Printed Circuit Boards are indispensable electronic devices, but sometimes engineers are faced with temperature constraints that force them to think outside the box. In this article, we will be giving a high-level overview of designing a high temperature circuit board, and along the way we will consider some of the options you may be given when you find yourself in this situation.
Material Choices for a High Temperature Environment
Before we ever consider our circuit itself, we need to think about the material that we will have our manufacturer make our board from. The vast majority of boards are made of a material called FR-4, a glass-epoxy laminate that can withstand between 90 and 110 degrees Celsius. If our environment falls below this threshold, we can generally disregard this consideration and order our board just the same as any other design, but if we are designing for an application that will experience temperatures above the boiling point of water, we will need to upgrade our board to high temperature FR-4 or polyimide. These upgraded materials can withstand between 130 and 260 degrees Celsius, varying by manufacturer and specific material type.
Moreover, traditional leaded solder paste melts around the 160 to 180 degree Celsius mark, so we need to instruct our assembler to utilize lead-free solder paste. If not, we run the risk of components literally falling off of our board. Likewise, we should consider certain types of conformal coatings to further protect our design from baking.
Design Rules in a High Temperature Environment
At last, we can begin to focus on our design. This is where large challenges begin to surface. Many logic components simply cannot function in extreme temperatures. Many silicon chips fail to work as intended in high temperature environments, creating a useless board. To ensure this will not happen, we need to add a temperature parameter when searching for a component, especially if our design needs to include logic chips or microcontrollers. Most supplier search tools feature this option, but there are significantly fewer options available.
After we have successfully picked our components, we need to consider our placement. Generally, when placing our components, we want to keep all heat-creating components as far spaced as possible, even if heat would not be a concern for those components in a normal environment. This is because these devices will create a pocket of additional heat within the environment. Parts to consider include voltage regulators, high power resistors and the like.
Finally, we may want to consider our enclosure. While a PCB can be designed to fit most environments, in most cases it is beneficial to us to modify our case design to insulate against heat as a protective measure. This does not take the place of the design considerations we already discussed, but should definitely be considered as a first line of defense. Some considerations may include thermal insulation on the interior walls of a case, temperature shielding materials such as titanium and kevlar, or, in less extreme cases, ABS plastic.
High Temperatures due to Component Dissipation
Some components require so much heat dissipation that we should consider design rules for situations of high temperature components irrespective of their environments. This is a major concern in a wide array of designs since executions of good design can influence reliability and service life.
Just like in heated environments, it is a good idea to place heat-creating components distanced from each other. Collectively, multiple heated parts can contribute to the collective baking of the entire board, netting us with an unusable device. Also, adequate ventilation is always necessary. If we were placing our board in a case, and the parts we chose generate a lot of heat, a small case-fan in conjunction with a heat sink could help ease our woes. The placement of the fan is also important, since we want it to blow cold air over the blades of our heatsink to dissipate the heat and therefore decrease the temperature of the component.
Strategically placed vias throughout our design also can help thermally taxing parts dissipate heat. For example, if we were using a flat surface mount package, we could use several vias to attach the thermal mass of our part to the ground plane. We can achieve even greater thermal conductivity by filling these vias with a conductive paste. This, in effect, turns the entire board itself into a heatsink, thereby eliminating the need for a standalone one. While this is not always possible, such as in most through-hole designs, this technique allows us to lower costs and reduce line items on our bill of materials.
While this has not been an exhaustive list, these practices, among others, can help us create boards that will survive the test of high temperatures. Although this takes quite a bit of practice to perfect, the general concepts are quite simple, can be learned rather quickly, and do not require too many concessions in terms of circuit design. As engineers, we will always be given difficult design constraints, but armed with this knowledge temperature constraints can be significantly simplified.