t y ch a l l en g es i n m a t e r i a l s an d i n t e r f ac i a l
s t ab il it y w i l l h av e t o b e ad d res s ed . Th e re d o es n o t
s eem t o b e an eq u a l p o s s i b i l i t y t h a t co n v en t i o n a l
s i l i co n IC s wi l l b e ab l e t o e f f i c i en t l y em i t l i g h t .
H o w e v e r , l o w c o s t , s m a l l s i z e d c o m p o u n d
s e m i c o n d u c t o r l a se r s a n d L E D ' s d o e x i s t a n d
h y b ri d _i ~ ti on t ech n i q u e s can b e u s ed t o a t t ach t h es e
t o s i l i co n IC s t o a l l o w t h em t o em i t l i g h t [7 ] .
I m p o r t a n t a m o n g t h e r e li a b il it y i s s u e s w h i c h n e e d
t o b e a d d r e s s e d i n c lu d e t h e i m p a c t o f t h e p i c k a n d
p l ace o p e ra t i o n o n o p t i ca l d ev i ce re l i ab il it y an d t h e
e f f e c t o f c y c l i c t h m n o m e c h a n i c a l s t r e s s o n t h e
re l iab i l it y o f t h e V C S EL i t s e l f an d i t s a t t ach m en t .
T h e d e v i c e a t t a c h m e n t i s h o w e v e r o n l y t h e f i r s t
s t ep i n b u i l d i n g th e o p t i ca l h y b r i d s y s t em . Th e i s s u e
o f i n t e r f a c i n g t h e V C S E L t o t h e o p t i c a l
i n te r c o n n e c t io n a n d e n s u r i n g t h e l o n g t e r m
m ec h an i ca l , th e rm a l an d e n v i ro n m en t a l i n t o g ri t y o f
t h a t i n t e r f ace m u s t a l s o b e ad d res s ed . Ad d t o t h i s
t h a t p r o j e c t e d c o m p l e x s y s t e m s m a y e v e n t u a l l y
r e q u i re t h e m o u n t i n g o f h u n d r e d s o f s u c h d e v i c e s
an d t h e s ca l e o f t h e r e li ab i li t y ch a l l en g e b eco m es
a p p a r m t . P a c k a g e a n d b o a r d l e v e l o p t i c a l
i n t e rco t m ec t i o n wi l l r eq u ir e , an d w i l l b e a v eh i c l e
t o d e m o n s t r a t e , m a n y n e w d e v e l o p m e n t s i n
re l iab i l i ty sc ienc e.
6 . G r e e n e l e c t ro a i e s
A m a r k e t a n d c o n s u m e r b a s e d d r i v e r f o r
a d v a n c e s i n p a c k a g i n g i s t h a t o f e n v i r o n m e n t a l l y
f r i en d l y e l ec t ro n i cs wh i ch i n c l u d es b o t h
m a n u fac t u r i n g an d u l t i m a t e d i s p o s a i/ r ecy c ii n g . Th e
p r i n c i p l e o f ex t en d ed p ro d u ce r r e s p o n s i b i l i t y
w h e r e , f o r e x a m p l e , a n I C p a c k a g i n g c o m p a n y
m u s t t ak e u l t im a t e en d o f l if e r e s p o n s i b i li t y fo r i t s
p ro d u c t s , h as l o n g t e rm t ech n o l o g i ca l i m p l i ca t i o n s
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f o r t h e I C p a c k a g i n g p ro c e s s. A c o m m o n e x a m p l e
i s & a t o f t r a n sf e r m o u l d e d p l a s t ic p a c k a g e s ,
f a b r i c a t e d i n v o l u m e s o f b i l l i o n s p e r y e a r . T h e
p l as t i c m a t e r i a l u s ed i s l a rg e l y t h en n o s e t ep o x y
wi t h a h i g h s i l i ca f i l l e r co n t en t an d o t h e r m i n o r
co n s t i t u en t s ( i n c l u d i n g en v i ro n m en t a l l y u n f r i en d l y
a n t i m o n y a n d b r o m i u m b a s e d f l a m e r e t a r d a n t
c o m p o u n d s ) . O n c e t h e e p o x y i s c u r e d i t b e c o m e s
l a rg e l y i m p e rv i o u s t o ch em i ca l a tt ack , w i t h o n l y
b o i l i n g fu m i n g n i tr i c ac i d ( i t s e l f n o w a r e s t r i c t ed
m a t e r i a l) c a p a b l e o f c h e m i c a l l y s tr i p p in g t h e e p o x y
s o th a t t h e c h i p a n d l e a d f r a m e c a n b e r e c o v e r e d a n d
recy c l ed . A l t h o u g h o rg an i c d i g es te r s a r e u n d e r
d e v e l o p m e n t f o r th e e p o x y , t h e p r i nc i p a l te c h n i q u e
fo r ep o x y r em o v a l a t en d o f l i f e i s i n c i n e ra t i o n ,
wi t h r e l eas e o f u n d es i r ab l e b y -p ro d u c t s . To
faci l i t a te r e y l i n g f p la s t i c p ack ag es , a p res s u re i s
b e i n g ex e r t ed t o u s e t h e rm o p l as t i c en cap s u l an t s , a
m o v e t h a t w i l l r a i s e m a n y n e w r e l i a b i l i t y
ch a l l en g es .
