Electronic assemblies for military hardware frequently undergo field conditions that extend beyond the commercial requirements for functional Class 3 specifications. The often harsh environments to which these assemblies are subjected, can preclude the more typical designs and circuit board constructions that are not equipped to meet the rigors of the inhospitable field conditions associated with mission critical operations. Communication and power devices along with other peripheral support equipment will need to withstand contamination from dust, sand, and water corrosion to operate reliably.

Portable Power – Environmental Requirement

A recent EMPF project, where a portable power supply was required to meet the aforementioned criteria, posed certain challenges in design, process, and quality diagnostics. The power source was to be protected against the seepage of water that would lead to inevitable corrosion. This was more difficult since the printed circuit board (PCB) transmitting the power is actually a part of an interface that must make repeatable electrical contacts to a removable unit. Additionally, the circuit board itself provided a functional mechanical sealing surface to keep the power unit assembly watertight. Since these boards are installed early in the assembly of the unit, failure of the boards to be watertight means that the customer would need to disassemble a significant amount of the unit to install a replacement board.

PCB Contact Board

The board assembly consists of seven gold plated button head pins. The two center pins function as the portable power supply transfer points to the adjoining conduit box, which contains the plug-in devices that supply the power to the respective unit. The shaft to the pins go through the printed circuit board and reduce in diameter to allow a flex circuit to be hand soldered in a subsequent assembly process. The shaft of the pin is also fluted along it’s length to allow better anchoring for the solder as it passes through the plated through-hole (PTH) in the PCB. The specification calls for a solder fill within the barrel of the hole, which will create a solid water tight seal on the underside of the pins. Migration of solder is not permitted on the top side of the pin, or around the perimeter of the PCB, where a rubber gasket functions as a mating surface to prevent water leakage into the power supply.

Since the PCB board is mated to a flexible circuit board, additional solder operations are performed to solidify the contact between the PCB and the flex circuit. The process of soldering will cause the existing pins on the PCB to “float” during the operation of applying the flex circuit. This condition can potentially cause the pins to rise above the surface of the PCB, violating the height requirements needed to avoid snapping of the pins during the “rotate and lock” maneuver to power the operational unit.

Soldering Processing

The original processing of the boards was done by conventional wave soldering which subjects the entire PCB to a wave soldering operation. There were two challenges with the wave soldering process:

• The dancer wave, which was to apply solder on the glued surface mount capacitors of the PCB, allowed solder migration on the underside of the soldering fixture where the board was being held in place during the wave soldering. This left a relatively rough solder bead around the perimeter of the annular ring which then had to be wicked off by hand, to form a flat sealing surface. This is a long and tedious rework when running the product in volume. The excessive solder deposition is exacerbated by the non-discriminate application of flux on the entire assembly, where excessive amounts of flux may pool.
  
  
  
  
• Reducing the aggressiveness of the dancer wave, and relying on the laminar wave to supply the necessary solder coverage needed to pass IPC Class 3 specifications, resulted in incomplete solder coverage in the PTH. Though the boards did meet class 3 specifications, problems did arise with a small number of boards due to voiding in the PTH. This “shadow effect” behind the pins, analogous to a rock in the current of a stream, produced solder joints that visually pass acceptability standards, but did not meet customer requirements.
  
  

X-Ray Diagnostics

Subsequent X-ray analysis showed that the solder did not completely fill the voids between the plated through hole and the fluting of the shaft. If the voids extended through the length of the shaft, a water leak source could be identified. These voids could have the potential of propagating even further during the flex circuit soldering operation to the pins.

The X-ray images show some of the obvious voids within the PTH that extends through the entire depth of the hole. The adjacent image below, exemplifies a completely soldered filled PTH. The void that was visible under X-ray was the culprit in the leaking pins that were found. These appeared as light areas that hugged the outline of the fluting on the pins.

When viewed from an angle, it was apparent that the bubble traveled the entire length of the barrel. These voids could be opened by the application of heat to the pin during a secondary soldering process, where the solder would become fluid, and air could travel freely.

Another example of solder voiding, as reflected in the crescent shaped light area in the fillet area of the solder joint, shows a surface air bubble that does not extend into the barrel of the hole. On examination of many of these types of voids, it was found that these were minor air bubbles just below the solder surface, and would not ordinarily be cause for concern if the air bubble remained in the pad area. This underlies the criticality of having non-floating pins during the flex soldering process under which air can migrate within the barrel of the hole. Fixtures have been adapted to provide compressive perpendicular pressure on the top side of the pins while the flexible circuit undergoes assembly to the bottom side of the pins. This will prevent the lifting of the pins as well as the migration of air into the barrel of the hole.

Process Solution

The uniqueness of this application required moving away from what would be a volume preferred method of wave soldering, to a more quality controlled method of selective soldering. Parameters that can be controlled are flux quantity, preheat time and temperature, solder placement, dwell time, travel speed, and withdraw parameters.

Some of the main advantages to using the selective soldering process for this application were the following:

• The elimination of excessive solder on the annular ring around the perimeter of the PCB board. This saved hours of rework and solder removal.
  
  
• The selective application of the flux prevented extraneous solder from forming onto areas beyond where it was required.
  
  
• Time controlled solder wicking into the PTH. A resident dwell time under each joint was established for the solder fountain. The correct diameter nozzle was chosen to apply proper amount of lateral solder application, while the amount of pressure in the solder fountain could be adjusted to assist the capillary action through the PTH. The additional dwell time assisted in the elimination of barrel voids.
  

Summary

Process issues may arise among assemblies that do not necessarily fall in the category of High Density Interconnects. Stringent environmental requirements can exceed the specifications applied to commercial products, and require a comprehensive approach in process development, mechanical fixtures, and diagnostic tools to resolve issues and apply them toward obtaining a good quality product. The EMPF continually engages the challenges of electronic manufacturing with a resolve to assist our valued customers using the latest in manufacturing, test, and analytical equipment. Most importantly, our EMPF associates bring experience, value, and innovation toward meeting present and future customer requirements.