Karen Tellefsen, Ph.D.
Alpha, an Alent plc Company
South Plainfield, NJ, USA
Manufacturers of soldering materials have been asked by electronics device manufacturers and contract electronics PWB assemblers for Surface Insulation Resistance (SIR) data of solder pastes and other soldering materials on IPCB-52 coupons. IPC-B-52 coupons were designed for evaluating the electrochemical reliability of the whole soldering process, not just the materials, used by PWB assemblers. The bare coupons should made by the same board fabricator using the same materials and processes as the boards used by the PWB assemblers. Additionally, the test coupons should be assembled and soldered by the PWB assemblers themselves. Having a soldering material manufacturer or a commercial test laboratory assemble and solder the coupons using a IPC-B-52 kit will not provide the complete benefit of evaluating the whole soldering process, i.e. all the materials and processes used by the manufacturers. The completely assembled and soldered coupons may then be tested for SIR to a standard test method, such as IPC TM 650 Method 2.6.3.7., by the assembler, a commercial laboratory or the material manufacturer.
The present SIR and ElectroChemical Migration (ECM) testing methods for soldering materials do not address the electrochemical failure mechanism concerns of flux residues trapped under component and the ever shrinking spacing between solder joints. However, requiring the use of complex and expensive IPC-B-52 coupons for material evaluation is a poor solution for this issue. New test vehicles for soldering materials that include occluded flux residue and tighter board spacing are needed.
This paper asks more questions than it answers. However:
- IPC-B-52 coupons were intended for entire electronics process evaluation, and are not appropriate for single soldering materials evaluation.
- SIR test vehicles for soldering materials evaluation need to have smaller conductor spacing than those presently specified.
- An SIR test vehicle needs to be developed that will evaluate the propensity of a material to allow ECM with occluded flux residue.
- SIR and ECM temperature and humidity conditions need to be reexamined as some materials have better SIR and less tendency for ECM at 40C / 90% R.H. and other materials have better behavior at 85C / 85% R.H.