T h e e n v i r o n m e n t a l i s s u e w h i c h h a s r e c e i v e d
t h e m o s t p u b l i c i t y i s h o w e v e r t h a t o f l e a d - b a s e d
(P b ) s o l d e r s . I t h as b een t h rea t en ed fo r a t l eas t t h e
l a s t f i v e y ea r s t h a t l ead wo u l d b e l eg i s l a t i v e l y
b a n n e d f r o m e l e c t r o n i c p r o d u c t s i n E u r o p e a n d
e l s ewh ere . Th a t t h rea t h as f a i l ed t o m a t e r i a li s e y e t
m a n y c o m p a n i e s ( e . g . N o r t e l a n d H i t a c h i ) h a v e
a n n o u n c e d t h e r e m o v a l o f l e a d f r o m t h e i r e l e ct r o n ic
p ro d u c t s . Th ere i s a l s o ac t i v e Eu ro p ean r e s ea rch
i n t o t h e d e v e l o p m e n t o f l e a d f r e e s o l d e r a l l o y s
co m p a t i b l e w i t h cu r ren t e l ec t ro n i cs a s s em b l y
p ro ces s es . Th e d r i v e r fo r th es e d ev e l o p m en t s i s
l a r g el y d r iv e n b y p e r c e i v e d m a r k e t d e m a n d , f u e l l e d
b y c o n s u m e r e n v ir o n m e n t a l c o n c e rn , f o r l e a d f r e e
p ro d u c t s . F ro m a r e l i ab i l i t y p o i n t o f v i ew, t h e re
ex i s t d ecad es o fa ccm n u l a t ed r e l iab i l i ty d a t a fo r t in -
l ead s o l d e r s an d r e l iab i f it y p red i c t io n s h av e b e co m e
i n c reas i n g l y accu ra t e . W i t h t h e i n t ro d u c t i o n o f l ead
f r e e al lo y s , u r g e n t w o r k h a s h a d t o b e c a r r ie d o u t
a n d w i l l c o n t in u e to b e r e q u i r e d to e v a l u a te t h e
re l i ab il i ty o f l ead - f r ee a l l o y s i n a l l t h e i r ap p l i ca ti o n s
f r o m l a r g e p o w e r d e v i c e a t t a c h d o w n t o f l i p - c h i p
s u k le r j o i n ~
P l as t i c IC p ack ag i n g an d s o l d e r a l l o y s a re
o n l y t w o e x a m p l e s - m o s t p r i n te d c i r c u it b o a r d s a r e
t h e r m m e t e p o x y b a s e d a n d c o n t a i n c o p p e r, n ic k e l ,
c a d m i u m , l e a d a n d g o l d w h i c h a r e u n d e s i r a b l e
p o l l u t an t s i f a l l o wed i n t o ch i n k i n g wa t e r . R ec y c l i n g
o f P C B s w i l l d e m a n d r e cl a m a t i o n o f th e s e m e t a l s
f r o m b o t h o u t e r a n d b u r i e d l a y er s a n d a m o r e e a s i l y
r e m o v e d m a t e r i a l t h a n t h e r m m e t e p o x y w i l l b e
n e e d e d . T a b l e 5 0 , w h i c h i s t o o l m g e t o r e p r o d u c e
h e r e , o f t h e S I A r o a d m a p f o r s e m i c o n d u c t o r s [1 ],
l i s t s fu r t h e r ex am p l es o f m a t e r i a l an d
m a n u f a c t u r i n g c h a n g e s t h a t w i l l b e n e e d e d t o m e e t
e n v i r o n m e n t a l r e q u ir e m e n t s . T h e c o n s m n e r c a n
h o p efu l l y l o o k fo rward t o a c l ean e r fu t u re ;
r e l i ab i l i t y p rac t i t i o n e r s can ce r t a i n l y l o o k fo rward
t o a c h a l l e n g i n g o n e w h e r e th e r e w i l l b e a d e m a n d
fo r r ap i d a s s es s m en t o f t h e r e l i ab i l i t y i m p l i ca t i o n s
o f u s i n g a l t e rn a t iv e m a t e r i a l s e ts . I t i s u n l i k e l y t h a t
t h e re wi l l b e t i m e av a i l ab l e t o d o ex t en s i v e
rel iab i l i ty t es t ing of many d i f ferent a l t ernat ives .
