PCBs Accommodate For Torsional Stresses

A printed circuit board (PCB) is the key to many electronic devices and equipment, including MP3 players, cell phones, calculators, digital watches and computers. The rapid pace of technological advances in the last two decades has allowed PCBs to shrink while also incorporating more and more functionality into these compact devices. This growth has also led to a need for PCBs that can accommodate high levels of vibration and shock, and that are able to resist the damage caused by dropping or mishandling them.

These factors create a challenge for designers, who must consider how to protect the integrity of a PCB while keeping its functionality and cost down. Using a rigid board with an adequate amount of copper is one way to reduce the risk of vibration-induced deformation. Another is to use a flexible pcb board, which can be designed to meet the specific requirements of a particular application, such as the aerospace or automotive industries, where vibrations are a common occurrence.

Rigid PCBs, which make up the vast majority of boards that a designer will encounter, are constructed from a rigid substrate such as FR-4 that has been coated with a conductive copper base and a protective solder mask. The layout of a PCB is then embedded within these layers through a complex lamination process that involves extreme heat and pressure.

How Do PCBs Accommodate For Torsional Stresses?

Multilayer PCBs, on the other hand, are made up of different materials that are bonded together. These materials have different rates of thermal expansion and contraction, known as the coefficient of thermal expansion (CTE). The differences between these CTEs can lead to stress fields in a multilayer PCB that can lead to deformation of the board. This can have severe consequences for circuit operation and reliability.

The primary sources of deformation in a multilayer PCB are thermal stresses and differential alignment. Differential alignment can result in a skewed structure that is not aligned with the layout of the component mounting holes. This can lead to misalignment of the components and resulting problems with solder joints. Thermal stresses can also cause warpage of the PCB, which can lead to poor quality solder joints and device failure.

To minimize these issues, a designer should follow good manufacturing practices to minimize the occurrence of both stress and warpage. The first step is to avoid large changes in the thickness of the copper layer. This can be achieved by designing the track work to avoid abrupt hard right-angles. Instead, routing the tracks with arc corner modes is preferred. This will help to reduce stresses that can cause weak spots in the flex circuit.

Another issue is to use strong adhesives where the flex circuit exits the rigid PCB. This can be done by dispensing epoxy, acrylic or hot-melt. However, this can be a time-consuming and labor-intensive process. Furthermore, it can be difficult to collaborate with assembly engineers to ensure that the correct amount of adhesive is dispensed.