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PCB Measling and Delamination: A Comprehensive Guide
Understanding Measling and Delamination
Measling in PCBs
Measling manifests as small, white, and discrete spots or crosses under the PCB laminate surface, resembling measles. These are typically observed around the glass weave of the laminate and are indicative of micro-cracks within the resin.
Causes of Measling
- Thermal Stresses: Repeated thermal cycling can cause the PCB material to expand and contract, leading to micro-cracks.
- Mechanical Stresses: Improper handling or excessive force during assembly can introduce stress fractures in the laminate.
- Chemical Processes: Aggressive chemical treatments during PCB manufacturing can weaken the laminate structure, predisposing it to measling.
Delamination in PCBs
Delamination refers to the separation between layers of the PCB laminate or between the laminate and the copper foil, resulting in a loss of structural integrity and potentially causing electrical failure.
Causes of Delamination
- Excessive Heat: Elevated temperatures during soldering or other manufacturing processes can weaken the adhesive bond between layers.
- Moisture Intrusion: Absorbed moisture expands when heated, creating internal pressures that can lead to layer separation.
- Manufacturing Defects: Inadequate curing of the laminate or improper application of adhesives during PCB fabrication can predispose the board to delamination.
Strategies for Prevention
Preventing measling and delamination requires a multifaceted approach, focusing on material selection, manufacturing processes, and handling procedures.
Material Selection and Handling
- High-Quality Materials: Opt for PCB substrates with superior thermal and mechanical properties to withstand stresses without cracking.
- Proper Storage: Maintain an appropriate environment to prevent moisture absorption by PCB materials, reducing the risk of delamination during soldering.
Manufacturing and Process Controls
- Controlled Thermal Cycling: Implement controlled ramp-up and cool-down rates during soldering to minimize thermal stress.
- Optimized Chemical Processing: Use milder chemicals and optimized exposure times during PCB etching and cleaning to preserve laminate integrity.
- Adequate Curing: Ensure complete curing of the laminate and adhesives to establish strong bonds between layers and with copper foils.
Detection and Testing of PCB Measling and Delamination
Advanced Detection Methods
To ensure the reliability of PCBs and prevent issues such as measling and delamination, advanced detection methods are essential. These methods help identify defects early in the manufacturing process, allowing for corrective measures to be taken before the PCBs are put into use.
X-Ray Inspection: This non-destructive testing method can detect internal flaws such as measling and delamination by producing detailed images of the PCB’s internal structure. X-ray inspection is particularly useful for identifying issues that are not visible on the surface.
Thermal Imaging: By using infrared cameras, thermal imaging can detect abnormal heat patterns that may indicate the presence of internal cracks or delamination. This method is effective in identifying areas with compromised thermal conductivity, which are often the result of structural defects.
Ultrasonic Testing: Ultrasonic waves can be used to detect delamination by measuring the reflection and transmission of sound waves through the PCB layers. Any disruption in the expected wave pattern can indicate the presence of delamination or other structural issues.
Reliability Testing
Environmental Stress Screening (ESS): This process involves subjecting PCBs to extreme environmental conditions, such as high temperatures, humidity, and vibration, to identify potential reliability issues. ESS helps ensure that the PCBs can withstand harsh operating conditions without failing due to measling or delamination.
Thermal Cycling Tests: PCBs are exposed to repeated cycles of heating and cooling to simulate the thermal stresses they will encounter during actual use. This testing method helps identify boards that are prone to thermal-induced delamination or cracking.
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