Th e t o o l s h o wev er a l r ead y d o ex i s t , an d wi l l
b e c o m e m o r e s o p h i s t i c a t e d , t o s i m u l a t e t h e
e l ec t ri c al , t h e r m a l a n d t h e r m o m e d u m i c a l e ff e c ts o f
u s i n g a l t e rn a t i v e m a t e r i a l s . W h a t i s n o t r ead i l y
av a i l ab le i s a f a s t an d accu ra t e m e t h o d o l o g y fo r
o b t a i n i n g m a t e r i a l an d i n t e r f ac i a l p ro p e r t i e s o f
m a t e r i a l s i n co n f i g u ra t i o n s wh i ch a re r ep res en t a t i v e
o f t h e i r en d u s e ap p l i ca ti o n . F o r ex am p l e , i f a
t h e rm o p l as t i c i s g o i n g t o b e u s ed a s a 2 5 t t m t h i ck
l a y e r , w e w i l l n e e d t o k n o w h o w i t b e h a v e s
e l ec tr i ca ll y , t h e rm a l l y an d m e ch an i ca l l y a t t h a t
t h i c k n e s s a n d n o t a s a b u l k s a m p l e . T h i s
i n f o rm a t i o n w i ll a l l o w t h e m o s t p r o m i s in g m a t e r i al
s e t s t o b e s e l ec t ed . Th es e can t h e n s u b j ec t ed t o t h e
re l iab i l it y t e s t in g w h i ch i s u n l i k e l y , a t l eas t i n t h e
n ea r t e rm , t o b e r ep l aced b y r e l i ab i l i t y s i m u l a t o r s
f o r c o m p l e x m u l t i - m a t e r i a l a n d m u l t i - c o m p o n e n t
s y s t em s wi t h m u l t i p l e p o t en t i a l f a i lu re m e ch an i s m s .
T h e d e v e l o p m e n t o f m e t h o d o l o g i e s f o r e f f i c i e n t
m e a s e r e m e n t o f m a t e r ia l p r o p e rt ie s , a n d h o w t h e s e
p ro p er t i e s ch an g e wi t h t cm p eraUae , h u m i d i t y e t c . ,
i s an i m p o r t an t ch a l len g e .
7 . M i c r m y a t e m s
M i c r o s y s t e m s t e c h n o l o g y (M S T ) , d e f i n e d a s t h e
i n t eg ra t i o n o f s en s o r s , ac t u a t o r s an d
m i c ro e l ec t ro n i cs i n a m o n o l i t h i c o r h y b r i d c i r cu i t ,
an d i t s co u n t e rp a r t m i e ro -e l ec t ro m ech an i ca l
s y s t em s ( IVlEM S ) a re r eg a rd ed as b e i n g a t a s i m i l a r
t e c h n o l o g y a n d m a r k e t s t a g e a s m i c r o e l e c tr o n i c s 3 0
y ea r s ag o [1 3 ] . R e l a t i v e l y s i m p l e d ev i ces , s u ch as
au t o m o t i v e a l rb ag s en s o r / ac t u a t o r IC s , a r e b e i n g
u s e d i n h i g h v o l u m e s w h i l e i n c r ea s i n g ly c o m p l e x
s y s t e m s a r e b e i n g d e m o n s t r a t e d i n m a n y
l ab o ra to r i e s an d a re b eg i n n i n g t o ap p ea r o n t h e
m ark e t . Ex am p l es i n c lu d e :
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J . B a r r e t t/ M i c r o e l e c tr o n i c s R e l i a b i li t y 3 8 { 1 9 9 8 1 2 7 ~ 1 2 8 6
Micro-total analysis systems OtTAS) for
chemical/biological analy sis wh ich contain
pumps, chemica l or biologica l sensors and
signal pro cessing on a single chip or hybrid.
