One of the most commonly ignored risks in designs is the impact of heat dissipation practices on PCB assembly processes.
PCB design is quite often a balancing act between function, size, cost and timelines. The drive to squeeze more into a smaller space while ensuring functionality, managing cost and hitting tight deadlines can dominate. Too often these imperatives can overshadow any consideration of manufacturability risks.
Understanding that designing for manufacturability can’t be the top priority, we see the consideration and mitigation of PCB manufacturing/assembly risk as an integral part of the design process. One of the most commonly ignored risks in designs is the impact of heat dissipation practices on PCB assembly processes.
Heat dissipation and the associated calculations have become increasingly complex and vital as components have shrunk in size and PCB designs have become more and more densely packed. Thermally conscious designs are vital to the reliability of any design so that components and bare board material can function within prescribed temperature ranges and ensure proper functionality and longevity.
Conduction of heat through thermal vias and copper pours, heat sinks, active heat exchangers and other tools are used to dissipate heat. The use of thermal vias and copper planes are most common as they typically have minimal cost and size impacts on a design.
The question of risk arises in how thermal vias and copper planes are designed on the PCB. Often, the sole focus of these features is how efficiently they can move heat from components. This ignores real assembly risks that can result in increased re-work, delayed projects or even reduced yields. These risks are particularly present in fast, small-run production volumes and prototype runs where highly customized assembly processes such as reflow profiles and paste applications are less feasible. And yet, there remains a misconception on this reality in the marketplace.
Some industry literature from leading multinational component manufacturers stress designs features that maximize heat dissipation without even acknowledging the associated risks to assembly processes. Other literature suggests over-riding CAD programs’ default design enhancements meant to lessen these risk.
We recommend first an understanding of the risks factors and how they arise as well as the incorporation of design elements where possible that will mitigate these risks.
At its most basic level, the use of thermal vias and copper planes for thermal relief have an inverse effect on solderability risks.
Calculations for the appropriate number and width of thermal vias to transfer heat can result in pads dotted with vias. Uncapped and unfilled vias – particularly those larger than ?? mil – thieve, or suck, solder from the pad during reflow. The more vias there are and the larger there are the more thieving. Thieving forces the assembly company into a battle to customize the volume of solder that will result in a solder joint meeting IPC standards.
Inspection via x-ray can be required and insufficient solder will result in rework of the component which in-turn introduces new risks.
For SMT pads in copper planes, the use of thermally relieved pads (spoke-and-wheel) is key. These pads are surrounded by soldermask with a design of spokes (or “traces”) radiate out from the pad. Basic examples below.
Without these reliefs, pads can heat unevenly during reflow increasing the chances of solder issues such as tombstoning or poor solder connections. These issues, in turn, require rework which is substantially more complicated and risky as the copper plane will drain heat from the soldering iron or heat gun. The technician must apply additional heat to fix the solder issue increasing the risk of damaging components or bare board material. These reliefs can dramatically reduce these issues.
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