Reflow Soldering with Tin Copper Eutectic

Lead-Free Alloy

Gerjan DiepstratenProcess Support ManagerCobar Europe BVMember of the Balver Zinn group

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Drivers for alternative for SACThere are a number of drivers to have a LF non SAC

alloy.

Technical:• Copper leaching (rework issues)• Reliabilty of SAC after ageing• Hardness• Voiding• Appearance of shrinkage cavities

Financial:• SAC305 contains 3% Ag

SAC305

SN100C

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SnCu vs SN100CTetsuro Nishimura found that additions of nickel and

germanium at low but quite specific levels dramatically change the behavior and appearance of SnCu.

Sn99,3Cu0,7 + Ni-GeSn99,3Cu0,7

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Challenges for a SnCu pasteDesign a flux system that enables:

• Good printing characteristics• Thermal stable structure• Tack force and time to prevent component movement• Compatible with specific processes: vapour phase,

intrusive reflow• Good cold and hot slump properties to avoid SMD

bridging• Residues that are harmless and not sticky• Long storage and stencil-life

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Paste - flux characteristicsXF3 flux is developed for reflow processes with

extended temperature profiles (lead-free)

- without nitrogen (cost issue)- incorporates organic materials with more advanced molecular structure

Modified wood resin Synthetic polymer4x larger molecular system

XF3 flux

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Print performance of a pastePrint performance is determined by the rheological

network of a paste:• Metal percentage and sphere

size• Topography of the powder • Resin system• Solvents• Property modified additives

Attention:This rheological network can be damaged by excessivehigh transport or storage temperatures.

Topography of SAC

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Print conditionsPrint conditions should by consitent to guarantee the

visco-elastic behavior of the paste.

Viscosity of the SN100Cpaste is temperature dependingwhich limits the printingtemperature 22 - 28 °C

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Printer performance• Maximum printing speed is process indicator for the

print performance of the paste• Differences in performance due to topographic

characteristics of the alloys

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Thermal Stability XF3

Low evaporation rate results in longer stencil-

lifeActivation starts

at 150°C.Remains during soak

High activity atpeak temperatureswhere oxidation becomes critical

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Full Factorial DesignDesign of Experiment to define solderability of SN100C

versus SAC305:

A. Solder paste – 4 levels:SAC3-XF3SAC4-XM5SSAC305 – competitor SN100C-XF3

B. Reflow profile – 2 levels:3 minutes – linear profile5 minutes – ramp soak spike profile

C. Reflow cycles – 2 levels:1 x reflow2 x reflow

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Solder paste

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Reflow profiles

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Spreading of solder paste

Dimensions aperture: 5 x 2.1 mm4 x 9 apertures per boardArea aperture = 10.2 mm²

Measure width and lengthto define solder spreading area.

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Solder spreading results

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Solder spreading microscope

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Solder spreading analysis

• XF3 solder paste flux results in better spreading on OSP.• SN100C-XF3 shows a good spreading with similar profileas SAC305.• 2 reflow cycles did not influence the spreading on small pads.• 6% more spreading for the 5 minute RSS heating profile.

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Parameters affecting spreading

DoE experiment:R-Sq = 85.70%

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Intermetallic thickness

Solder (SN100C)

Intermetalliclayer

Copper

FR4Printed Circuit Board

(Ni, Cu)3Sn4, (Cu,Ni)6Sn5, and (Cu, Ni)3Sn; (Ni, Cu)3Sn4 was the most commonly encountered IMC.

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Intermetallic thickness

DoE experiment:R-Sq = 78.24%

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IMC analysis• No significant differencebetween 3 and 5 minutereflow profile (only 1%)• One heating process before reflow results in a 4% thinner intermetallic layer(OSP is more oxidized)

• The average IMC thickness of SN100C is more or lessequal to the SAC305. (Alloy with higher melting point wetsfaster)• Ag will accelerate the IMC growth during ageing.• Ni will inhibit the IMC growth within the SnCu system.

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Spreading factor

D - HS = ---------- x 100 %

D

S = Spread factor [%]D = Diameter “visual circle” [mm]

SAC305 – spread factor 99.54%

SN100C – spread factor 99.45%21/32

(Klein Wassink – Soldering in Electronics pg. 79)

Vapor phase soldering

SN100C-XF3Dull surface, clear,small amount of flux residues

SAC3-XF3No significant difference

between the two alloys.

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Vapour phase - tombstoning

Remarks to the vapour phase soldering:Tombstoning was observed for both SAC305 and SN100C Minimal small flux residues on the solder mask aftercleaning for both alloys

SN100C-XF3

No significant differencein yield between SN100C and SAC305 vapourphase process.

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VP – Solder joint appearance

Cleaning standard components:Very good cleaning results : no residues sticking to the

solder mask or componentsPaste is designed to be no-clean. Residues are harmless

(REL0)

SN100C-XF3SAC3-XF3

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Intrusive reflow-printingStencil thickness = 0.006” (150um)

Stencil aperture size = 0.080” (2.03mm)Solder paste printed into through hole and over

solder mask

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Reflow – Pin in paste

SN100C-XF3Top and bottom side show nice solder fillets acc. to IPC-610D

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Feasibility SN100C BGA’s

Voiding: 100% X-ray inspection sample size 4096

BGA Spheres Solder paste 1 SAC305 SAC3052 SAC305 SN100C3 SN100C SN100C

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Voiding SN100C vs SAC305

SAC3-XM5S13% voids

SAC3-XF311% voids

SN100C-XF34% voids

Voiding:- Solder paste flux- Oxidation of solderpowder

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Reliability solder joints

Thermo cycling:-40 °C + 125 °C

2512 Resistor

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Boeing Recommendations

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Corrosion resistanceAfter Exposure to a Marine Environment:

SN100C SAC305

Silver increases susceptibility to corrosion

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Conclusions• SN100C is not a drop in for SAC305.

• SN100C solder paste is a lead-free alternative for SAC305;The question that has to be answered is whether that extra cost of the SAC305 can be justified in terms of the performance of the alloy.

• SN100C at least matches the performance and reliabilityof the SAC alloys in most situations.

• While it is possible to reflow SAC alloy at temperatures as low as 232°C most reflow is done at temperatures more like 245-250°C at which temperature SN100C also can be used.

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