AN2035 - Application note

Whiskers and thermal cycles

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Whiskers and thermal cycles

In thermal cycles, a different cause of whisker growth exists, which is linked to the mismatch

in thermal coefficient of expansion (TCE) between base material and coating layer, rather

than to localized internal stress. The TCE of Tin, Copper and A42 are 23ppm/°C, 17 ppm/°C

and 4 ppm/°C, respectively. Therefore, whisker growth will be more significant on A42 than

on Copper. Whisker length depends also on the extreme lower limit of the thermal cycles:

the lower this limit, the longer the whisker.

It must be noted that PbSn finishings, too, have a TCE that is close to the one of Matte Tin.

They are exposed to the same whisker growth, and give the same whisker length.

Therefore, Matte Tin behaves in a very similar way to the traditional SnPb coating.

Figure 5.

Temperature Cycling SnPb on FeNi42 (250 Cycles of –35 to 125°C)

Figure 6.

Temperature Cycling Sn 100% (500 Cycles of –35 to 125°C)

Whisker on

SnPb3 (18µm)

(over FeNi42)

Whisker on

SnPb15 (10µm)

(over FeNi42)