Other Parts Discussed in Thread: LMC555
I have been planning to use the LMC555 in uBump package. I read in International Rectifier's AN-1050 that early failure modes were encountered using encapsulants and conformal coatings with the DirectFet package, which is similar.
Does National/TI have any experience with failures in coated/encapsulated uBump packages?
I have been planning to use a 35 Shore A hardness encapsulant.
The only info I've found on TI's site is App Note 1412 http://www.ti.com/lit/an/snva131j/snva131j.pdf
that specifies that the large bump ic's (49 bump and higher) don't require underfill.
Also, AN 1412 specifies 1oz copper plating or thinner on the pcb. The board I'm laying out is plated 2 oz.
Any recommendations regarding the LMC555?
Thanks much for help,
Paul Toffolo
AdvancedPWM
International Rectifier’s original tests initially complied with the manufacturer’s instructions, which advised against allowing the coating to flow under devices because the thermal expansion of the coating might lead to fatigue in the solder joints. This caused a problem: material flowing up to but not under devices trapped air beneath them. In high- temperature testing, the expansion of the trapped air caused a failure in a medium-can device with a small die (and therefore more space for air beneath it).
The tests were repeated, this time disregarding the instructions and allowing the material to flow under the devices. Although this solved the problem of device failures, signs of increased fatigue in the solder joints were observed — as predicted by the manufacturer.
Results
As the summary table shows, mean RDSon shift increased from 130μΩ for an uncoated FR4 board to 300μΩ for a coated board. Even this substantial rise did not fully reflect the potential problems of using some coatings with some early DirectFET devices.
There are two potential solutions to these problems:
Use of underfill before applying coatings, preventing air from being trapped under devices
Use of a coating material with a lower modulus of elasticity, allowing trapped air to expand without causing damage to devices
Conclusion
Several customers are using conformal coatings successfully with the latest DirectFET devices. Automotive-grade devices are expected to pass a new series of tests. However, International Rectifier recommends careful evaluation of the properties of proposed coatings and, where necessary, the use of underfill before coating. Thorough testing is advised.
International Rectifier would like to thank Techsil
http://www.irf.com/technical-info/appnotes/an-1050.pdf