Evaluation criteria for post-welding PCB surface cleanliness

Today, when the no-clean process is widely adopted, it is extremely difficult to obtain a very clean post-welding PCB surface. The presence of flux residues on the post-welding PCB surface is inevitable. The problem lies in how to define cleanliness. If the PCBA board surface is visually compared to be “dirty” as shown in the figure, with obvious corrosion marks or accumulation of flux residues, then it can be judged as NG (not qualified) by visual observation. In most cases, it is between the two extremes mentioned above. At this time, it is necessary to rely on tests to inspect cleanliness. There are mainly two evaluation methods for board surface cleanliness:
(1) SIR testing
(2) Ion contamination testing.

Next, we will focus on the second type of ion contamination testing. As mentioned earlier when discussing the definition of no-clean, whether flux residues need to be cleaned or not ultimately depends on whether they will affect electrical safety performance. And SIR testing is the most effective test method. The basic principle of ion contamination testing is to measure the resistivity of the extract of flux residues on the board surface. The standard criterion is that the ion contamination value should not be greater than 1.56 grams of NaCl equivalent per square centimeter. For specific details, please refer to IPC-TM-650. In the no-clean process, a certain white residue is very common.

Under the volatilization of certain fluxes and changes in temperature and humidity, after SMT processing, it is normal for some white residues to appear on the surface of the finished PCBA board. If the customer has special requirements for the cleanliness of the surface process, after the product inspection is completed, a surface cleaning workflow can be arranged for the SMT factory. Then, three-proofing paint is sprayed for moisture-proof and anti-oxidation treatment.