Recently, a customer contacted the EMPF helpline to determine the cause of the high bulk ionic contamination and green color imparted to the isopropyl alcohol/water extract solution during Ionograph testing of a particular lot of assemblies.

During Ionograph testing, the customer discovered an ionic concentration greater than 100 µg NaCl equivalent/in2 from their printed circuit boards. Also, the printed circuit board leached green dye into the IPA/DI-water bath. With high ionic concentration and green extract in solution, the boards from this particular assembly were rejected for quality issues. As a result, the customer immediately contacted the EMPF to diagnose the problem.

To consider all possibilities, both bare and populated boards were requested from the customer to identify/quantify the ionic contaminant and possible non-ionic contaminant. Ion chromatography (IC) was performed to confirm the high ionic contamination as well as identify the ion contaminants. Fourier Transform Infrared (FTIR) spectroscopy was utilized to investigate possible organic contaminants and assist with indentifying a cause for the extract solution The extraction process resulted in a similar green extract solution.

Also, “tenting” at various vias was observed on the printed circuit boards (Figure 2-1). “Tenting” refers to the shape forming behavior of via masks into tepees (Figure 2-2) which lift off from the printed circuit board. The IC results from the bare and populated boards indicated excessive levels of chloride and bromide based upon the EMPF’s recommended maximum levels for boards and assemblies and the IPC5701 “Users Guide for Cleanliness of Unpopulated Printed Boards”.

The extract from the ion chromatography testing was concentrated and analyzed through FTIR spectroscopy along with one of the tents that dislodged from the board. These results indicated a number of common peaks shown in the spectrum below (Figure 2-3). The green line represents the spectra for the tented via. The red line corresponds to the spectra from the concentrated green residue collected during the IC extraction.

Conclusions:

The high bulk ionic cleanliness results and green tint imparted to the extract solution are the result of improperly or incompletely cured masking at the tented vias. This could have resulted from contaminants left on the PCB prior to masking or improper controls during curing. This conclusion is based upon the following:

• The mask does not normally succumb to attack from benign solvents like isopropyl alcohol and water.
  
  
• The tented vias were observed on annular rings.
  
  
• High levels of specific anionic residues were identified on the bare PCB. These residues can vary in corrosive nature, with the halides (fluoride, chloride and bromide) being the most detrimental agents. The chloride could have come from residual etchants or from direct handling of the substrate. Bromide, in a freely ionized form (i.e. flux residues) can be detrimental; however organo-bromide used in laminate flame retardant is somewhat benign. Sulfates can come from a number of sources including solder mask, plating bath chemicals (methane sulfonic acid, MSA), or tap water.
  
• The FTIR analysis of the collected extract matched well with the analysis of the masking at the tented vias, indicating the extract solution did contain masking at some point.

Recommendations:

The EMPF recommended a review of the PCB assembly process. It was not clear what solder resist was applied and how. As a result, any further recommendations would require additional information regarding the PCB assembly process. Focus should be applied to the solder mask step. This should include the manufacturing steps, materials, and an evaluation of manufacturing equipment.

Many manufactures follow a “just-in-time” process which follows the concept of lean manufacturing. As a common practice, the manufacturer will assemble parts that have been recently acquired. This reduces the in-process inventory of parts and improves the return on investment for the company. However, some manufacturers do not follow this practice and store parts in the stockroom for a prolonged period. As a result, material being used beyond its shelf life can result in defective product.