|
|
 |
 |
 |
 |
 |
 |
| A publication of the National Electronics Manufacturing Center of Excellence | January 2011 |
|
Recently, a customer called the EMPF Helpline and asked for advice regarding tin whisker qualification and long term reliability tests for electronic assemblies. The first part of the inquiry can be answered with a reference to the number of JEDEC and GEIA standards that delineate acceptance requirements for tin whiskers (Figure 2-1) in both commercial and military hardware. The second part of the question is much more difficult to answer since none of the methods outlined by the standards can guarantee a total mitigation of whisker growth.
Whisker Growth Mechanism There is no single accepted theory that explains the tin whisker growth mechanism. The leading theories postulate that the growth mechanism is powered by a release of internal stress in the tin finish or by re-crystallization and abnormal growth in the tin grain structure1. Since the growth mechanism is not fully understood, the required environmental stress tests used for qualification such as thermal cycling and thermal aging do not necessarily simulate tin whisker growth in an assembly. A linear growth rate for tin whiskers has not been observed during long term studies of whisker growth. There is a latency period after the coating is applied during which tin whiskers do not grow, which can range from a period of days to years. Once a whisker starts growing, it will most likely grow at a constant linear rate, but not all whiskers on a particular finish will grow at the same rate. This unpredictable whisker growth rate which leads to inaccurate acceleration models makes qualification testing of high reliability electronics assemblies a challenge. Qualification Tests For tin whisker acceptance testing, the commercial electronics industry has utilized JEDEC standard JESD201 (Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes). Due to the long service life (>10 years) required for the high reliability electronics assemblies used in the aerospace and defense (A&D) industry (Table 2-1, Class 3), JESD201 requirements are not adequate. While valuable as a process evaluation tool for data comparison, the environmental test conditions do not correspond with actual A&D service environments. Tin whisker testing in JESD201 focuses on humidity, thermal cycling (1500 cycles maximum), and calendar aging for a maximum of only 4000 hours (5.5 months). The A&D environment includes vibration, shock, corrosion, and even probe marks in the surface finish left behind during troubleshooting. These environmental conditions induce stress on the thin tin finish which promotes the growth of tin whiskers.
The GEIA-STD-0005-2 (Mitigating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems) is presently the adopted standard used to manage (but not eliminate) tin whisker risk. These mitigation methods include PbSn solder dipping of component leads, conductor spacing control, and conformal coating. Unfortunately, the current knowledge base does not include decades of experience to verify the success of these mitigation methods and their affect on system reliability. Mitigation Techniques While pure tin and high tin content finishes are not recommended for Class 3 electronics assemblies, the push to keep A&D electronic assembly costs down has precipitated the use of COTS (commercial off the shelf) components whenever possible. Unfortunately, since pure tin finished devices are common and inexpensive, they have inadvertently made their way through A&D supply chains. Component manufacturers and distributors frequently use the lot number as the only discriminator to distinguish the lead-free finish. Even reputable distributors have difficulty tracking lots of pure tin finished components versus the same component with an alternate finish. As a first level of tin whisker mitigation, it is critical to screen for pure tin finished components at incoming receiving. Conformal coating, which is required for most military applications, is another whisker mitigation technique frequently used to help retard the effects of whisker growth. By isolating the individual leads, the coating can prevent arcing due to whisker bridges. The more elastic coatings rely on elongation properties to prevent penetration. Other coatings rely on their hardness to restrain or redirect whiskers to grow back on themselves, preventing them from breaking through and potentially shorting. It must be reiterated that currently conformal coating is not a fool proof method to prevent whisker growth. There are various methods and materials available for conformal coating. The following have been the most commonly used, with most having been qualified under the IPC CC 830 specification for conformal coating. The coatings are listed in increasing order by their expected tin whisker mitigating value.
The EMPF has utilized and tested many of the more commonly used coatings. With further research still pending, the ALD-Cap high alumina conformal coating has shown some encouraging signs as a reliable method for tin whisker mitigation. An overview of the challenges of lead free electronics manufacturing and tin whiskers can be found in the publications of the Lead Free Electronics Manhattan Project4. More information about coatings and tin whisker mitigation techniques can be found on the EMPF website, www.empf.org or obtained by calling the EMPF Helpline at 610.362.1320. References
|
|||||||
The EMPF is a U.S. Navy-sponsored
National Electronics Manufacturing Center
of Excellence focused on the development, |
|
| ACI Technologies, Inc - - www.aciusa.org - - (610)362-1200 |  |
