Death of a Battery -- Review your facility's hand-soldering process-it could be the cause of product failure.

Recently, a client experienced product failure in the field. A preliminary inspection indicated the four AA batteries, used in the product, were drained within 2 to 3 weeks of field operation, but no visible short circuit existed on the board. A clear residue was present near the hand-soldered battery terminals on all boards.

Board Analysis

To discover the cause of the failure, a comparative analysis was conducted of a bare board, two failed boards and two current boards. The bare board consisted of a FR-4 epoxy glass laminate, LPI solder mask and hot-air solder leveling (HASL) metalization. The bare board also used a no clean manufacturing process to complete the finished assemblies.

The battery area of each finished assembly with wave solder and hand solder flux residues sampled and compared to a reference area on the opposite side of the board. The board was subjected to wave soldering only-not to hand soldering. Analytical evaluation was performed using ion chromatography per IPC-TM-650, method 2.3.28, to characterize process residues.

Data Discussion

The most significant difference indicator was discovered to be the large delta in the weak organic acid (WOA) levels between the reference area and the battery area (Table 1).

Weak Organic Acids

WOAs, such as adipic or succinic acid, serve as activator compounds in many fluxes, especially in no-clean fluxes. WOAs are benign materials and are not typically a threat to long-term reliability. In general, water-soluble fluxes have a much lower WOA content than do low-solids (no-clean) fluxes, and the amount of residual WOA is proportional to the amount of residual flux. Typically, bare boards do not contain WOA residues. When WOA levels are under 400 mg/in2, the residues are generally not detrimental.

Excessive or additional amounts of flux and WOA (appreciably greater than 400 mg/in2) can present a significant reliability threat to finished assemblies. An excessive amount of flux can produce an electrical current and can leak across the conductive flux residue. This condition exists when a lack of sufficient thermal energy, from hand soldering, prevents nearby flux from reaching full activation temperature. Unreacted flux residues readily absorb moisture, which promotes the formation of conductive pathways for potential current leakage failures.

Problem Found

High levels of flux-WOA activators-indicate a thick layer of flux, which is difficult to fully activate. Uncomplexed flux residue is conductive and moisture absorbing. Conductive moisture absorbing flux, when in the proper location (between contact terminal leads), was the root cause of leakage failures that drained product batteries. Examination of the hand-soldering process revealed that operators were using a squeeze bottle to apply liquid flux to the board-before hand soldering the battery terminals. Furthermore, the amount of flux applied varied from operator to operator.

Problem Solved

Operators were trained to use a minimum-and consistent-amount of flux using a flux pen and a heat gun. Thermal feedback was used to establish a heating profile that would activate or complex any remaining flux on the surface of the board near the connectors.

Terry Munson is with Contamination Studies Laboratory (CSL), Kokomo, IN; [phone]; e-mail: Residuguru@ aol.com; Web page: www.residues.com.

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