BY CRAIG HUNTER & CHRIS REYNOLDS
Some connector manufacturers began supplying tin (Sn)-based lead-free products in 2001 in preparation for compliance with the new RoHS regulations that go into effect on July 1. But with the market quickly changing to lead-free products, tin whiskers have become a significant concern.
Tests have shown that whiskers can appear on the Sn contact areas not long after the connectors undergo mating and unmating motions, particularly in connectors with shorter leads. Japanese connector suppliers are very strong in this miniature connector market, with products targeted for the consumer electronics market. While these applications are purely commercial, many connector manufacturers switched to tin-copper (Sn-Cu) plating as a possible alternative to pure Sn.
The Japanese Electronics and Information Technologies Industries Association (JEITA) studied the problem and determined that while whisker growth on Sn-Cu plating is more problematic than on other lead-free platings (such as Sn-reflowed, tin-bismouth [Sn-Bi], and tin-gold [Sn-Ag]), it could be mitigated by heat-treatment. Studies for whiskers typically require a series of accelerated environmental tests, using conditions predetermined by the International Electronics Manufacturing Initiative (iNEMI). The studies are performed on virgin product for comparative purposes but do not reflect actual performance in solder-in applications.
While soldering materials are standardized into a Sn-Ag-Cu base, many manufacturers are researching the practical use of low-temperature solder. They hope to find a stable plating material that mitigates the whiskering effect even under typical mounting temperature conditions.
Experiments by AVX involved the preparation of various kinds of Sn-Cu-plated samples to verify the mitigation effect. We applied treatments of alkaline degreasing, electrolytic decreasing, and acid activation on phosphor bronze coupons stamped out in the form of connector contacts and undercoated with sulfamic acid nickel. Next, we applied electrolytic plating to create both matte Sn-Cu plating and semi-bright Sn-Cu plating. The samples included six variations in thickness ratio of matte plating and semi-bright plating, while the Sn-Cu film thickness was maintained on all samples.
We prepared three types of flat-flexible-cable (FFC) connectors: one subjected to no reflow, one subjected to a reflow temperature of 230° C, and one subjected to a reflow temperature of 250° C. Various flexible printed circuits (FPCs)/PPCs were connected to each type of connector and then exposed at the room temperature (25±10° C) at which whiskers are expected to grow.
The result after 72 hours was the formation of the longest whiskers on the connector and the dummy FPC (see Fig. 1). The thicker the Sn-Cu film, the shorter the longest whisker measured. Since the film hardness of the matte Sn-Cu plating is lower than that of the semi-bright Sn-Cu plating, the matte Sn-Cu plated layer presumably absorbs the external stress. The thickness ratio that showed the best whisker resistance was a matte film thickness ratio of 0.8.
 FIGURE 1. A filament-type whisker. (Photo Credit: AVX) |
By using the X=0.8 thickness ratio showing the best whisker resistance, we conducted a test with the Sn-plated FFC for 100 hours. Whiskers were effectively mitigated on every sample. The length of whiskers generated on the contact point of the coupons was 50 μm or less, regardless of reflow conditions. When comparing the samples with no reflow to the ones subjected to reflow at 230° C, the whiskers generated were slightly fewer and shorter. We believe this is most likely due to the annealing effect of the reflow itself.
Also noted were the effects on two kinds of Sn-Cu plating layers in samples subjected to reflow at 230° C. Due to the short period of peak temperature retention time, the Sn-Cu plating did not start to melt. Also, crystals in both layers grew bigger in grain size than those with no reflow. (This Sn-Cu grain is probably the result of the softer coating.)
The Sn-Cu plating forms the biggest grain from re-crystallization after melting during reflow at 250° C: it is believed that this is when the Sn-Cu layer is softest. We also observed that a needle-shaped, inter-metallic compound is formed on the border of the matte Sn-Cu layer and the nickel layer. This alloy layer is made up mostly of Ni3Sn2 or Ni3Sn4, which may be the cause of the film hardness increasing in the reflow at 250° C. AVX found that this phenomenon resulted in whisker generation on the conductor of the FPC/FFC. (Fig. 2)
 FIGURE 2. Whisker growing from a nodule. (Photo Credit: AVX) |
It’s clear that plating that provides high whisker resistance and good corrosion resistance could be achieved by controlling the size of the crystal grain. (Fig. 3) This can be done by using either semi-bright or matte Sn-Cu plating. Semi-bright is essential in securing solderability, while matte reduces the hardness of the film and inhibits whisker generation.
 FIGURE 3. A scanning electron micrograph image of an SMT terminal of a connector shows no tin whiskers. (Photo Credit: AVX) |
The best whisker resistance is obtained when the film thickness ratio is X=0.8. Whiskers generated in the mated condition become saturated after 100 to 500 hours.
For typical applications, our data shows that semi-bright Sn-Cu plating strikes the optimum balance for whisker mitigation and solderability in Pb-free and traditional reflow systems. Since the mechanism of whisker generation is not yet fully understood, mitigation will have to be further studied with attention paid to bath composition, fabrication condition, contact pressure, shape of contact point, and other variables.
CRAIG HUNTER (hunterc@avxus.com) is strategic marketing manager, and CHRIS REYNOLDS is field applications engineering manager at AVX, Myrtle Beach, SC.
