What causes mechanical stress in PCB, and how do you prevent it? These are the questions this write-up will answer, among others. We will also discuss how engineers test for the strain and why it matters.
What is Mechanical Stress in PCB?
Mechanical stress in PCB is the strain generated when the circuit board is under load. The load can be a force that bends, twists, or causes excessive vibration.
Mechanical stress is often introduced during PCB fabrication, assembly, and other manufacturing processes. Other situations where damage may occur include improper packaging, storage, and transportation.
Causes of Mechanical Stress in PCB
Mechanical strain in printed circuit boards results from bending, shearing, vibrations, or thermal expansion. Situations that can cause it include the following.
Poor Design
Poor PCB design choices compromise its structural integrity and manufacturability. They can also make the board more prone to damage. Examples include choosing weak laminates, mismatching CTE values, or placing components incorrectly.
In other words, the design doesn’t align with its structural integrity requirements and factory processes. Luckily, you can alter the choices before CAD files enter the production stage.
Manufacturing Steps
Some manufacturing processes involve impact forces and high temperatures. These conditions can cause damage. Key procedures that can cause stress include lamination, component placement, and soldering.
Depaneling/Cutting
Separating individual PCBs from panels can induce stress. The same applies to sizing procedures, such as cutting. Damage occurs if the method is inappropriate for the board material, thickness, and layout.
Thermal Cycling
The board heats unevenly, causing different sections or layers to expand at varying rates. The heat may come from the PCB itself or from external sources, such as manufacturing temperatures and application conditions.
Handling and Transportation
Rough handling and transportation are also among the reasons why a PCB suffers mechanical strain. Examples include dropping, over-packing/stacking, harsh storage conditions, and excessive vibrations in shipping trucks.
Resource: https://hackaday.com
Effects of Mechanical Stress on a PCB
Severe damage can occur if a printed circuit board is under strain or stress, whether from physical loads or thermal cycling. The following are common effects if the problem occurs during manufacturing, storage, or transportation.
Failed Solder joints
Solder joints crack, separate from the board, or suffer other forms of damage. It affects their reliability and may cause electrical discontinuities that cause the board to malfunction or fail.
Component Damage
Components get damaged or misaligned. Some parts, such as ceramic capacitors, are more susceptible to mechanical forces. Stress can also cause components to shift, affecting their reliability.
Board Delamination
The layers may separate, leading to structural, thermal, or electrical issues. The board becomes weaker, increasing the likelihood of further damage. Delamination also creates spaces that affect signal quality and heat loss.
Trace and Pad Damage
Traces and pads break or lift, leading to open circuits. Such occurrences can require costly interventions, including design changes and rework, for the affected board.
Resource: https://www.youtube.com/watch?v=Q7tQd1SXAlo
How to Prevent Mechanical Stress in PCB Manufacturing
Mechanical strain is a critical concern in PCBs. It can affect a board’s overall quality or necessitate repair and rework. The aim, therefore, is to minimize its chances and save time and costs. Minimization strategies include:
Design Optimization
Design optimization is an early way to prevent stress in printed circuit boards. As we saw earlier, improper component placement and layout strategies are major causes of board strain.
Designers must know where to position parts and why. For instance, heavy components exert mechanical load on the board, causing strain. Improving the design for manufacturing, such as applying DFM principles, is essential.
Process Control
Intervention during the manufacturing process prevents forces that could damage the PCB. It involves using appropriate and safe technologies.
It also includes monitoring production steps to detect risks. One of them is testing forces to ensure they are within safe limits. Bare PCBs also undergo baking to remove moisture and prevent thermal stress.
Using Adhesives and Stiffeners
Adhesives anchor heavy or sensitive components, improving their reliability and resistance to stress. They can be component-level underfills (essential for BGAs and CSPs) or entire-assembly encapsulations, depending on the specific requirements.
On the other hand, stiffeners reinforce delicate substrates, making them mechanically robust to withstand forces. Some stiffeners may also dissipate heat and prevent thermal damage.
Depaneling Choices
Breaking PCBs apart manually or using a guillotine cutter induces massive stress near the break lines.
Manufacturers should choose low-stress methods based on the substrate type. For example, Laser Depaneling and Router Milling impose zero mechanical stress compared to V-Cut scoring, making them safer for boards with ceramic capacitors near the edge.
Proper Handling
Safe handling reduces the risk of bending or twisting the PCBs during manufacturing, packaging, or transportation. Manufacturers ensure proper board support during production.
Other practices include avoiding dropping boards, using racks and trays, and implementing proper packaging for shipping. Factories must provide handling guidelines for machine operators and other workers.
Resource: https://www.mdpi.com
Testing Mechanical Stress in PCB
Testing mechanical strain in PCB manufacturing is a common quality assurance practice. It involves various techniques, equipment, and detection technologies. The following are key tests.
Strain Gauge Test – (Based on IPC-9704) Tiny sensors measure slight deformations in the board. The test is useful when undertaking various manufacturing steps, including assembly and testing.
Drop Testing – It involves dropping the board from a height to measure its structural integrity when subjected to extreme g-forces.
Vibration Test – This test subjects the board to vibrations. The aim is to identify its weaknesses and determine its ability to operate in the application environment.
Flex Test – The test applies a load to the board to determine the maximum stress that can cause damage.
Thermal Stress Test – The thermal test simulates rapid temperature changes that can cause mechanical strain. Results indicate the board’s ability to withstand heat changes.
Conclusion
Mechanical stress in PCBs can cause unexpected damage, whether during manufacturing, transportation, or use. Applying measures that reduce strain ensures safety and long-term reliability.
Ensure your PCBs are built to withstand the rigors of production and use. Submit your design to Jamindo’s engineering team for a comprehensive reliability assessment and stress-mitigation review today.
