Due to the complexity of the PCB manufacturing process and the difficulty of process maintenance,
particularly in chemical wet treatments, preventing blistering defects on the board surface is relatively challenging.
Blistering on the PCB surface is essentially a problem of poor adhesion on the board,
which can be further understood as an issue with the surface quality of the board. This includes two aspects:
1. The cleanliness of the PCB surface;
2. The microscopic roughness (or surface energy) of the PCB surface; all blistering issues on circuit boards can be attributed to these causes.
Poor or insufficient adhesion between layers makes it difficult for the plating layers to withstand stresses generated during subsequent production, processing,
and assembly, such as plating stress, mechanical stress, and thermal stress, ultimately leading to varying degrees of separation between layers.
Factors causing poor PCB surface quality during production and processing can be summarized as follows:
1. PCB substrate — issues in the copper-clad laminate processing; especially for thinner substrates (generally under 0.8mm), due to their lower rigidity,
it is not suitable to use a brushing machine. This may prevent the effective removal of protective layers applied
during substrate manufacturing to prevent copper foil oxidation.
Although this layer is thin and relatively easy to remove by brushing, chemical treatment of this layer is significantly difficult.
Therefore, it is important to carefully control this during production and processing to avoid blistering
caused by poor adhesion between the substrate foil and the chemically deposited copper.
This issue can also result in poor darkening or browning on thin inner layers, uneven color, and areas where darkening or browning fails locally.
2. PCB surface contamination during machining (drilling, lamination, milling edges, etc.),
such as oil or other liquids attracting dust, resulting in poor surface treatment.
3. Poor PCB copper deposition brushing: excessive pressure during pre-deposition board grinding can deform the hole edges,
causing rounding of the copper foil at the hole or even exposing the substrate. During copper plating, soldering, and other processes,
this can lead to blistering at the hole. Even if the brushing does not expose the substrate,
excessive pressure increases the roughness of the hole copper, making it prone to over-roughening during micro-etching,
creating potential quality risks. Therefore, it is important to strengthen control of the brushing process,
such as optimizing brushing parameters through scratch tests and water film tests.
4. PCB Washing Issues: Because the electroless copper plating process involves extensive chemical treatments with various acids, bases,
organic chemicals, and solvents, inadequate rinsing of the board surface, especially after using degreasers during copper deposition adjustments,
can not only cause cross-contamination but also lead to localized poor treatment or suboptimal results.
Uneven defects can occur, causing bonding-related problems. Therefore,
it is important to strengthen control over washing, mainly including controlling the flow rate of rinse water, water quality, rinse time,
and board drip time. Special attention should be paid in winter when low temperatures significantly
reduce washing effectiveness, necessitating stricter washing control.
A brief discussion on the causes of blistering on the surface of electroless copper-plated PCBs:
5. Micro-etching in PCB pre-deposition treatment and photo-patterned plating pre-treatment:
Excessive micro-etching can cause the base material to be exposed at the hole openings, leading to blistering around the holes.
Insufficient micro-etching can result in poor adhesion and blistering issues. Therefore, micro-etching must be carefully controlled.
Generally, the micro-etching depth for pre-deposition treatment is 1.5–2 microns, while for photo-patterned plating pre-treatment, the depth is 0.3–1 micron.
If possible, it is best to control the micro-etching thickness or etching rate through chemical analysis or simple weight testing.
Normally, the color of the board after micro-etching should be bright and uniformly pink without reflection.
Uneven color or reflection indicates potential quality issues in the pre-treatment process,
requiring increased inspection. Additionally, factors such as copper content in the micro-etching bath, solution temperature,
load, and micro-etchant concentration must be carefully monitored.
6. PCB Electroless Copper Solution Being Overly Active: When a new bath is started or the solution has excessively high levels of the three main components,
particularly high copper content, the solution becomes overly active. This can result in rough chemical copper deposits,
and excessive inclusion of hydrogen, cuprous oxide, and other impurities can reduce the physical quality of the plating and cause poor adhesion.
The following measures can be taken as needed: reduce the copper content (by adding pure water to the bath), adjust the levels of the three main components,
appropriately increase the concentration of complexing agents and stabilizers, and slightly lower the bath temperature.
7. Oxidation of the PCB surface during production: If a copper-clad board oxidizes in the air, it can not only result in holes without copper and a rough board surface,
but also cause blistering on the board surface. If the copper-clad board is left in acid solution for too long,
the board surface will also oxidize, and this oxide layer is very difficult to remove.
Therefore, during production, the copper-plated board should be thickened in a timely manner and should not be stored for too long;
generally, the copper plating should be completed at the latest within 12 hours.
8. PCB copper plating rework defects: Some reworked boards after copper plating or pattern transfer
can develop blistering on the board surface due to poor stripping, incorrect rework methods,
improper micro-etching time control during rework, or other reasons. If copper plating defects are detected online,
rework can be done directly after washing the board and de-oiling on the line, without stripping.
It is best not to perform a new de-oiling or micro-etching. For boards that have already undergone electric thickening,
the stripping should be done in the micro-etching tank, with attention to time control.
One or two sample boards can be used to roughly estimate the stripping time and ensure effectiveness. After stripping,
use a soft brush on the board machine, then proceed with normal copper plating, but th