In the whole production process, there are many control points, if a little carelessness, the board will be broken, and the quality problems of PCB circuit boards are endless, which is also a headache, because only one of them has problem, then most of the devices will also fail to use.
We have compiled a total of ten problems, listed here and attached some experience in dealing with them:
1. Layering
Delamination is the oldest and most difficult problem in PCBs, and it ranks first in common problems. The reasons for its occurrence may be as follows:
a. Improper packaging or storage, damp;
b. The storage time is too long, exceeding the storage period, and the PCB board is damp;
c. Supplier material or workmanship issues;
d. Poor design material selection and copper surface distribution.
The problem of moisture is relatively easy to occur. Even if good packaging is selected, there is a constant temperature and humidity warehouse in the factory, but the transportation and temporary storage process cannot be controlled. However, most factories have many problems with temperature and humidity management. Some even have leaking roofs, and the boxes stay directly in the water. However, it can still be dealt with when it is wet. Vacuum conductive bags or aluminum foil bags can effectively prevent the intrusion of water vapor. At the same time, a humidity indicator card is required in the packaging bag. If the humidity card is found to exceed the standard before use, it can be solved by baking before going online. The baking condition is usually 120 degrees. If there is a problem with the material or process of the mobile phone and other products provided by the supplier at the mobile phone exhibition, it may be scrapped. Sex is bigger.
Common possible causes include: poor browning (blackening), damp PP or inner board, insufficient PP glue, abnormal lamination, etc. In order to reduce the occurrence of this problem, special attention needs to be paid to the management of the corresponding process and the layered reliability test of the PCB circuit board supplier. Taking the thermal stress test in the reliability test as an example, a good factory must pass the standard more than 5 times and cannot be layered. It will be confirmed at the sample stage and in each cycle of mass production, while an ordinary factory may only pass the standard twice. It's only been confirmed every few months. The IR test of analog placement can also prevent the outflow of defective products, which is a must for excellent PCB manufacturers.
Of course, the design company itself will also bring hidden dangers of stratification. For example, the choice of Tg of the plate is not required in many cases. In order to save costs, the PCB factory must choose the material of ordinary Tg, and the temperature resistance performance will be relatively poor. In the era of lead-free becoming the mainstream, it is safer to choose Tg above 145°C. In addition, the large open copper surface and the too dense buried hole area are also hidden dangers of PCB delamination, which need to be avoided in the design.
2. Poor solderability
Solderability is also one of the more serious problems, especially batch problems. The possible causes are board surface contamination, oxidation, black nickel, abnormal nickel thickness, solder mask SCUM (shadow), too long storage time, moisture absorption, solder mask on PAD, too thick (repair).
The problems of pollution and moisture absorption are relatively easy to solve, but other problems are more troublesome, and there is no way to find out through the incoming inspection. At this time, we need to pay attention to the process capability and quality control plan. For example, for black nickel, you need to see whether the PCB factory is outsourcing gold, whether the analysis frequency of its own gold wire is sufficient, whether the concentration is stable, whether regular gold stripping test and phosphorus content test are set to detect, and internal solderability test Is there a good implementation etc. If it can be done well, the possibility of batch problems is very small. For PAD on solder mask and poor repair, you need to understand the standards set by PCB suppliers for maintenance, whether inspectors and maintenance personnel have a good assessment system, and clearly define that areas with dense pads cannot be repaired (such as BGA and QFP) .
3. Plate bending and warping
The reasons that may cause the plate to bend and warp include: supplier selection problem, abnormal production process, poor heavy industry control, improper transportation or storage, insufficient broken hole design, and too large difference in copper area of each layer. The last two design problems need to be avoided by conducting a design review in the early stage. At the same time, the PCB factory can be required to simulate the IR conditions of the placement to test to avoid the bad board bending after the furnace. For some thin boards, it can be required to press the wood pulp board up and down during packaging before packaging to avoid subsequent deformation, and at the same time, add a clamp to prevent the device from being too heavy to bend the board.
4, scratches, exposed copper
Scratches and exposed copper are the defects that most test the management system and execution of the PCB factory. This problem is not serious or serious, but it does bring quality concerns. Many PCB companies will say that this problem is difficult to improve. The author has promoted the improvement of scratches in many PCB factories, and found that many times it is not that the improvement is not good, but whether it is necessary to change it, and whether there is any motivation to change it. Any PCB factory that seriously promotes the project has significantly improved the DPPM delivered.
5. Poor impedance
Impedance is an important indicator related to the RF performance of mobile phone boards. The common problem is that the impedance difference between PCB batches is relatively large. Since the impedance test strips are generally made on the edge of the PCB and will not be shipped with the board, the supplier can be asked to pay the batch of impedance strips and test reports for reference each time they ship. At the same time, it is also required to provide The comparison data of the wire diameter of the board edge and the wire diameter of the board.
6. BGA solder voids
BGA solder voids may cause poor function of the main chip, and may not be found during testing, and the hidden risk is high. So now many SMT factories will go through X-RAY for inspection after placement. Such defects may occur due to residual liquid or impurities in the PCB hole, vaporization after high temperature, or poor laser hole pattern on the BGA pad. Therefore, many HDI boards now require electroplating to fill holes or semi-fill holes, which can avoid this problem.
7. Solder resist blistering/falling off
Such problems are usually caused by abnormal control of the PCB solder mask process, or the selection of solder mask ink is not suitable (cheap, non-chemical gold ink, not suitable for mounting flux), or the temperature of patch and rework is too high. To prevent batch problems, PCB suppliers need to formulate corresponding reliability test requirements and control them at different stages.
8. Poor plug hole
The poor plug hole is mainly caused by the lack of technical ability of the PCB factory or the simplification of the process. It may cause problems such as insufficient solder volume, short circuit with SMD or assembled devices, and residual impurities in the hole. This problem can be found in the appearance inspection, so it can be controlled in the incoming inspection, and the PCB factory is required to improve it.
9. Bad size
There are many possible reasons for the poor size. It is easy to expand and shrink in the PCB manufacturing process. The supplier has adjusted the drilling program/graphic ratio/molding CNC program, which may cause problems such as placement and misalignment and poor coordination of structural parts. Since such problems are difficult to detect and can only be controlled by the supplier's good process, special attention should be paid to the supplier selection.
10. The Galvanic Effect
The Mevani effect, also known as the galvanic cell reaction learned in high school chemistry, appears in the OSP process of selective gold plate. Due to the potential difference between gold and copper, the copper pads connected to the large gold surface during the OSP process will continue to lose electrons and dissolve into bivalent copper ions, resulting in smaller pads, affecting subsequent component placement and reliability. sex.
Although this problem occurs infrequently and does not appear in Plato, once it does, it is a batch problem. Board factories with experience in mobile phone PCB production will screen out this part of the pads through computer software, pre-compensate during design, and set special rework conditions and limit the number of rework in the OSP process to avoid problems. Therefore, this problem can be confirmed in advance when auditing the board factory.
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