Ten quality problems and solutions for PCB products that novices must know

Ten quality problems and solutions for PCB products that novices must know

In the entire production process, there are many control points. If you are not careful, the board will be broken. PCB quality problems are endless. This is also a headache because only one of them has problems. Then most of the devices will be unusable.

At the Embedded System Exhibition, I collected some quality problems that often occur in PCB through questionnaires combined with my own experience, and they are organized as follows:

In addition to the above issues, there are also some potential risks. The author has sorted out ten major issues. Here are a list and some experience in handling, and share with you:

1. [Layered]

Delamination is the biggest problem of PCB, and it ranks first among common problems. The possible causes are as follows:

(1). Improper packaging or storage, damp;

(2). The storage time is too long, the storage period is exceeded, and the PCB board is damp;

(3). Supplier material or process problems;

(4). Poor design material selection and copper surface distribution.

Moisture problems are relatively easy to occur. Even if you choose a good packaging, there is a constant temperature and humidity warehouse in the factory, but the transportation and temporary storage process cannot be controlled. The author was once "fortunate" to visit a bonded warehouse. Don't count on temperature and humidity management. The roof is still leaking, and the boxes are directly in the water. However, dampness can still be dealt with. Vacuum conductive bags or aluminum foil bags can well protect against water vapor intrusion. At the same time, the packaging bag requires a humidity indicator card. If the humidity card is found to exceed the standard before use, it can usually be solved by baking before going online. The baking condition is usually 120 degrees 4H.

If there is a problem with the materials or processes of the mobile phone and other products provided by the supplier at the mobile phone exhibition, the possibility of scrapping is relatively high. Common possible causes include: poor browning (black), damp PP or inner layer, insufficient PP glue, abnormal pressing, etc. In order to reduce the occurrence of such problems, it is necessary to pay special attention to the PCB supplier's management of the corresponding process and the layered reliability test. Taking the thermal stress test in the reliability test as an example, a good factory requires more than 5 times to pass the standard and cannot be stratified. It will be confirmed in the sample stage and each cycle of mass production, while the ordinary factory may only pass the standard twice. Confirmed only once in a few months. The IR test of simulated placement can also prevent the outflow of defective products more, which is a must for excellent PCB factories.

Of course, the design company's own PCB design will also bring hidden dangers of delamination. For example, the choice of plate Tg is often not required. In order to save costs, PCB manufacturers must choose ordinary Tg materials, which will have poor temperature resistance. In the era when lead-free becomes the mainstream, it is safer to choose Tg above 145°C. In addition, large open copper surfaces and too dense buried via areas are also hidden dangers of PCB delamination, which need to be avoided during design.

2. [Poor solderability]

Solderability is also one of the more serious problems, especially the batch problem. The possible causes are board surface contamination, oxidation, abnormal black nickel, nickel thickness, solder mask (shadow), long storage time, moisture absorption, solder mask on PAD, too thick (repair).

Pollution and moisture absorption are relatively easy to solve, and other problems are more troublesome, and there is no way to find out through incoming inspection. At this time, you need to pay attention to the PCB factory's process capability and quality control plan. For example, black nickel, you need to see whether the PCB factory is out of gold, whether the frequency of analysis of its own gold wire is sufficient, whether the concentration is stable, whether there is a regular gold stripping test and phosphorus content test for testing, internal solderability test Whether there is good execution, etc. If all can be done well, the possibility of batch problems will be very small. As for the PAD and poor repair of solder masks, it is necessary to understand the standards set by the PCB supplier for the maintenance, whether the inspectors and maintenance personnel have a good evaluation and induction system, and clearly define that the areas with dense pads cannot be repaired (such as BGA and QFP) .

3. [Board bending and warping]

The possible reasons for the bending and warping of the board include: supplier material selection, abnormal production process, poor heavy industry control, improper transportation or storage, insufficient design of the broken hole, and excessive difference in copper area of each layer. The last two design problems need to be avoided in the early stage of the design review. At the same time, the PCB factory can be required to simulate the placement IR conditions for testing to avoid bad plate bending after the furnace. For some thin boards, it may be required to press the wood pulp board up and down during packaging to avoid subsequent deformation. At the same time, clamps are added to prevent the device from bending the board excessively during placement.

4. [Scratches, exposed copper]

Scratches and exposed copper are the defects that most test the PCB factory's management system and execution. This problem is not serious, but it does bring quality concerns. Many PCB companies will say that this problem is difficult to improve. The author has promoted the scratch improvement of many PCB factories, and found that in many cases, it is not that the improvement is not good, but whether it is necessary to do it, and whether there is motivation to do it. The DPPM delivered by all PCB factories that are earnestly promoting the project has improved significantly. So the trick to this problem lies in: push and press.

5. [Poor impedance]

The impedance of the PCB is an important index related to the RF performance of the mobile phone board. A common problem is that the impedance difference between PCB batches is relatively large. Since impedance test strips are generally made on the side of the PCB and will not be shipped with the board, the supplier can be asked to provide the impedance strips and test reports of the batch for reference each time they ship, and it is also required to provide Comparison data of board edge wire diameter and board inner wire diameter.

6. [BGA solder voids]

BGA solder voids may cause poor function of the main chip, and may not be found in the test, and the hidden risk is high. So now many placement factories will go through X-RAY for inspection after placement. This type of failure may occur because of the residual liquid or impurities in the PCB hole, which vaporizes after high temperature, or the poor laser hole pattern on the BGA pad. Therefore, many HDI boards require electroplating filled holes or semi-filled holes to avoid this problem.

7. [Solderproof blistering/falling off]

Such problems are usually caused by abnormalities in the control of the PCB solder mask process, or the selection of solder mask inks is not suitable (cheap, non-chemical gold inks, not suitable for mounting flux), or it may be due to excessively high temperature in the patch and heavy industry. To prevent batch problems from occurring, PCB suppliers need to formulate corresponding reliability test requirements and control them at different stages.

8. [Poor plug hole]

Poor plug holes are mainly caused by insufficient technical capabilities of the PCB factory or simplified processes. The plug holes are not full, and the hole ring has exposed copper or false exposed copper. It may cause problems such as insufficient solder quantity, short circuit with the patch or assembled device, and residual impurities in the hole. This problem can be found by appearance inspection, so it can be controlled during incoming material inspection, and the PCB factory is required to make improvements.

9. [Poor size]

There are many possible reasons for poor size. The PCB production process is prone to expansion and contraction. The supplier has adjusted the drilling program / graphics ratio / molding CNC program, which may cause problems such as misalignment of placement and poor coordination of structural parts. Since this type of problem is difficult to detect and can only rely on the supplier's good process control, special attention should be paid to the supplier selection.

10. [The Cavani Effect]

The Avani effect is the galvanic reaction learned in high school chemistry and appears in the OSP process of selective gold plates. Due to the potential difference between gold and copper, the copper pad connected to the large gold surface in the OSP process will continue to lose electrons and dissolve into divalent copper ions, resulting in smaller pads, affecting subsequent component placement and reliability Sex.

Although this problem does not happen often, it has not appeared in Plato, but once it occurs, it is a batch problem. Board manufacturers with experience in mobile phone PCB production will screen out this part of the pads through computer software, compensate in advance during design, and set special rework conditions and limit the number of rework in the OSP process to avoid problems. So this problem can be confirmed in advance when auditing the board factory.