Always compare soldering interactions to the bare board’s cleanliness.

Per J-STD-001, a new solder paste, cored wire and wave solder requires process requalification. We designed a board to help qualify primary and secondary soldering operations processes. Using the Umpire Test assembly, we are able to look at a variety of interactions such as top- and bottom-side SMT, wave solder, selective solder, hand solder and repair. All these processes should be evaluated against incoming bare board residues from a primary supplier. The best evaluation conditions are to compare the residue impact, soldermask porosity and soldering residue effects that make up the finished product to ensure the soldering processes and bare board cleanliness have no detrimental effect on product performance.

The Umpire Test board permits a look at process interactions. The test coupon was designed as a mixed technology assembly, but has been used to evaluate double-pass SMT reflow in combination with selective wave or selective robotic soldering.


Here are data from a process qualification conducted on an internal mixed-technology assembly in three areas:

• An 80-pin TQFP (with every other pad connected to make SIR measurements pad to pad) (Figure 1).


• A 0.006" comb pattern below the TQFP (to assess entrapped residues with four via opens next to comb pattern).

• A B-24 comb pattern on the assembly bottom side (next to vias below the TQFP) (Figure 2).


This qualification was to assess the residues of an assembly process for a Pb-free no-clean with selective wave soldering. Each area was pasted and reflowed, then set for 24 hours. Selective solder with a no-clean liquid flux was sprayed to the assembly bottom side, then wave-soldered using a selective soldering pallet.

Figures 3 and 4 show the electrical effects of topside TQFP components with large vias near and below the component.


The areas of the TQFP, 0.006" comb and the selective solder B-24 comb all showed failing (less than 1.0e8V of resistance). None of the SIR failures produced a dendrite, but visible flux residue was seen in all areas before and after environmental exposure. In this case, selective soldering created failures for the assembly in the area around the selective solder sites, while the B-24 control and 68-pin LCC showed good electrical performance of the solder paste and selective solder areas exposed to the solder. Areas around the selective solder site and on the board topside near vias that were fluxed permitted flux to remain in areas that were not completely heat-activated. This flux is conductive and moisture-absorbing, and in some cases has proved to be sufficiently corrosive to grow dendrites.

Terry Munson is president of Foresite Inc. (residues.com); tm_foresite@residues.com. His column appears monthly.

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