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January 2017 • SMT Magazine 15 EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS the 12 µin limit in IPC-4556. One paper sug- gests a limit of 7.8 µin 4 , although one study found that Pd thickness as high as 20 µin had no impact on solder joint shear strength 5 . In this study, PBs plated with a typical EN- EPIG surface finish (per IPC-4556) will be assem- bled with a minimum amount of solder to cre- ate a very small solder joint where the Pd con- centration becomes larger relative to the over- all solder joint. Solder shear strength and solder joint analysis will be used to investigate the ac- ceptability of using ENEPIG with SnPb solder in these very small solder joints. Experimental Procedure For assembly and testing, an existing test vehicle was selected for use with a small LGA package. Using an LGA package limits the sol- der volume in the solder joint since there is no contribution from a solder ball as there is in a BGA package, resulting in a higher Pd concen- tration in the solder joint. A small LGA package of 6 x 6 pad array was selected and will be in- stalled at the four corners of a larger BGA foot- print in a manner so that only the middle 4 x 4 array of pads on the part are soldered. This will be done to increase the quantity of data points for the shear test, and to limit the number of solder joints which will limit the shear force re- quired to remove the part. The test vehicles (24 total) were separated into four sets, with three of these sets going to three separate vendors (each of which used a different plating chemistry) for application of the ENEPIG finish, and the remaining set coat- ed with HASL (control set). Once returned, the PBs were subjected to XRF for measurement of the tri-metal layer thicknesses. The data from these measurements is shown in Figure 2 (each vendor represented by a different color). A stencil of 3 mil thickness was used for sol- der paste application. The apertures of the sten- cil were matched to the PB pads (as designed) at 14.8 mil diameter. Half of the assemblies were Figure 2: Palladium thickness measurements (color indicates each vendor).