Pb-free solder assembly for mixed-technology boards: tests show no joint degradation related to mixing SN100C and SAC alloys.

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Several Pb-free alloys have been investigated for SMT assembly. Many users have migrated to a family of alloys known as SAC (tin/silver/copper), and within this group industry has narrowed its focus to SAC305 (SnAg3 Cu0.5). The Solder Products Value Council (SPVC), under the guidance of IPC, recently completed a $1 million, yearlong study that recommended SAC305 as the default SMT alloy. (1)

It is known that SAC305 can cause negative results when used for wave, hand or selective soldering. For example, due to solder volume and solder joint configuration, SAC alloys often suffer from "hot tears/shrink holes." (2) These shrink holes can make it difficult to distinguish between a "cold" joint and a good joint, thus causing inspection difficulties. In addition, the cost of SAC305, although the lowest of the SAC family, is higher than other alloys due to its silver content. For these reasons, industry is looking for alternative alloys for non-SMT applications. Common alternative alloys are SN100C (3), SAC-LOW and SACX. (4)

Choosing a wave solder or selective solder alloy is more complicated than choosing an SMT alloy because they not only have to form a reliable solder joint, but they also are used in a molten solder bath. How these alloys react to molten solder baths is what differentiates them from one another. The SAC and SACX alloys dissolve most metals they come into contact with over time. For these alloys, new soldering equipment parts are necessary to keep equipment maintenance to a minimum. On the contrary, SN100C and SAC-LOW alloys contain additives (dopants) to reduce their aggressiveness to the contact parts in the solder pot. This also reduces copper corrosion, important when running selective soldering for rework applications. Figures 1 and 2 demonstrate that SN100C is less likely to remove pads and traces from boards during extended working times. Although SACX is similar in composition to SN100C, SACX is more prone to copper erosion during rework or extended soldering times.

The dopant added to SN100C also slows wetting slightly. Figure 3 demonstrates how the solder pot temperature affects wetting and solder joint performance. When the recommended soldering temperature is used, wetting is very similar between SN100C and SACX. Wave pot temperatures should be from 265[degrees] to 270[degrees]C to achieve maximum wetting and, depending on board layout, the dwell time in the wave should be ~5 sec. This dwell time is longer than conventional SnPb profiles; however, the extended dwell time will help topside solder fillet formation.

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SACX also has a dull appearance (Figure 4) and suffers from micro-cracks or the aforementioned hot tears/shrink holes. This also can lead to inspection problems.

Voiding in PTH joints. Another issue to consider is voiding in the PTH barrels. The SAC alloys are more prone to voiding than alternative alloys. This may affect reliability. SAC alloys have more than ample joint strength, even with the voids. However, if the leads are used for high-power applications, resistance or temperature dissipation might be an issue.

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Figure 5 shows a comparison of ENIG boards soldered with SAC and SN100C. The SN100C solder joints have consistently lower voiding and good fillet formation.

In addition to voiding less, SN100C is also less prone to surface cracks that form due to the cooling rate along intermetallic grain boundaries. This results in fewer cracks, shinier joints and an appearance very similar to SnPb solder joints.

Cross-Contamination

Pb-free alloys. Many assemblers want to use a single alloy throughout all soldering processes to avoid cross-contamination. The contamination of Pb-free solder joints from exposure to SnPb can be detrimental and has been documented by Seelig et al and others. (5) Little work, however, has been done on the mixture of different Pb-free alloys in the same solder joint. This mixture could stem from using the wrong repair solder or from board designs that do not provide sufficient space between SMT and wave pads. Fear of contaminating one Pb-free alloy with another has led manufacturers to use a single-alloy system despite the fact that SAC may not be the best choice for wave, selective or hand soldering. This paper addresses the issue of mixed Pb-free solders.

During typical SMT assembly there are few places that solder can cross-contaminate other than BGA assembly, in which the solder connection itself is a combination of solder ball and solder paste. Many of these Pb-free BGA balls are composed of a proprietary alloy mixture similar to SAC alloys. SAC is not used is because the drop shock test properties of these alloys is inferior to SnPb. Therefore, alloys doped with antimony, nickel, germanium, indium, silicon and other elements are used to improve this characteristic. These solder balls are then soldered with SAC during SMT applications without any issue. However, soldering with SAC to a SnPb solder ball gives much poorer results due to excessive voiding and a segregated joint that will fail at lower forces than either alloy independently.

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PTH. As Figures 6 to 8 demonstrate, the only alloy combination found to be detrimental to solder joint life during the wave soldering test for cross-contamination was Pb in SAC or any other Pb-free alloy.

SAC and SN100C are compatible when mixed by using a different touchup wire alloy from the alloy used in the wave. Therefore, no detrimental effect would arise from accidentally mixing these alloys.

SMT. Because they are found on both sides of the board and can be soldered with solder paste in an SMT reflow process or glued and soldered in a wave solder process, chip resisters and capacitors are the SMT components most likely to become cross-contaminated. If touchup or repair is required, these solder joints may have the wrong solder wire applied to them. To determine if this is a problem, SMT components were soldered with solder pastes comprised of SN100C, SAC305 and an SN100C and SAC mixture. These assemblies were then thermal cycled from 0[degrees] to 100[degrees]C for 1000 cycles to determine if these combinations would cause a failure. Figure 9 (online) shows the various solder joints.

The grain structures of the alloys are different and easily distinguishable in an etched cross-section. The grain of the SAC alloy is very large while the SN100C is finer and more uniform (Figure 10, online). The combination alloy falls between these.

Thermal Cycling Results

The SAC, SN100C and SN100C + SAC mixture performed the same under thermal cycling testing. None of the boards that were assembled and thermal cycled had any solder joint failures. From this initial testing on SMT joints and more extensive testing on through-hole joints, it does not appear that the cross-contamination between SAC and SN100C alloys has any negative effect on solder joint reliability based on thermal cycles.

The SAC, SN100C and SN100C + SAC mixture were each tested for shear strength of the components before and after thermal testing. The components were tested using a destructive shear test to measure the relative solder joint strength. The test consisted of a Chatillon TCM 201 SS coupled with a Chatillon DFIS-50 Force Gauge. Downward force speed was 0.25 in./min. Table 1 shows results of the shear testing.

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The solder joints made with SAC, SN100C and the SN100C + SAC mixture are all comparable to SnPb solder joints made on the same board. In addition, thermal cycling does not seem to degrade these solder joints.

Based on results from this testing, no degradation of the solder joint was found due to mixing SN100C with SAC alloy. Therefore, a two-alloy system may be successfully used for mixed technology boards without compromising reliability.

Karl Seelig is vice president of technology at Aim Solder (aimsolder.com); [e-mail].

Table 1. Shear Test Results

Alloy             0 cycles   1000 cycles

SAC305            15.3 lbs.  15 lbs.
SN100C + SAC mix  14.1 lbs.  14.3 lbs.
Sn100C            14.2 lbs.  14.0 lbs.
SnPb              14.8 lbs.  14.5 lbs.