What is the cause of blistering on the circuit board surface

The blistering on a circuit board is primarily caused by poor bonding force on the board's surface, which is fundamentally a surface quality issue. This problem generally stems from two aspects:

1. The cleanliness of the board surface.
2. Surface micro-roughness (or surface energy).

If the bonding force between the coatings is insufficient, it becomes difficult for the board to resist coating stress, mechanical stress, and thermal stress generated during manufacturing. This ultimately leads to varying degrees of separation between the layers. The specific manufacturing factors that cause poor bonding and subsequent blistering include:

1. Substrate Processing Issues

For thinner substrates (generally below 0.8mm), poor rigidity makes it unsuitable to use a brushing machine. As a result, it may be difficult to effectively remove the anti-oxidation protective layer applied to the copper foil. While chemical treatments can be used, this layer is often resistant to them. Close attention must be paid during processing to avoid poor bonding between the copper foil and the chemical copper, which leads to blistering.

2. Machining Contamination

Poor surface treatment can occur if the board becomes contaminated with oil stains, dust, or other liquids during machining processes such as drilling, lamination, or milling.

3. Poor Plating Through-Hole (PTH)

Excessive pressure on the board before electroless copper deposition can cause the holes to deform. This deformation often leads to blistering in or around the holes during subsequent processes like plating, hot air leveling (HASL), and soldering. Additionally, overly aggressive brushing (often done to prevent voids) can unnecessarily increase the roughness of the hole copper. Therefore, the brushing process must be strictly controlled; parameters should be optimized using wear scar and water film tests.

4. Washing and Cleaning Problems

The copper electroplating process involves multiple chemical treatments that are not easily washed away by water. Inadequate washing causes cross-contamination and uneven surface treatments, directly reducing bonding force. Manufacturers must control water flow, water quality, washing time, and board dripping time. Particular attention is needed in winter, as lower water temperatures significantly reduce cleaning efficiency.

5. Micro-Etching Control

Micro-etching is a critical pre-treatment before copper immersion and pattern plating. Excessive micro-etching can expose the underlying substrate in the holes, causing blistering around the orifices. Conversely, insufficient micro-etching fails to provide enough roughness for a strong bond. Generally, the micro-etching depth for copper deposition should be 1.5 to 2 microns, and 0.3 to 1 micron before pattern plating. The etched board should display a bright, uniform pink color with no reflections. Uneven colors or reflective spots indicate hidden pre-processing issues. Factors like copper content in the bath, bath temperature, load, and micro-etching agent concentration must be closely monitored.