Case 5

ASSEMBLY MATERIALS

CE Analytics Case Study #5

Customer: Major

Network

Board

Manufacturer

Problem: Connectors

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after

soldering

in

the

wave

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suspected,

.

a

new

source and

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Background: Customer was experiencing discoloration of Au plated connector surfaces after completing its soldering operation. It appeared that the staining on the connector surfaces was creating unacceptable resistance through the mating parts. The customer requested assistance to identify the source, and determine the cause of the discoloration on gold plated mating surface of pins located on the “female” portion of the pin connector.

was

high

showed

dark

staining

resistance

failures

Material

investigated

contamination and

was

confirmed.

Investigation Details: The customer sent the following components for analysis: six and one half connectors as received from the connector manufacturer, plus one half of a connector, brushed with the water soluble spray flux currently used in their process, and dipped into a solder pot containing eutectic solder at about 480˚F. The customer also provided a sample of a lubricant that had been previously determined by an outside lab of being present on the components. The half of a connector, fluxed and solder-dipped, was thoroughly examined on a stereo microscope. It was confirmed that the gold plated pins possessed a faint residue that more closely resembled a brownish-black stain. As a starting point, a solder pot dip test was conducted to determine if the staining was related to a specific flux. One of the connectors was cut in half. One half was dipped into the currently used flux and the other half was dipped into a water soluble rework flux as a reference. Each part was shaken to remove excess flux, then each was dipped into the solder pot, which was filled with eutectic tin/lead solder at approximately 480˚F. Each sample was immersed in the solder for about 2 seconds, the amount of time necessary for the leads to heat up and for the solder to flow up and wet the surface of the leads. Subsequently, the leads were rinsed with warm deionized water and examined with a stereo microscope. The part fluxed with the reference flux did not appear to have the brown/black staining that was present on the part fluxed with the currently used flux. However, it should be noted that the reference flux is an extremely viscous rework flux and not practical for use as a spray flux. Next, a section of a part was cut off, and put into mounting compound and metallurgically prepared for cross-sectional microscopic analysis to inspect the gold plating. The gold plating revealed no obvious abnormalities, such as large amounts of occluded organics that might be released and play some role in the formation of the residue and/or contribute to the observed staining. Since a lubricant had been previously determined to be present on the connectors, an experiment was conducted to clean the connectors prior to fluxing and solder dipping. The first solvents tested were isopropyl alcohol and ethyl alcohol. The parts

ASSEMBLY MATERIALS CE Analytics Case Study #5 were cleaned in both alcohols, separately, for five minutes, in an ultra-sonic cleaning bath, then removed and hot air dried. The amount of staining observed after fluxing, solder dipping, and deionized water rinsing was similar with the two alcohol solvents, but the alcohol pre-cleaned components showed slightly less staining than the components soldered as is. The next solvent tested was trichloroethylene (TCE). The same cleaning procedure was followed with this more aggressive solvent system. Upon examination, there was a noticeable difference in the amount of staining on the uncleaned part after soldering and rinsing compared to the part pre-cleaned in TCE. The TCE part was found to be free of the residue/stain. The same test was repeated with two uncleaned and six TCE pre-cleaned parts. The same results were obtained. The black residue was not present on any of the parts pre-cleaned in the TCE, but was still present on both uncleaned parts. The photographs in Figures 1a through 2b, are representative of the solder dip test results. The uncleaned parts have the same residue/brownish-black stain occurring on the gold plated surface of the pins on the connector. The trichloroethylene cleaned parts do not have any evidence of residue or staining on the surface of the gold plated pins on the connector.

Figure 1a – “Uncleaned” connector, fluxed, solder dipped, water rinsed and dried before examination

Figure 1b – “Uncleaned” connector, fluxed, solder dipped, water rinsed and dried before examination

Figure 2a – “Cleaned” connector, fluxed, solder dipped, water rinsed and dried before examination

Figure 2b – “Cleaned” connector, fluxed, solder dipped, water rinsed and dried before examination

Conclusions: The currently used connectors supplied by the component manufacturer are now being sourced from a new supplier. Those from a previous supplier did not exhibit any staining, even though the process and soldering materials were exactly the same as those currently being used. Since the time of this transition, the staining problem has become progressively more pronounced.

ASSEMBLY MATERIALS CE Analytics Case Study #5 For this reason, it can be deduced that there must be something specifically related to the differences in the connectors that is associated with the problem. Among the differences to be noted are the holes in the housing into which the pins are pushed. These have been increased in diameter and there is no longer an epoxy material being used to “seal” these openings around the pins. The epoxy was previously used to keep excess flux from entering the housing. In addition, there is now analytical (FTIR analysis) evidence of a synthetic lubricant being present on the connector pins. The lubricant has the signature of a commercially available product used in the connector fabricating shop. It is not clear whether or not the lubricant is intentionally used in processing, or if it is deposited by some means from the environment in the area where the connectors are fabricated or stored. Regardless, FTIR analysis has revealed clear evidence of its presence on the pin surfaces of the connectors prior to soldering. Based on all of the accumulated information and observations gathered from the tests conducted during this investigation, we have reached this conclusion: It is most likely that the high resistance connector failures have occurred as a result of some sort of interaction between the currently used flux and the residual lubricant on the surface of the currently used connector pins. This interaction has obviously taken place during the soldering process. It is clear that there is a synthetic lubricant present on the currently used connectors that was not present on the previously used connectors. Incidentally, the residue/staining problem was not encountered with the previously used connectors, only with the currently used connectors. Upon ultrasonic cleaning of the connector in trichloroethylene, the lubricant has likely been removed. The removal of the lubricant residue prior to soldering seems to have resulted in the elimination of the unwanted and deleterious residue/staining. Recommendations: It is recommended that the connector supplier provide the customer with connectors free of lubricant residues. This will thereby eliminate the problem by preventing the deleterious interaction with the flux, and the resulting formation of the residue and/or stain. Periodic incoming FTIR analysis could also be used as a spot check to verify compliance with the customer’s requirements. Outcome: Since the implementation of the recommendations, there have been no recurrences of the high resistance connector failures and no evidence of any undesirable staining of the connector mating surfaces following the soldering operation. Cookson CE Analytics has a long history of solving electronics assembly problems. Our talented laboratory staff applies its in-depth analytical and diagnostic experience to effectively detect the root causes of problems and offers realistic solutions that help our customers meet their process goals. We are committed to sharing our expertise – we call it “Shared Intelligence”.

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