PCB Tech - Causes of poor PCB surface and PCB repair sequence

The bonding force between the PCB coatings is poor or too low, and it is difficult to resist the coating stress, mechanical stress and thermal stress generated during the production and processing during the subsequent production and assembly process, which will eventually cause different degrees of separation between the coatings. Some factors that may cause poor board quality during production and processing are summarized as follows:

1. The problem of substrate processing:

Especially for some thinner substrates (generally less than 0.8mm), because the rigidity of the substrate is poor, it is not suitable to use a brushing machine to brush the board.

This may not be able to effectively remove the protective layer specially treated to prevent the oxidation of the copper foil on the board during the production and processing of the substrate. Although the layer is thin and the brush is easier to remove, it is difficult to use chemical treatment, so in production It is important to pay attention to control during processing, so as to avoid the problem of blistering on the board caused by the poor bonding between the copper foil of the board substrate and the chemical copper; this problem will also be blackened and browned when the thin inner layer is blackened. Poor, uneven color, local black browning and other problems.

2. The phenomenon of poor surface treatment caused by oil or other liquids contaminated with dust during the machining (drilling, lamination, milling, etc.) process of the board surface.

3. Poor sinking copper brush:

The pressure of the front grinding plate of the sinking copper is too large, which causes the hole to be deformed. The board does not cause leakage of the substrate, but the excessively heavy brushing board will increase the roughness of the hole copper, so during the microetching roughening process, the copper foil at this place is very easy to produce excessive roughening, and there will also be a certain quality. Hidden dangers; therefore, pay attention to strengthening the control of the brushing process, and the brushing process parameters can be adjusted to the best through the wear scar test and the water film test;

4. Washing problem:

Because the electroplating treatment of copper sinking has to undergo a lot of chemical treatment, various kinds of acid, alkali, non-polar organic and other pharmaceutical solvents are more, the surface of the board is not cleaned, especially the sinking copper adjustment degreasing agent, which will not only cause cross-contamination, but also cause cross-contamination. The local treatment of the board surface is poor or the treatment effect is not good, and the uneven defects cause some bonding problems; therefore, it is necessary to strengthen the control of the washing, mainly including the flow of the washing water, the water quality, the washing time, and the dripping of the plate. Time and other aspects of the control; especially in winter, the temperature is low, the washing effect will be greatly reduced, and more attention should be paid to the strong control of the washing;

5. Micro-etching in the pre-treatment of copper sinking and the pre-treatment of pattern electroplating:

Excessive micro-etching will cause leakage of the substrate in the orifice, resulting in blistering around the orifice; insufficient micro-etching will also cause insufficient binding force and cause blistering; therefore, it is necessary to strengthen the control of micro-etching; generally the micro-etching of copper pretreatment The corrosion depth is 1.5-2 microns, and the micro-etching before the pattern plating is 0.3--1 microns. If possible, it is best to control the thickness of the micro-etching or the corrosion rate through chemical analysis and simple test weighing; in general, micro-etching The surface of the etched board is bright in color, uniform pink, no reflection; if the color is not uniform, or there is reflection, it means that there is a quality risk in the pre-processing; pay attention to strengthen the inspection; in addition, the copper content of the micro-etching tank, the temperature of the tank, and the load capacity, Microetching agent content, etc. are all items to be paid attention to;

6. Poor rework of heavy copper:

Some reworked boards after copper sinking or pattern transfer are poorly plated during the rework process, incorrect rework methods or improper control of the micro-etching time during the rework process, etc., etc., or other reasons will cause the board surface to bubble; if the rework of the copper sink board is found online Poor copper sinking can be directly degreasing from the line after washing with water, then pickling and reworking without commissioning; it is best not to re-degreasing, micro-etching; for the plates that have been thickened by the board, the micro-etching tank should be deplated now, Pay attention to time control. You can first use one or two plates to roughly measure the deplating time to ensure the deplating effect; after the deplating is completed, apply a set of soft brushing machine and lightly brush, and then sink the copper according to the normal production process, but the corrosion is slight. The eclipse time should be halved or adjusted if necessary;

7. The board surface is oxidized during the production process:

If the immersion copper plate is oxidized in the air, it may not only cause no copper in the hole, the plate surface is rough, but also may cause the plate surface to bubble; the copper immersion plate is stored in the acid solution for too long, and the plate surface will also be oxidized, and This kind of oxide film is difficult to remove; therefore, during the production process, the heavy copper plate should be thickened in time, and it should not be stored for too long. Generally, the thickened copper plating should be completed within 12 hours at the latest;

8. The activity of the copper precipitation liquid is too strong:

The newly opened tank of the copper sinking solution or the high content of the three components in the bath, especially the high copper content, will cause the bath to be too active, the electroless copper deposition is rough, hydrogen, cuprous oxide, etc. are mixed in the chemical copper layer Excessively caused the defects of the quality of the coating's physical properties and poor bonding; the following methods can be appropriately adopted: reduce the copper content, (add pure water to the bath) including three major components, and appropriately increase the complexing agent and stabilizer Content, appropriately reduce the temperature of the bath liquid, etc.;

9. Insufficient water washing after development during the graphics transfer process, too long storage time after development or too much dust in the workshop, etc., will cause poor cleanliness of the board surface, and slightly poor fiber processing effect, which may cause potential quality problems;

10. Organic pollution, especially oil pollution, is more likely to occur in the electroplating tank for automatic lines;

11. Pay attention to timely replacement of the acid bath before copper plating. Too much pollution in the bath or too high copper content will not only cause problems with the cleanliness of the board surface, but also cause defects such as rough board surface;

12.In addition, when the bath liquid is not heated in some factories in winter, it is necessary to pay special attention to the electrification of the plate during the production process, especially the plating tank with air agitation, such as copper-nickel; it is best for the nickel tank in winter Add a warm water washing tank before nickel plating (the water temperature is about 30-40 degrees) to ensure that the initial deposition of the nickel layer is dense and good;

The general sequence of PCB maintenance

(1) Carefully observe whether there are obvious traces of failure on the surface of the faulty circuit board. For example, whether there are burnt and cracked integrated ICs or other components, and whether there are traces of disconnection and cracking on the circuit board.

(2) Understand the process of failure, analyze the cause of the failure, and infer where the faulty device may exist.

(3) Understand and analyze the application nature of faulty circuit boards, and count the types of integrated ICs used.

(4) Sort according to the location of various integrated ICs and the probability of failure.

(5) Various detection methods are used to detect in order of probability, and gradually reduce the scope of failure.

(6) Determine the specific faulty device. When replacing a good integrated IC, it is best to install an IC device socket for trial replacement.

(7) If it is still abnormal after the installation test, it should be tested again until all the faults on the faulty circuit board are repaired.