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92 PCB007 MAGAZINE I FEBRUARY 2020 ment can be a good process step. Keep in mind that the boards will need to be completely dried after developing. With constant production at higher outputs, the use of developing as the only pretreatment process will disturb the normal solder mask process. The developer is key equipment in the solder mask process. It can be used for devel- oping or pretreatment, but not for both at the same time. Additionally, pretreatment must be carried out at a lower transportation speed; otherwise, the PCBs will not be dry. If the ENIG process is performed at a sub- contractor facility, hold time might be longer. Sometimes optical inspection is performed be- fore solder mask coating; other times, electrical testing is performed before coating with solder mask. In all these cases, the gold surface may be contaminated with fingerprints, dust, sa- liva, and/or grease, so an additional cleaning process prior to solder mask coating is neces- sary. Figure 1 shows a dirty gold surface next to a cleaned gold surface. For cleaning gold surfaces, an alkaline-based cleaning process is recommended. The alka- line cleaning agents deliver the best clean- ing results when compared to neutral or acid cleaning agents. The disadvantage of alkaline cleaning agents, however, is a tendency toward strong foaming. If surface contamination is minimal, then cleaning in a neutral or slightly acidic medium is possible. Although there are strong acid cleaning agents available, I do not recommend a strong acid cleaning process for a nickel-gold surface. At the microscopic level, three or more metals come in contact (gold, nickel, copper). I am concerned that, in a strong acid medium, a gal- vanic element is created with accelerated cor- rosion of the less noble metal, despite the use of inhibitors. Alkaline Cleaners There are several types of alkaline cleaners commercially available. These cleaners also have the advantage of removing fingerprints leftover from inattentive handling of the pan- els. It must be stressed that you need to fol- low the technical datasheets, as concentrations and operating temperatures vary. After clean- ing with alkaline, rinsing with demineralized water is necessary. These cleaners are also suitable for cleaning gold surface finish PCBs before shipment. This last cleaning step will reduce ionic contamina- tion and significantly improve the solderability and bondability of the boards. Neutral or Slightly Acid Cleaners There are also a number of slightly acid or neutral cleaners available. This class of clean- ing agents provide reduced foaming while pro- viding a good cleaning capacity. The products can be applied by spraying or dipping. Again, concentration and temperature may vary by supplier. Figure 1: SEM picture of ENIG surface (L) and ENIG surface with visible contaminants (R), both at 1000X magnification.

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