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NO-CLEAN SOLDER PASTE Recent research has unveiled several options for no-clean reflow soldering. he cleaning of electronic assem- blies is', zero-value-added opera- T tion which frequently involves :nvironmentally harmful solvents such IS chlorofluorocarbons (CFCs). Elimi- iation of cleaning is therefore economi- :allyand environmentallydesirable. AT&T has recently madegreat progress Dward eliminating the need for circuit ack cleaning through the use of low-solids flux for wave soldering. For reflow pro- cesses, although the solder paste residues are noncorrosive and nonconductive, cleaningis still performed for testability or cosmeticreasons. To eliminate this need, the amountof residuemust be sigmficantly reduced. This article swnmafizes several approaches to no-clean reflow soldering being evaluated by AT&T and the BOC Group. Tests used to ensure that fluxes figure Flux residuesfrom rosin-based solderpaste under discrete components. 1 Nikhiles Bandyopadhyay, PI and residues are not detrimental to E uct reliability when left on the cii board are described. Next, low-so, flux-based solder pastes (LSSPs) their limitations are discussed. Fin1 the developmentand ongoing characc ization of soldering processes involii a proprietary solder paste and contra, reflow atmospheresare summarizec Characterization of Flux Residues The simplestno-clean reflow pmce:: the one currently available for users 0) BellcoreTR-TSY-00078' and, most lik; IPC-SP-819? type L qualified materiii All such solderpastes havebeen thomug tested to ensure that their flux residu are not detrimental to the reliability of' product. The residues, shown pho) graphically in Figure 1, have been evai ated for corrosion on copper mirrcc and/or plates, halide content, pH, ai conductivity, as measured by surfa insulation resistance. The followii sectionsdescribe the tests currently us; at AT&T and by most of the electronii industry to qualify solderingmaterial1 Corrosion %sting lbo types of corrosiontesting are tyil caUy performed on liquid fluxes and soldl paste fluxes: copper mirror and coppml plate evaluations. Both determinethe pn I pensity of the flux to attack bare coppe' The copper mirror test, as describe in IPC-SF-818: determines the remow effect, if any, of a flux on a bright coppr film vacuum-deposited on clear glass. , , small amountof flux (a drop of liquid flu or a 0.25cdiameter dot of paste) i depositedon the mirror. The mirrors I then placed in a temperature-humidit) controlled chamber at 23 f 2 "C and 5r If: 5% relative humidity for 24 f 0." hours. At the end of the 24-hour period the test flux is removed by immersion ii isopropyl alcohol and the coupons art evaluated visually. If there is any com plete removal of the copper film, a: evidenced by the background showing through the mirror, the flux has failed Discoloration of the copperby superficia reaction or only a partial reduction of tk film thicknessis not considered a failure. 26 PRINTED CIRCUIT ASSEMBLY. FEBRUARY 1990
Transcript
Page 1: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

NO-CLEAN SOLDER PASTE

Recent research has unveiled several options for no-clean reflow soldering.

he cleaning of electronic assem- blies is', zero-value-added opera- T tion which frequently involves

:nvironmentally harmful solvents such IS chlorofluorocarbons (CFCs). Elimi- iation of cleaning is therefore economi- :ally and environmentally desirable.

AT&T has recently made great progress Dward eliminating the need for circuit ack cleaning through the use of low-solids

flux for wave soldering. For reflow pro- cesses, although the solder paste residues are noncorrosive and nonconductive, cleaning is still performed for testability or cosmetic reasons. To eliminate this need, the amount of residue must be sigmficantly reduced. This article swnmafizes several approaches to no-clean reflow soldering being evaluated by AT&T and the BOC Group. Tests used to ensure that fluxes

figure Flux residuesfrom rosin-based solder paste under discrete components. 1

Nikhiles Bandyopadhyay, P I and residues are not detrimental to E uct reliability when left on the cii board are described. Next, low-so, flux-based solder pastes (LSSPs) their limitations are discussed. Fin1 the development and ongoing characc ization of soldering processes involii a proprietary solder paste and contra, reflow atmospheres are summarizec

