AN2035 - Application note

Whisker assessment and process qualification

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Whisker assessment and process qualification

Standardized Methodology for the assessment of whisker risk: JESD 201 (Environmental

Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface

Finishes) to be implemented together with JESD 22A121 (Test Method for Measuring

Whisker Growth on Tin and Tin Alloy Surface Finishes).

STMicroelectronics will abide by this standard and re-qualify the Tin plating line in

accordance with it.

Since no international standards were available 4 years ago when STM began preparing for

the RoHS initiative, customer specifications guided our testing strategy to qualify both our

internal and subcontractor plating lines. This qualification plan satisfies more than eighty

percent of our customers.

The qualification plan used the following stresses and criteria:

All customer specifications imposed a maximum permissible whisker length of 50µm.

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Ambient storage (15°C to 30°C, at 60 % RH) for 6 months

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Dry air: 50°C to 55°C for 6 months

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Thermal cycles: (–35°C to +125°C) for 500 cycles

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Temperature & Humidity: 85°C at 85%RH for 500 hrs.

A total of about 1500 units from 40 different package types have been tested in this phase,

with no evidence of whiskers.

The electronics industry’s search for an appropriate set of stresses for testing the

susceptibility of Tin-surface finishes to whisker formation has continued over the last 4

years. This search has been strongly influenced by several commercial consortia.

Most attention has been focused on the High Humidity stress since some studies have

indicated that this stress may in fact favor whisker growth. However, there is concern that the

conclusions of some of these studies were flawed, the results being confounded by the

presence of corrosion.

Unfortunately, now as more industry data is being generated, it is apparent that the high-

humidity environmental stress state has the ability to introduce corrosion in the test sample

and the corrosion may in turn produce spurious secondary whisker formation that is not truly

representative of the integrity of the surface finish under study. As a result, the various

commercial organizations providing input to the JEDEC task group have changed their

recommendations for the humidity stress state used for whisker evaluations at least 4 times

(60°C/95%RH, 60°C/93%RH, 60°C/87%RH and 55°C/85% RH) over the last year.

It is still not clear whether applying high-humidity stress conditions for a long period of time

either relates to a whisker growth mechanism or to actual field usage. For example, a review

of the literature shows no examples of whiskers associated with corrosion and it is clear that

whiskers have been observed even in vacuum.