What is Surface Blistering on Circuit Boards?
Surface blistering is a defect that can occur on the surface of printed circuit boards (PCBs) during the manufacturing process or during use. It appears as raised, dome-shaped protrusions or bubbles on the surface of the PCB substrate or solder mask.
Blistering can range in size from very small, pinhead-sized bumps to larger areas several millimeters in diameter. It compromises the integrity and reliability of the circuit board by:
- Lifting and separating copper traces
- Breaking solder joints
- Allowing moisture and contaminants to penetrate the board
- Causing intermittent connections
- Leading to early failures in the field
Therefore, it’s important for PCB Manufacturers and electronics assemblers to understand the causes of blistering and take steps to prevent it. Even a small amount of blistering is usually considered unacceptable in industries with high reliability requirements like aerospace, medical, and automotive electronics.
What Does Circuit Board Blistering Look Like?
Blisters on a circuit board can be identified visually as raised areas or bumps disrupting the normally smooth, flat surface:
| Characteristic | Description |
|---|---|
| Shape | Circular or irregular domes, bumps, and bubbles |
| Size | Pinhead-sized to several millimeters in diameter |
| Location | Surface of PCB substrate, solder mask, or around solder joints |
| Color | May be same color as PCB surface or appear whitish or discolored |
| Texture | Raised and uneven compared to flat PCB surface |
Other signs that may accompany blistering include:
- Solder mask discoloration or degradation around the blister
- Exposed or broken copper traces if the blister has ruptured
- Cracking or cratering in the PCB surface
Cross-sectioning a blister may reveal a hollow space or void underneath where the PCB substrate layers have delaminated. Analytical techniques like Scanning Electron Microscopy (SEM) and Fourier-Transform Infrared Spectroscopy (FTIR) can provide additional details about the composition and structure of the blister.
What Causes Blistering on PCB Surfaces?
There are several potential causes of surface blistering on circuit boards:
Moisture Absorption
One of the most common causes of PCB blistering is moisture absorbed by the board substrate material, typically woven glass fibers impregnated with epoxy resin (FR-4). PCBs can absorb atmospheric moisture during storage and transport before assembly.
If this moisture is not adequately baked out in a drying cycle prior to solder reflow, it can vaporize and expand rapidly when the board is exposed to Soldering Temperatures around 200-250°C. This sudden vaporization causes the moisture to force its way out, pushing up on the PCB surface and creating a blister or void.
Blistering due to moisture is most likely to occur when:
- PCBs are stored in high humidity environments above 60% RH
- Boards are not baked prior to assembly or baking is insufficient
- Reflow soldering temperatures are too high or applied too quickly
- The Glass Transition Temperature (Tg) of the PCB material is exceeded
Contamination
Chemical contamination on the surface of the PCB or within the substrate can also lead to blistering. Contaminants may be introduced during PCB fabrication processes like cleaning, etching, plating or applying solder mask.
Common contaminants that can outgas and cause blistering when heated include:
- Plating or etching residues
- Flux residues
- Solder mask solvents
- Silicone oil or grease
- Fingerprints and skin oils
If these contaminants are not thoroughly cleaned prior to solder reflow, they can vaporize and create blisters similar to the moisture mechanism. Contamination sources can be difficult to isolate since there are many steps and chemicals used in PCB manufacturing.
Thermal Stress
Thermal stresses from uneven heating or rapid thermal cycling of the PCB Assembly can also contribute to blistering in some cases. During reflow soldering, areas of the board may heat up faster than others, creating localized thermal expansion mismatches.
If the Coefficient of Thermal Expansion (CTE) differs between the PCB substrate, copper, and solder mask materials, these uneven forces can cause delamination and blistering. Boards subjected to multiple solder reflow cycles or rework are especially susceptible to heat stress blisters.
Thermal induced blistering is more likely when:
- The PCB design has large copper planes or heat-sink areas
- Components are closely spaced, restricting even heating
- Reflow oven conveyor speed and temperature profiles are not optimized
- Boards are subjected to multiple reflow cycles or rework heating
Materials and Manufacturing Defects
In some cases, blistering can arise from defects and inconsistencies in the PCB materials and manufacturing process themselves, such as:
- Improperly cured or out of date prepreg resin
- Unevenly dispersed or broken glass fibers
- Incompletely cured or too thick solder mask
- Drilled holes with rough edges or poor wall quality
- Micro-voids or inclusions in the resin from manufacturing
Problems with the incoming PCB substrate, solder mask, or hole drilling can create discontinuities and weak spots that are prone to delamination and blistering under thermal stress. Issues with resin composition, curing, and application are especially critical since the resin bonds the layers together.