Characterization of Flux Residues The simplest no-clean reflow pmce::

the one currently available for users 0) Bellcore TR-TSY-00078' and, most lik; IPC-SP-819? type L qualified materiii All such solder pastes have been thomug tested to ensure that their flux residu are not detrimental to the reliability of' product. The residues, shown pho) graphically in Figure 1, have been evai ated for corrosion on copper mirrcc and/or plates, halide content, pH, ai conductivity, as measured by surfa insulation resistance. The followii sections describe the tests currently us; at AT&T and by most of the electronii industry to qualify soldering material1 Corrosion %sting lbo types of corrosion testing are tyil

caUy performed on liquid fluxes and soldl paste fluxes: copper mirror and coppml plate evaluations. Both determine the pn I

pensity of the flux to attack bare coppe' The copper mirror test, as describe

in IPC-SF-818: determines the remow effect, if any, of a flux on a bright coppr film vacuum-deposited on clear glass. , , small amount of flux (a drop of liquid flu or a 0.25cdiameter dot of paste) i deposited on the mirror. The mirrors I then placed in a temperature-humidit) controlled chamber at 23 f 2 "C and 5r If: 5% relative humidity for 24 f 0." hours. At the end of the 24-hour period the test flux is removed by immersion i i isopropyl alcohol and the coupons art

evaluated visually. If there is any com plete removal of the copper film, a: evidenced by the background showing through the mirror, the flux has failed Discoloration of the copper by superficia reaction or only a partial reduction of tk film thickness is not considered a failure.

26 P R I N T E D C I R C U I T ASSEMBLY. F E B R U A R Y 1990

Page 2: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

Copper corrosion test samples (left = PMS, right = FAIL).

e properties of fluxes under and the coupon is heated to melt the

ersulfate, tap water, sulfuric water, deionized water and

f 2% relative humidity. Samples are removed after 72 and 240 hours, and are

Su face insulation resistance comb pattems.

examined for evidence of corrosion. The IPC defines corrosion as: “Ex-

crescences at the interface of the flux residue and copper boundary, or the residues or discontinuities in the resi- dues, or as discrete white or colored spots in the flux residues.” Any material that exhibits any evidence of corrosion is said to have failed this test. Figure 2 illustrates the pass/fail criteria. Surface Insulation Resistance

This test is designed to characterize the degradation of electrical insulation resistance of rigid printed wiring board specimens by a flux due to the deleterious effects of high-humidity and -heat condi- tions. Figure 3 shows two types of inter- digitated comb patterns that are used for general applications and telecommunica- tions products. The test procedures are detailed, and are different for various appli~ationsP3~ In general, the pattern is coated with the flux, heated to above the liquidus temperature of the solder, and tested both in soldered (uncleaned) con- difion and after cleaning.

An unprocessed (control) coupon is usually tested with the samples. A rosin flux with little or no activator typically has a higher surface insulation resistance (SIR) than the control, and the addition of activators or other components reduces the SIR. A certain amount of SIR degra- dation is allowed by the various specifica- tions, with telecommunications products requiring tighter standards than general electronics or consumer products. Silver Chromate Paper and pH

Two chemical evaluations, pH mea- surement and silver chromate paper test for halides, are typically performed on solder paste fluxes. The pH is specified in the slightly acidic to neutral range, typically from 3 to 7.5.

The silver chromate paper test deter- mines the presence of chlorides or bromides in soldering materials. A small sample (a drop of liquid flux or a 0.25 diameter dot of solder paste flux) is placed on a silver chromate paper and allowed to remain for 15 seconds. The paper is then immersed in isopropanol to remove residual organics. After drying, the test sheet is examined for possible color changes. A change to off-white or yellow-white indicates halide presence.

27

Page 3: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

figure Silver chromate paper test results. Two left photos: Little or no chlorides or bromides. Two rightphotos: Large amounts of chlorides or bromides.

igure 4 illustrates the presence of little * no chlorides or bromides vs. large nounts of chlorides or bromides. An additional chemical test is some- nes performed which uses the color mge of a zirconium alzarin purple lake indicate the presence of fluorides. easolls for Removing mign Residues The next question one might ask is, Why do we clean circuit boards after ch a thorough evaluation to ensure that e residues are harmless?” The answer twofold: cosmetic appearance and

rtability.

