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Why Do White Spots Always Appear on PCB Surfaces? Analyzing the Problem of Solder Mask Blisters in Production
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Aug 13.2025, 09:33:34
The PCB industry is developing at a remarkable pace these days, with technological upgrades happening as quickly as flipping through the pages of a book. Just look, devices related to artificial intelligence, like servers and data centers, have a huge demand for high - density interconnect (HDI) and high - layer count PCBs. The electric vehicle market is also booming, requiring PCBs that can carry large currents, are reliable, and durable. However, as the industry charges forward, there are quite a few troubles. The constant appearance of "white spots" on PCB surfaces is a major headache, and this is mostly related to blisters in the solder mask layer.
What Are "White Spots" and Solder Mask Blisters?
The "white spots" on the PCB surface are visible discolored areas, and there are many reasons for this, with solder mask blisters being a common one. The solder mask is usually green and plays an important role in PCB manufacturing. It can protect the copper circuit from oxidation, prevent solder bridges during welding, and improve the insulation of the circuit. If there are blisters in the solder mask, white spots may appear. These blisters are, in essence, air or vapor pockets trapped between the solder mask and the PCB substrate.
Causes of Solder Mask Blisters
1. Moisture Entrapment
Moisture is really a troublemaker in PCB manufacturing. During production, if the substrate is not completely dried after processes such as etching and cleaning, moisture may get trapped inside. Later, during welding, the PCB has to withstand high temperatures (reflow soldering usually reaches 240 - 260°C, and wave soldering is around 250 - 270°C). The trapped moisture turns into steam when heated. The steam expands suddenly, generating a force that can lift the solder mask off the substrate, and thus blisters are formed. For example, in humid places, if the PCB is left for a long time before welding, it will absorb a lot of moisture from the air.
2. Incompatibility Between Solder Paste and Solder Mask Ink
The solder paste used for welding contains something called flux, which is responsible for cleaning the metal surface and allowing the solder to adhere better. But if the flux in the solder paste doesn't get along with the solder mask ink, problems arise. Some fluxes with high acidity or special compositions may react with the solder mask ink. For instance, if the solder mask ink can't resist the organic acids in the flux, the flux may seep into the ink, damage it, and then blisters will form. This is more common in high - precision work, where the solder mask is thinner and not as durable.
3. Improper Curing of the Solder Mask
The solder mask needs to be properly cured to form a strong and durable coating. If something goes wrong during curing, whether it's insufficient temperature, too short a time, or uneven heating, the solder mask won't fully set. A poorly cured solder mask has a loose structure and is particularly prone to blistering. Sometimes, the temperature in the curing oven varies, and some parts of the PCB are not properly cured, which is where blisters are likely to appear.
4. Contaminants on the PCB Surface
If there are contaminants on the PCB surface, such as dust, oil stains, or residues from previous processing, they will affect the adhesion of the solder mask. If oil stains are left on the PCB due to careless handling during production, the solder mask won't stick firmly in those areas. When heated during welding, the poorly adhered areas will lift up, forming blisters. Even a little dust can act as a "seed" for blisters because it disrupts the close contact between the solder mask and the substrate.
Impact on PCB Performance
Solder mask blisters and the resulting white spots are not just unsightly; they have a significant impact on the performance of the PCB. Blisters expose the underlying copper circuit, and once exposed, the copper is prone to rusting (oxidation). Oxidized copper has increased resistance, which affects the conductivity of the circuit. In severe cases, the circuit may break directly or work intermittently. In addition, if the blisters are near components, they will affect the firmness of the connection between the components and the circuit board. This is especially critical in things like automotive electronics, where there is constant vibration and large temperature changes in cars.
Prevention and Solutions
1. Strict Moisture Control
To solve the blister problem caused by moisture, PCB manufacturers must strictly control moisture. After wet processing, the substrate must be thoroughly dried and stored in a place with appropriate humidity. Before welding, baking the PCB at around 100 - 120°C for a few hours can drive out the absorbed moisture. In humid places, putting desiccants during storage also works.
2. Compatibility Testing
Before mass production, manufacturers must carefully test whether the solder paste and solder mask ink are compatible. Small - scale welding tests can be done on sample PCBs using different combinations of solder paste and solder mask, and then checked under a microscope for signs of ink damage or blistering. Once a suitable combination is found, production should follow this standard.
3. Precise Curing Process
The curing of the solder mask must not be taken lightly. Manufacturers must regularly calibrate the curing oven to ensure stable temperature. Using equipment to monitor the temperature during curing can help detect problems in a timely manner. Moreover, the curing time must be adjusted according to the type of solder mask ink and the thickness of the coating. Nowadays, some advanced curing ovens can automatically adjust the temperature and time for different batches of PCBs, which is very convenient.
4. Surface Cleaning
Thoroughly cleaning the PCB surface before applying the solder mask is a simple and effective method. It can be done by solvent cleaning, ultrasonic cleaning, or plasma cleaning. Solvents can remove oil stains and organic residues; ultrasonic waves shake off small particles with high - frequency sound waves; plasma cleaning is more advanced, using plasma of reactive gases to chemically clean the surface, making the solder mask adhere more firmly.
At PCBgogo, we know the "white spot" problem caused by solder mask blisters like the back of our hand. In our production process, we have very strict quality control. Our expert team regularly checks incoming substrates for moisture to ensure they are all dry. All the materials we use, such as solder paste and solder mask ink, undergo comprehensive compatibility testing. Our curing ovens are the most advanced, with extremely accurate temperature control to ensure the solder mask is properly cured. In addition, we have spent a lot of money on advanced surface cleaning equipment to make sure every PCB surface is clean before applying the solder mask. Through these methods, we strive to reduce the occurrence of white spots and solder mask blisters, providing customers with high - quality PCBs that meet the high requirements of the current electronics industry.
