A post-selective soldering heating step helps flux reach activation temperature.

Last month we reviewed conditions of a new process qualification. The data showed poor electrical and ionic residue performance from the selective solder process. This month we will qualify a new step (post-heating the selective solder area after soldering), which involves the following: After selective solder, boards are placed over the preheat zone to heat-activate the flux (the time and temperature will vary depending on flux volume and board thermal mass).

Let us review data from a process qualification conducted on a mixed technology assembly in three areas:

• An 80-pin TQFP (with every other pad connected in order to make pad-to-pad SIR measurements).
• A 0.006" comb pattern below the TQFP (to assess entrapped residues with four via opens next to comb pattern) (Figure 1).
• An IPC-B-24 comb pattern on bottom side of assembly (next to the vias below the TQFP) (Figure 2).

This qualification assessed assembly process residues for a Pb-free no-clean with selective wave soldering. Each area was pasted and reflowed, then sat for 24 hrs. Selective solder using a no-clean liquid flux was sprayed on the assembly bottom side, and then wave-soldered using a selective soldering pallet. During SIR testing at 85°C/85% RH and 40°C/93% RH, topside TQFP components with large vias nearby and below the component showed WOA levels in the selective solder area of 49.65 µg/in², while levels in areas outside the selective solder area (no solder contact) were 194.22 µg/in².

Initial conclusions. The areas of the TQFP, 0.006" comb and selective solder IPC-B-24 comb all showed failures (less than 1.0e8 ohms of resistance) under 85°C/85% RH and 40°C 93% RH conditions (Figures 3 and 4). 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 assembly failures in the area around the selective solder sites. The IPC-B-24 control and 68-pin LCC showed good electrical performance of the solder paste and selective solder areas exposed to solder. Areas around the selective solder site and on the board topside near vias that were fluxed showed flux remaining in areas not completely heat-activated. This flux is conductive, moisture-absorbing, and in some cases has proven to be corrosive enough to grow dendrites. The amount of partially heat-activated flux created a moisture-absorbing condition that caused the leakage path and subsequent electrical failures.


Selective soldering optimization. After the first qualification failures and high WOA values from the selective solder process, it is clear the process residues are causing poor electrical performance. We chose a unique approach to optimizing the flux residue. Instead of trying to clean the flux or reducing the amount, which would jeopardize solder joint quality, we chose to apply heat (using the preheat section of the selective solder process) for 45 sec. to reach the flux activation temperature and drive off the carrier. WOA levels showed a large reduction following post-heating of the area away from the selective solder. The selective solder area showed a WOA level of 15.29 µg/in², while the area outside the soldering area was an order of magnitude lower, at 18.35 µg/in². The SIR of these test boards is shown in Figures 5 and 6.


The optimized selective solder areas showed great electrical performance in high humidity environments (greater than the 1.0 e8 V of resistance required by J-STD-001). This was achieved by adding a heating step after selective soldering that permitted flux to reach its activation temperature and drive off the carrier. The optimized process shows no visible difference in the selective solder area under visible conditions, but reacts differently under humid conditions. No-clean fluxes are supposed to be insulative and protective; however, this is only upon reaching the point of complete activation, and when they are completely complexed by the removal of the carrier. To that point, these areas become conductive pathways that continue to absorb moisture over time, even below conformal coatings.

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