Large amounts of flux residue are unsightly, and some customers will not accept product exhibiting this character- istic. More importantly, flux residues can hinder bed-of-nails testing by fouling the test pins. Thus, if flux residues are to be left on the board, the residues must be made less visible and reduced signifi- cantly in volume. A final concern is the formation of solder balls, which is an intermittent problem usually associated with poorquality boards or paste or poor soldering process control. Thus, any no- clean reflow process must be carefully controlled to avoid the formation of

figure Poor rejlow of typical lowsolids solder paste in air. 5

solder balls. The following sectii describe methods of reducing res& volume.

Low-Solids Solder Pastes The simplest but least promisii

approach to reducing residue volume ii direct replacement of the rosin-bas solder paste with a low-solids-basil material. This would involve minim process changes and would be easii implemented. Unfortunately, no acceF able low-solids-based solder paste exisu and the probability of one being intm duced in the near future is low.

There are several difficulties with thi existing LSSPs. The rheology and stencil1 printing characteristics of these material1 are typically different from rosin soldex pastes, due to the difficulty in attaining ii good dispersion of solder particles in E

vehicle with very few (organic) solids.. We have evaluated the rheology of LSSPs; using the tests described in a recent paper! and have found them unsuitable for high-volume, high-yield printing. They are typically difficult to print con- sistently, due to the fact that their rheol- ogical properties are strongly influ- enced by shear conditions. LSSPs usually have lower t a c k i n e s ~ ~ ~ ~ ~ than conven- tional rosin-based materials. Finally, reflow using a standard IR profile is dif- ficult because of the volatility of the ac- tivators used in LSSPs. Figure 5 shows the poor reflow characteristics of a ty- pical LSSP which has been IR-reflowed in air.

PRINTED CIRCUIT ASSEMBLY, F E B R U A R Y 1990 28

i

Page 4: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

--a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi-

reflow and minimal residues. ng optimization studies

,"tion line in our laboratory, c -- aining process compatibility

soldering and to reduce residue. Figure 6 shows

ere is a small dot of resi-

is no residue under discrete , as Figure 7 illustrates. ponent fillets are shown represent a worst-case

i

d to reduce the

f residue. For smaller joint

e is visible on the joint e, as the amount of residue is rela- surface-to-volume ratio. ogravimetric Analysis (=A)

quantify the effect of process phere on residue content, TGA

ents were performed on several

The solder paste was tested as de- SCribed in an earlier paper;' under differ- entwntrolled atmospheres using a Cahn

rmogravimetric Microbalance. experiment, 0.05 gram of solder as heated at 100"C/minute to

25oy, followed by rapid cooling to room temperature. The weight loss in the solder paste represents volatilization of

anic components. Figures 8 and 9 the weight loss with time as the was heated to 250°C under two

different controlled atmospheres. The dopants decreased the final weight in comparison to pure nitrogen reflow. As shown in Figure 8, changing the concen- tration of dopant A from concentration X to concentration Y (Y < X) decreases the amount of initial weight gain and the

degree of paste slump. However, this decrease does not affect the total weight loss nor the soldering performance. Reliability Characterization

To ensure product reliability, any residues left on the board must pass the industrial qualification tests. AT&T has

6 Good reflow and minimal residue from proprietary solder paste reflowed in controlled atmosphere A.

fiuure 7 No residue under components from proprietary solder

paste reflowed in controlled atmosphere A.

29 TeDCIRCUIT ASSEMBLY, F E B R U A R Y 1990

Page 5: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

begun such testing, and the preliminary results are encouraging.

We have performed copper corrosion tests on proprietary solder pastes re- flowed in several controlled atmos- pheres. No evidence of corrosion was observed after ten days at 35 T and 90 % relative humidity, as Figure 10 illustrates. These test conditions are slightly differ- ent from those of IPC SP-819, due to chamber limitations. Although we be- lieve that there will be little difference when the tests are run under the specified conditions, the test will be rerun for IPC compliance.