• M icro-m etrology system s such as miniaturised
spec t rometers and gyroscopes .
Micro-optical systems containing integrated
lasers, adjustable lenses, mirrors and filters
such as the Texas Insmnnents micro-mirror
project ion IC [8].
M any other exam ples exi s t but tw o pr imary b arr ie rs
to vo lum e comm ercialisation are consistently
mentioned: cos t efficien t pac kag ing and reliability.
The reasons for thi s can be readi ly seen i f we
examine some of the new re l iabi l i ty cha l lenges
which M ST/M EM S present .
hundreds o f microns in s ize , the presence o f
particulates or contaminants can comp rom ise
system operation and reliability. Indeed, wear in
moving micro-components can i t se l f genera te
particulates even if they are absen t from the
package as fabr ica ted [13] . In components which
use biological layers for sensing, biological
contaminat ion from packaging materials and the
effec t of packaging thermal processes on the bio-
layers must be taken into account . A micro-sensor
which i s intended to measure he avy meta l (Pb, Cu,
Zn, C d) content in water to an accuracy of par t s per
bi l l ion cannot be contaminated by leaching of such
metals fro m pac kaging materials [ 10].
7 1 Lo w o r z e r o s tr e s s p a c k a g i n g
Ma ny s i l i c on ba se d MST/ MEMS c ompone nt s
conta in micro-machined mem branes or beams, used
for sensing and/or actuation, w hich are o f the order
of a f e w mi c rons t h ic k bu t w hi c h ma y b e hundre ds
or thou sands o f micro ns in length/width. These
structures are ver y sens itive to mechanical slress
which may cause incorrect operat ion or complete
fai lure. At the same t ime, these components must
undergo the normal packaging processes of die
attach, electrical connection and encapsulation, all
of which impose bending s t resses of var ious sor t s
on the component , especial ly as the preferred route
for low cost is to use organic packaging materials.
Figure 5.(a) shows a stress simulat ion o f on e com er
of a micro-machined m embrane subjec ted to plas t ic
encapsulation stresses. Critical stresses were
predicted to exist in the membrane (lower left )
which exceeded the memb rane s t rength and in a
micro-m achined channel (low er right). Fai lure
analysis confirm ed the simulat ion and the cracke d
me mb rane is show n, after decapsulation, in Figure
5. (b) [9] . Methodo logies mu st therefore be found
which mechanica l ly and the tmomechanica l ly
de c oupl e t he mi c ro-ma c hine d s ~c t u re s f rom the
pack age w ithout com prom ising reliabil ity.
Figu re 5.(a). Thermo mechanical stress simulation
o f a micro -mac hined si licon structure
7 2 P a r t i c u l a te a n d c o n t a m i n a n t f r e e p a c k a g i n g
In MST/MEMS components which conta in micro-
mechanica l moving par ts of the order of tens o r
Figu re 5.Co). Stress induced cracks in th e micro-
mac hined si licon structure of Fi~n'e 5.(a).
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285
7. 3 .
