Does localized brush cleaning really eradicate flux residues?

Localized brush cleaning is one method to remove residues from areas with a heavy buildup of flux from hand soldering, selective pallet soldering or sensitive areas that need to be cleaned. After evaluation of a large number of brush cleaning processes, no data from any manufacturers we worked with indicated they qualified the brush-cleaning process using SIR (surface insulation resistance) or ionic analysis other than a ROSE tester. We wanted to see the effect, and understand why we saw so many problems with flux residues on brush-cleaned processes causing electrical leakage, corrosion and dendritic growth failures.

Using the umpire assemblies that passed a recent process qualification of a no-clean, low solids mixed technology process here at Foresite, we were able to assess areas with selective soldering, SMT replacement and brush cleaning.

Test Matrix Using Umpire Assemblies

• Three unprocessed controls.
• 20 processed assemblies (10 without brush cleaning and 10 with brush cleaning).
• Selective solder on three comb patterns (same as IPC-B-24 test coupon).
• Removal and replacement of 68-pin LCC (same as IPC-B-36 test coupon).
• Then, on 10 boards, these areas were cleaned with a brush and alcohol (using a beaker with 100 mls of IPA).

Test Results

The unprocessed control showed great electrical performance and low ionic residue (Table 1). The 10 processed assemblies with no brush cleaning and proper heat to activate flux showed great electrical performance and good low ionic and organic levels in the selective solder and 68-pin LCC areas when using the C3 localized extraction system. Brush-cleaned results showed two assemblies with marginal electrical, eight assemblies with failing electrical performance and moderate to high levels of weak organic acid (WOA) levels on the localized and 68-pin LCC areas.


The first two brush-cleaned assemblies marginally passed SIR (168 hr. of 85°C/85%RH), while the remaining eight assemblies showed poor electrical performance and higher WOA levels on the assembly surface. What we learned is that brush cleaning doesn’t remove the residue, but rather redistributes the flux residue and breaks down insulative residues formed during soldering. Because flux levels increase on the sequenced assemblies’ surfaces, it is clear the brush is transferring flux from one board to the next. This residue is solubilized WOA residues: succinic, malic, adipic and glutaric on these samples. These residues are conductive and moisture-absorbing in this chemical state. ROSE analysis on the boards showed the brush-cleaned samples had similar levels to the non-brush-cleaned samples: all below 3.6 µg/in2 of NaCl equivalents on an 8 x 12" assembly with only two locations brush-cleaned. Ion chromatography, SIR and C3 results showed a large difference between the groups, while it appears that the ROSE tester was unable to see the localized areas of contamination when averaged across the entire board surface.

Conclusions

Localized brush cleaning shows various levels of flux residue, and when no volumes of rinsing solution are used to flush solubilized residues off the board, what happens is the redistribution of flux residue to nearby areas of the assembly. When using localized cleaning without rinsing, the risk is high and poses a great risk for leakage and corrosion problems.

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

• Prev
• Next