How to Identify the Cause of PCB Surface Blistering
Determining the root cause of blistering requires a systematic approach to gather data, analyze the failure, and narrow down the variables. Key steps include:
- Inspect the location, appearance, and distribution of blistering
- Check if blistering correlates to certain PCB designs, material lots, or manufacturing dates
- Review storage conditions and pre-bake procedures for moisture control
- Analyze PCB cross-sections to identify delamination, voids, and contaminants
- Inspect drilling and wall quality of vias and through-holes
- Assess solder mask coverage and thickness over problem areas
- Verify the moisture sensitivity level (MSL) of the PCB material
- Audit suppliers and incoming inspection records for material defects
- Validate the reflow soldering profile and oven setup
- Consider the thermal expansion properties and glass transition temp of materials
It’s important to involve the PCB design, manufacturing, and assembly teams in the troubleshooting process. Blistering issues often span multiple steps in the production process.
Preventing PCB Surface Blistering
Based on the identified causes, there are several best practices that can help prevent PCB blistering:
Control Moisture Absorption
- Bake PCBs that have absorbed excess moisture at 125°C for 4-6 hours prior to assembly
- Use Moisture Barrier Bags (MBB) with desiccant and a Humidity Indicator Card (HIC) for storage and transport
- Maintain factory floor conditions below 60% relative humidity
- Assemble PCBs soon after opening MBB packaging
Minimize Contamination
- Implement a comprehensive PCB cleaning process to remove flux, solder mask residues, oils, and other contaminants
- Use clean, lint-free gloves when handling PCBs
- Audit chemical processes and baths for contamination levels
- Consider no-clean solder mask and assembly processes
Optimize Thermal Profile
- Design reflow oven thermal profile with a gradual ramp to a peak temp below the PCB material’s glass transition temp (Tg)
- Use thermocouples and profiling software to measure and optimize actual PCB temp
- Reduce the number of reflow cycles and rework on boards
- Increase spacing between large components for more even heating
Improve Materials and Processes
- Select high-quality, consistent PCB substrate materials
- Verify incoming materials meet requirements for resin content, glass distribution, and curing
- Optimize solder mask thickness, registration, and curing
- Use vacuum lamination to remove air and improve bonding during fabrication
- Reduce drilling speeds and inspect holes for wall quality
By understanding and controlling these variables, PCB manufacturers and assemblers can significantly reduce instances of surface blistering. Building in protection against moisture, contamination, and thermal stress leads to more reliable circuit boards.
FAQ
How can you tell if a PCB has absorbed too much moisture?
Moisture absorption can be measured directly by weighing PCBs and comparing to their baseline dry weight. A moisture pickup of more than 0.2% is usually considered too high for safe assembly. Many PCB suppliers mark the Moisture Sensitivity Level (MSL) on the packaging which indicates the floor life and recommended bake-out procedures.
What is the best way to bake out PCBs before assembly?
PCBs should be baked in a temperature and humidity controlled oven at 125°C for 4-6 hours, depending on the board thickness and MSL rating. The boards should be separated to allow air flow. Baking at higher temperatures can oxidize the surface finish or degrade the material. Nitrogen convection ovens are preferred to prevent oxidation.
Can blistering be repaired after assembly?
In most cases, blistering is not repairable after reflow soldering. The delamination and voids under the surface are permanent defects. For critical applications, the affected PCB may need to be scrapped and replaced. If the blistering is minor and not over critical traces or solder joints, the board might be used as-is with extra inspection and testing.
How much blistering is acceptable on a PCB?
The acceptable level of blistering depends on the industry, product, and reliability requirements. In general, any measurable blistering is cause for concern and should be addressed. Even if the blisters do not initially cause functional problems, they can grow and lead to early failures in the field. Most high-reliability PCB standards consider any blistering as a rejectable non-conformity.
What are some common tests to analyze PCB blisters?
Analytical tests that can provide useful data on PCB blisters include:
- Scanning Electron Microscopy (SEM) to image the blister structure and composition
- Energy Dispersive X-Ray Spectroscopy (EDX) to identify the elemental makeup and check for contaminants
- Fourier Transform Infrared Spectroscopy (FTIR) to determine the molecular structure and look for degradation byproducts
- Ion Chromatography (IC) to detect and measure ionic contamination
- Cross-sectioning and microscopy to visualize the depth and extent of delamination
By combining visual examination with these advanced analysis techniques, PCB engineers can better characterize the failure mode and narrow down the root cause of blistering.
