Military and Aerospace PCB Assembly
During mil/aero board assembly phase, several key steps can be enhanced to increase board reliability. Those include applying thermal compound to insulate heat sinks, soldering swages to the pad and PCB, pre-tinning stranded wires, extra spacing between component and PCB, keeping thermal profiles current to assure sufficient solder quality, and conformal coating.
On a mil/aero circuit board, applying thermal compound insulates heat from transferring to other components thereby acting as an heat insulator, reduces vibration, which can create cracking on the lead solder formations. Applying thermal compound to insulate heat sinks make this assembly step an estimated 5-10 % more reliable.
Flange mounts are normally press fit on the board for commercial applications. However, with a mil/aero PCB, once flange mounts are press fit, each swage is hand soldered to the pad and the PCB to minimize vibration. Reliability is thus increased by about 2 – 5 %.
Pre-tinning is a process involving dipping stranded or braided wire into hot melted solder, removing it, and drying it so that the solder formation solidifies. Soldering transforms stranded wire into one solid wire to eliminate air gaps and to allow current flow to be more stable. Thus, electromagnetic interference (EMI) effects and attenuation are reduced and mil/aero PCB reliability is increased by about 2 – 5 %.
If certain components on a mil/aero PCB dissipate more than one watt of heat, one of two remedies may be applied to minimize the damage that could be caused by excessive heat. Components can either be mounted about 20.00 mils. from the surface of the board or a spacer can be used between the heat sink and the component, itself. Both these strategies allow component heat to be thermally dissipated to avoid damaging the PCB. Spacing also acts as stress relief because there is sufficient clearance to protect components and PCB surface. Here, an estimated 5 – 10 % reliability increase is realized.
Mil/aero PCB quality and reliability assurance is applied when PCBs with components undergo wave soldering and reflow. Here, it’s important to perform a first article thermal profile. Running first article through before running the actual job ensures thermal profiles are properly studied and temperatures are correctly dialed in, as well. The objective is to get the correct temperature on the PCB to make sure there is more than sufficient solder wicking up to the board. This ensures solid and reliable connections and provides an estimated 2 – 5 % increased reliability.
Some mil/aero products are frequently subjected to long and continuous exposure to water, moisture, and extreme humidity and temperature conditions. PCBs can be more reliable by an estimated 2 to 5 % by making sure that sufficient conformal coating is applied with high quality acrylic-based sprays.
Mil/aero PCB reliability is increased by an estimated 2 to 5 % by increasing solder paste volume by 25 percent more per square inch of area unit compared to commercial PCBs. Also, for acceptable quality levels, (AQL), mil/aero boards must undergo 100 % inspection to assure 100 percent reliability compared to the commercial AQL standard of 0.65. To meet MIL standards, PCBs must have zero defects.
To learn more about Military and Aerospace for PCB Assembly, visit our PTH Reliability page.