Mult imedia
i n p u t o u t p u t
The problem of deal ing with elect r ical input-
outputs w as discxmsed n Section 2.1 abov e and the
issue of optical input-ontput was re ferred to in
Sect ion 6. I f added to this is f luidic and/or gaseous
input-output, the challenge is considerably
complicated. At the simplest level is a sensor IC
which has to be immersed in ambient temperature
aqueous analytes and even this represents a
cons iderable chal lenge in def ining and proving the
reliability o f a low cost packa ging process w hich
exposes the sens ing element whi le protect ing the
microelectronics [10 ] . The extension of such a
packaging process to higher temperature or more
chem ically aggressive analy tes is a considerable
reliability challenge. H owe ver, m an y MST/M EMS
comp onents are being developed which wil l include
act ive f low-throogh sys tems dr iven by integrated
micro-pumps. Suc h com pone nts requ ire micro-
plum bing input-output and internal packaging to
isolate the fluid from micfo electron ics sections o f
the component. Som e of these com ponents wil l a lso
use m icro-spectrometers to p erform analysis of the
passing fluid and an optical interfac e will therefore
also have to be provided. Add the compfications
that such a system should be compact, portable,
low-cost and recyc lable and th e scale o f the
reliability challen ge becom es apparent.
MST/MEMS component s p resen t mos t o f the
reliability cha llen ges associated with conv entional
microelectronics and add m any new ones . A good
exam ple of the additional challenges to b e
considered, and discovered, in bringing a MEMS
com ponen t to ma rket is given in the IRPS'98 paper
on the rel iabi l i ty engineer ing of the Texas
Instruments mic ro-m irror projection system [8].
Rev i ew nd summ ry
This paper has at tempted to forecas t the major
reliabili ty challenges that will need to be faced in
the com ponen t and sys tem s packaging f ields in the
com ing years. I t is obviously d ifficult to discuss all
the possible challen ges as i t is s imilarly difficult to
be quantitatively specific on their nature or t iming;
how ever som e overal l t rends can be forecast :
• Reliability is mo ving and will con tinue to
move fur ther back in the sys tem development
c y c le w i t h d e s i g n f o r re l i a b i l it y b e c o m i n g an
in teg ra l pe r t o f t he cyc le f r om t he beg inn ing
Increasingly, pedatging and reliabili ty will be
appficafion driven and system focused. The
traditional sequential,
a n d
not necessar i ly
interaetive, W oc ~s o f chip design -~ chip-fab
--~ package -~ board --~ box --~ burn-
in/reliability test will not be adequate.
Con current multi-disciplinary effort will be
required to deliver a reliable system to tim e
and budget.
The re is a growing expectation o f increased
reliability a t reduce d cost and o f shortcr system
developme nt cycles . The t im e and budget for
prototyping and tes t ing wil l therefore
progressive ly shrink. Simulation and virtual
prototyping will therefo re increasing ly beco m e
important reliability activities. T he y will
how ever only be viable i f the inputs on
ma terial data an d beh aviour are accurate.
Reliability cha lleng es in conventional
mictoeleetronics and opto-electronies
packaging can be reasonably forecas t but the
pace o f progress wi ll be v ery fast over the next
ten years . Th e reliabi li ty com mu nity therefore
needs to begin work now on developing i ts
own roadmaps to meet these chal lenges . The
recent ly published European p ackaging
rcadm ap is an exam ple [11] al though i t i s not
specifically focused o n reliability issues.
New chal lenges are l ikely to cont inue
appearing from sources such as environmental
legislation and consumer concern.
Methodologies to q uick ly assess th e reliabili ty
impact o f these and to r ecommend op t imum
solutions w ill be required.
MST and M EMS technology of fe rs p robably
the greates t new chal lenge in packaging
reliabili ty. New materials will be used and
ma ny ex tra mater ial parameters and behaviours
will need to be cons idered in des igning for and
assessing reliability. The multidisciplinary
concurrent development teams that are
important for microe lectronic systems will be
essent ial for MST and MEMS packaging. As
we ll as consisting o f the traditional electronic,
physics, materials and mechanical disciplines
which, to at least some extent share a com mon
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technical language, the team s w ill also contain
disciplines such as micro-chem islry and m icro-
biology. The integration of these disciplines to
produce a reliable product will itself be an
interesting challenge. To quote the IEEE
Transactions on C PM T: '~lectronic Packaging:
Cross-Discipline is the Only Discipline [12].
A e i m e w l e d g e m e n t s
The content of this paper has been informed by
discussions with m any colleagues in N MR C, with
partners in collaborative projects and at
conferences. Too many to mention individually,
their collective enthusiasm to discuss these issoes is
gratefully acknow ledged.
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