August 24, 2009 — A new application note from Johnstech International Corporation, released in conjunction with a new test contact product, describes the advantages of NiPdAu packages and outlines the remaining challenges.
The company notes that a significant percentage of IC packages are migrating from matte tin pads and leads to those using NiPdAu plating. The cost of pre-plated NiPdAu packages has decreased significantly over the last several years making NiPdAu a more cost-effective plating option. In addition, NiPdAu is an excellent alternative to matte tin, particularly when tin whiskering is a concern.
NiPdAu-plated IC packages can present testing challenges. Without the proper socket design, material, and forces, the extreme hardness of NiPdAu material will result in faster pin wear rate, higher spare pin usage and higher overall cost of test. Advantages of NiPdAu
There are several reasons for this transition to NiPdAu plated packages, according to the new app note:
1. Device users in high reliability applications such as medical, military, automotive and telecom had concerns about tin whisker growth in some matte tin packages under certain conditions. To limit the liability in high-reliability end-user applications, more testing and inspection is often required. This can lead to higher cost of test. While it is true that there are ROHS exemptions in some of these high reliability applications, many related applications still do not allow the use of tin-lead plated packages. Therefore, the manufacturer must choose between matte tin and NiPdAu for their package plating.
2. Matte tin-plated packages are prone to the buildup of tin oxides during package testing. When testing these parts, the contacting (socket) technology can build-up with these oxides, which increases the contact resistance (in an absolute sense) and increases the variation of the contact resistance (standard deviation). On the other hand, NiPdAu plated devices do not include tin, and therefore are not susceptible to the same tin oxide build-up as their matte tin package counterparts. Test Contactors such as those provided by Johnstech that include a self-cleaning wipe function significantly reduce the fluctuation in contact resistance, whether from oxide buildup in matte tin or other material residues associated with other platings.
While many manufacturers initially opted for matte tin packages due to the perceived lower acquisition costs, the bottom line is that many manufacturers are migrating to NiPdAu packages. In many cases, their overall costs are actually reduced due to the elimination of the additional tin whisker monitoring that was required with matte tin and the higher OEE that is gained by the reduction in cleaning frequency on the test floor.
Challenges of NiPdAu
While NiPdAu packages exhibit the benefits described earlier, there remain numerous challenges.
1. The hard and often rough surface finish of NiPd and NiPdAu packages can cause excessive contact wear in Test Contactors and sockets..
2. Package singulation is often accomplished by sawing the NiPdAu plated lead frames. This can leave a sharp burr on the edge of the package, leading to potential additional contact wear. Additionally, if the burr is dislodged during the handling or contacting process, the debris can cause more frequent cleaning cycles. Refer to the photo below.3. NiPdAu packages are not always clean and free of residues, adhesives, and debris. While oxides are not the issue like in matte tin packages, the presence of these contaminants can result in more frequent cleaning cycles. Having a self-cleaning contact wipe can extend the cleaning cycle interval, thus improving the MTBA.
There are also variations in NiPd and NiPdAu package platings Some package vendors use NiPd plating while others add a thin layer of gold to produce a NiPdAu plating. The texture and consistency of NiPd or NiPdAu can differ significantly among manufacturers. For example, the photos shown here illustrate how the texture and grain structure can vary between two different package platings.
Contact life depends on many factors
Since NiPd and NiPdAu platings are much harder and rougher than their matte tin counterparts, Test Contactors or sockets must be designed to withstand these challenges. Contact life depends not only on the package material plating properties, but it can
vary even more significantly based on the handler-to-Contactor interface, handler velocity, whether auto alignment plates are used, alignment plate opening size and tolerances, and test temperature.