Recently, the Helpline responded to a call concerning residue and contamination on the solder joints of a circuit board. The customer complained of a white residue on the solder joints, but the circuit boards passed ionic cleanliness testing. The EMPF worked with the caller to identify the source of this residue and prevent its recurrence.

Typically, when the Helpline team answers a residue and contamination question, the solution is related to ionic contamination. Ionic contamination can cause corrosion and intermittent short circuits, particularly in fine pitch board designs. Common sources of ionic contamination are flux residue, perspiration, and plating chemistry. Many ionic contamination issues can be resolved by making changes in materials and to the cleaning process.

The customer’s boards were soldered with Sn63 solder and had been stored for 3 to 6 months before the white residue appeared. The residue appeared only on the solder joints and was not found on the substrate or any other area of the assembly. When viewed under a microscope, the residue appeared flakey (Figure 3-1). Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was performed on the surface of the solder joints. Additionally, Fourier transform infrared spectroscopy (FTIR) was performed on the residue.

SEM and FTIR analysis showed that the white residue was likely lead carbonate [PbCO3·Pb(OH)2]. The formation of lead carbonate is the result of chloride reacting with lead in the solder, leaving an insoluble lead chloride (PbCl2). Carbon dioxide water (moisture, humidity, etc.) reacts with the lead chloride, forming the lead carbonate constituent. The growth of lead carbonate occurs slowly at room temperatures but increases rapidly with increased temperature. Lead carbonate is insoluble in water and typically remains on the circuit board. The chlorine continues the cycle of corrosion of the solder surface, leaving a white residue.

The EMPF performed an on-site process audit and investigated the board assembly process and its affect on the white residue formation. The audit revealed the following:

1. Incoming inventory and storage were tightly controlled. All components, boards, and assembly materials were inspected and indexed. There were no apparent sources of cross-contamination.

2. The manufacturer isolates the production for the boards in question. The facilities used to assemble these boards are not utilized for other commercial production; therefore, the likelihood of cross-contamination and/or materials incompatibility is low.

3. The manufacturer displayed good assembly practices. Component placement, stencil printing, reflow, and rework were not considered sources of contamination.

4. After review of the data sheets and contacting the product manufacturers for further information, the EMPF
determined that the assembly materials did not provide a possible source of contamination.

5. The cleaning process, although adequate for most water soluble flux residues, required further investigation.

The manufacturer utilized a stainless steel in-line aqueous cleaner for all their assemblies. The cleaner has three cleaning zones and one drying zone. The pre-wash and wash zones were self-contained. Clean, deionized water was added to these zones daily, but the water was recycled within each zone. The water temperature in the pre-wash and wash zones was heated to 105°F and 140°F, respectively. No saponofier or surfactant was used. The final rinse used a deionized water flush at room temperature. The water in this cycle was held in a tank and expunged after the cycle. The deionized water used in the final rinse measured approximately 7 MW at room temperature. Boards were run through the in-line cleaner as many as 5 times during a normal assembly process. Omegameter cleanliness testing was performed during each shift as a process control tool.

The manufacturer was using a water soluble flux that required higher wash and rinse temperatures than those used in their normal in-line cleaning process. The flux has been reported to be difficult to clean at temperatures below 160-170°F. The flux residue contained chloride and was a likely source for the creation of lead chloride.

Discussions with the paste manufacturer, regarding cleaning of their water soluble flux, revealed that the following improvements can be made to the cleaning process to ensure removal of all flux residues:

• Reduce belt speed
• Increase pre-wash temperature
• Increase wash temperature to 160-170°F
• Increase final rinse temperature
• Increase cleanliness of incoming deionized water to 17 MW

The SEM/EDAX and FTIR analyses confirmed that the white residue was lead carbonate. This white residue may not show up during normal cleanliness tests since it is not soluble in water and alcohol. The residue left by the soldering process and the bare boards likely contributed to the formation of the lead carbonate.

The EMPF recommended that the cleaning process be examined for effectiveness at removing these residues before insoluble lead chlorides are formed. Experiments should be performed with water flow rate, peak temperature, and belt speed to ensure that all soldering residues are removed.