Surface insulation resistance measure- ments were also performed. We prepared coupons by stencil printing solder pastes on the comb pattems and reflowing them in the appropriate atmospheres. All uncleaned samples had SIR values in excess of 10" ohms and approaching those of the unprocessed control coupons, as Figure 11 shows.

These results indicate that a no-clean reflow process employing proprietary solder paste in a controlled atmosphere can yield noncorrosive and nonconduc- tive residues. The process could easily be qualified for consumer electronics, but some additional testing will be needed after pastelprocess optimization to ensure the reliability needed for telecom- munications and other high-reliability applications.

Summary To achieve a no-clean reflow process,

the residues left on the circuit board must be noncorrosive and nonconductive. This article summarizes the types of testing performed to ensure that solder paste residues will meet these qualifications. Using these tests, the following no-clean dtematives were evaluated.

Do nothing except turn off the :leaner - a viable alternative for all isers of qualified materials, except in :ases in which the customer requires :leaning for cosmetic reasons or requires :lean pads for testing, or if the process 3ften produces unacceptable amounts of ;older balls.

Substitute a low-solids solder paste n the existing process. This would be the deal solution; however, no acceptable

-0.0046 I . , . , \~ , J O Time (Hrs: Min: Sec) 0: 40: 0 EA analysis of

proprietary solder paste under controlled atmosphere A.

O: O: 0

300

. . . . . . . . Bgure

s . . . v

E F

P

. . . 3

m w . . . e

8 . . .

0 0: 0: 0 Time (Hrs: Min: Sec) 0.40: 0

E A analysis of proprietary solder paste under controlled atmosphere C.

I figure

paste reflowed in controlled atmosphere A (top), controlled atmosphere B {bottom).

1

Page 6: No-Clean Solder Paste Reflow Processes · --a Retlow Processes ,.; is involved in a development m to investigate controlled reflow e-, Preliminary results indi- reflow and minimal

alternative will be a new a simple material solution. atmosphere reflow process

rietary solder pastes produces benign residue. With opt&-

-up, this process would pro- w capability for total no-

m

ical Reference TR-TSY-

ents for Electronic-

irements for Electronic-

818, Institute for Interconnecting Electronic Circuits, Lin-

,” Japanese Industrial Standard 97, October 1986 update.

nical Reference TR-TSY-

ng of Solder Paste for Fine-Pitch

“Solder Paste Tackiness

for Surface-Mount Technology,”

n R. Morris, Ph. D., is with the En- gineering Research Center of AT&T Bell Laboratories, Princeton, NJ. Nikhiles Bandyopadhyay, Ph. D., is

BOC Group Technical Center,

Soldering - m I Equipment Performance

with exact measurement of: Tip Temperature Voltage Leakage ESD Resistance

Hexacon’s Soldering Equipment Analyzers will test these three critical parameters and tell you exactly how your soldering stations, irons, pots and automatiC soldering equipment are performing. They will also test dynamic tip temperature under actual working conditions. The sophisticated level of today’s military and commercial electronics puts high demands on soldering equipment performance. Tip temper- ature, voltage leakage and ESD resistance from tip-to-ground must be held to very close tolerances. Hexacon’s Soldering Equipment Analyzers will test all makes of equipment to insure that these high performance standards are being met.

Model G-210 The industry’s finest analyzer. Features: W Uses both J and K type thermocouples W Imbedded thermocouple for precise temperature measurements W Contact sensor tip temperature measurements W Chart recording and computer data logger intexface

W Provides digital readout of all test values port for statistical process control

Model G-310 The ultimate in portability and convenience. Features: W Compact, hand-held unit without

compromising test accuracy W Has many of the features of the

Perfect tool for production line G-210

testing

Request catalog SEA-3 for information on our complete line of Soldering Equipment Analyzers and auxiliary equipment. Hexacon Electric Company, RO. Box 36, Roselle Park, NJ 07204-1946. Phone 201-245-6200 FAX 201-245-6176.

See Us At NEPCON WEST ’90 HEX N Booth #701

Circle Na 31


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