A customer called the EMPF Helpline and asked for assistance in evaluating their rework process for a new Class 1 assembly process which failed
reliability testing.

In this case, visual inspection revealed the issue to be excessive fluxresidue remaining around the reworked solder joints, most likely due to improper cleaning after rework. Properly cleaning an assembly after rework is extremely important in maintaining board reliability. Ionic contamination left by flux residues can lead to corrosion and dendrite growth, two common causes of electronic shorts and opens. Identifying and implementing a cleaning solution for this assembly was necessary to greatly improve the reliability of the assembly. The EMPF offers various analytical techniques (ROSE, IC, FT-IR, SIR) to determine the root cause of contaminant problems, and to evaluate the effects of process or materials changes on cleanliness.

Figure 2-1 shows an example of a solder joint with acceptable flux residue per IPC-A-610D Acceptability of Electronic Assemblies guideline. The joint is clean, with little or no evidence of solder residue. On the contrary, it is easily seen in Figure 2-2 that a large area surrounding the solder joint is covered in flux residue. This solder joint clearly was not cleaned properly. According to the Joint Industry Standard requirements, IPC J-STD-001D, the maximum rosin flux residue levels for process compliance are as follows:

• Class 1 assemblies less than 200 micrograms / cm2
• Class 2 assemblies less than 100 micrograms / cm2
• Class 3 assemblies less than 40 micrograms / cm2

The recommended RMA flux removal method requires a saponifier heated from 100 to 150ºF to properly remove any residues. Deionized water is recommended for the final rinse. Reagent grade isopropyl alcohol (IPA) can also be used for manual cleaning by agitating with an acid brush. The affected board was cleaned using these methods and retested to verify Class 1 compliance. The more extensive cleaning process was proposed for implementation at the customer site in order to improve future reliability testing results.

The EMPF has the equipment and the experience to assist with our customer’s process development needs. For customers interested in learning more about flux selection and choosing the right cleaning process for performance and cost effectiveness, we recommend participation in the EMPF’s Electronics Manufacturing Boot Camp and IPC certification classes. Students of these courses gain an understanding of the concerns about cleanliness and residues common with electronic devices, the types of residues that are considered benign or harmful, the various cleaning chemistries and processes, safety issues, and the most commonly used cleaning equipment.

For more information please contact the registrar at 610.362.1295, via email at registrar@empf.org, or find course descriptions on the web at http://www.aciusa.org/courses.