CUSTOMER ISSUE: The EMPF Helpline responds to calls spanning a wide range of issues encountered by electronics manufacturers. One common area of concern is solderability. Arecent call reported a soldering problem involving a process utilizing a unique form of solder mask, namely Milk of Magnesia. The problem arose when trying to solder lugs to a cable composed of stranded wire. The lugs were placed very close together with the requirement that the cable remain flexible in between the lugs. As a result, the soldering had to be performed with very little wicking of solder from the lugs. It was observed that the soldering area was turning very black and the solder was not wicking between the strands, nor flowing out from the lugs.

The Helpline team approached the problem by observing the wires firsthand, critiquing the current soldering process, offering soldering suggestions, and performing energy dispersive spectroscopy (EDS) in conjunction with scanning electron microscopy (SEM) to determine if contamination was present on the wires composing the cable. Figure 1shows a SEM micrograph of a wire from a charred area.

The EDS, used to determine the elements present on the sample, was performed at multiple locations including the charred areas, areas near the charred areas, and wire locations from the main spool of cable.

Magnesium, thought to be from the Milk of Magnesia used as a solder mask, was present near the charred area. That is, the magnesium was present very near to the areas in which soldering was attempted. It is possible that the Milk of Magnesia, used as a solder mask, was excessively applied, which supports the theory that soldering was attempted on areas covered with the Milk of Magnesia. It was the reaction of the soldering temperatures with these covered areas that could have caused the charring.

The details of the current soldering process were conveyed to the EMPF Helpline team. Suggestions included using a lower activity flux than that currently in use, and to discontinue the use of Milk of Magnesia as a solder mask.

It was postulated that if Milk of Magnesia was overly applied to areas that were to be soldered, then an unknown interaction could occur that could cause the charring appearance noticed as well as the lack of soldering. A member of the EMPF Helpline team was able to solder a lug to the cable using RMA flux and Kapton tape at appropriate areas to prevent wicking. Further details of this experiment were conveyed to the caller for their consideration. The Helpline team also recommended that using a resistance soldering iron would be best for this sort of application .

Information on an aluminum fixture that could be constructed and used as a heat sink to prevent solder wicking was also offered. A subsequent response by the caller claimed soldering success, mentioning information gained about a resistance soldering iron as very helpful.

We were glad to respond to this inquiry, and aid in the problem resolution. That is the purpose of the EMPF Helpline. Whether Milk of Magnesia is the source of your problems or an anecdote, the EMPF Helpline is available to help you get your electronics manufacturing process going again.