Rescue cleaning of no-clean assemblies: saving hardware after a failure analysis.

After a failure analysis, we are routinely asked if the current inventory can be cleaned or recovered in some fashion. Our answer: It depends. With the issues of dirty incoming bare boards for a no-clean assembly process, a cleaning protocol that cleans no-clean flux and residues from board fabrication must be used. DI water only has not proven to be as effective in cleaning no-clean residues. Cleaning with a soaping agent works well, but cleaning water intolerant components or switches can be an issue. Saponifiers are designed to wet the wash water to get into tight, low standoff places. This works on open relays of mechanical switches, while cleaning below the components. This cleaning is critical for the product performance, but may require replacement hardware on the mechanical open architecture.

The following case study is a review of hardware cleaning after failure analysis showed the root cause of field failures as a combination of dirty bare boards and heavy fluxing of a no-clean VOC-free material. Bare-board contamination showed high chloride from an immersion silver process using hydrochloric acid etch prior to silver plating.

The heavy flux residue levels on the surface of the assembly are shown in Figure 1. Flux absorbed moisture on the surface of the assembly and created leakage and corrosion pathways, causing poor circuit performance.

Cleaning with a saponifier at 10% in a heated wash (140[degrees]F) using an inline aqueous cleaner reduced the chloride and WOAs to acceptable levels (Table 1). By reducing the chloride residue from the bare-board fabrication and the WOA residue from the flux, the customer was able to put the assemblies into the field without moisture-absorbing and corrosive residues on the board surface. Since the cleaning recovery two years ago, no returns have been documented on that product program. Before, hardware in the field saw a 22% return rate after six to nine months in use.

[FIGURE 1 OMITTED]

Terry Munson is with Foresite Inc. (residues.com); [e-mail]. His column appears monthly.

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Table 1. Residue Levels Before and After Cleaning

                                            Ion Chromatography
Sample     Description              CI    N[O.sub.2]  Br    N[O.sub.3]

Sample 1   Supplier 1 bare PCB      9.51  0           1.09  0.17
Sample 2   Supplier 2 bare PCB      7.62  0           1.13  0.11
Sample 3   Supplier 3 bare PCB      9.96  0           1.17  0.16
Sample 4   Failed assembly area 1   9.56  0           2.54  0.08
Sample 5   Failed assembly area 2   9.16  0           2.36  0.13
Sample 6   Failed assembly area 3   8.55  0           3.04  0.13
Sample 7   Failed assembly area 4   9.74  0           2.84  0.11
Sample 8   Cleaned assembly area 1  1.21  0           0.12  0
Sample 9   Cleaned assembly area 2  1.35  0           0.23  0
Sample 10  Cleaned assembly area 3  2.04  0           0.33  0
Sample 11  Cleaned assembly area 4  1.24  0           0.27  0

           Ion Chromatography    C3 Results
Sample     P[O.sub.4]  WOA     Results  Time (sec)

Sample 1   0             0     Dirty     44
Sample 2   0             0     Dirty     49
Sample 3   0             0     Dirty     40
Sample 4   0           218.63  Dirty     11
Sample 5   0           221.39  Dirty     10
Sample 6   0           219.57  Dirty      9
Sample 7   0           209.87  Dirty     13
Sample 8   0            18.32  Clean    178
Sample 9   0            16.98  Clean    180
Sample 10  0            17.14  Clean    177
Sample 11  0            15.69  Clean    180