Production and acceptance of steel mesh

Steel Mesh For SMT

The production and acceptance of steel mesh required in our PCBA process is actually a large topic, and IPC7525 has a set of relatively rigorous methods and suggestions. We mainly talk about some of Chee Mau's own experience. Of course, a large part of the content or key points actually come from IPC-7525 and our internal specifications.

Let's talk about acceptance first. If you are just starting to engage in SMT technology, you will most likely come into contact with steel mesh acceptance. If there is no relevant standard within your company, or you have not found the relevant standard, the following content can be used for your reference.

Generally, micrometers, plug gauges, tensiometers and steel mesh films are used for steel mesh acceptance.

1) The number/product model/thickness/production date of steel mesh etching, these contents are generally available on steel mesh, and you need to pay attention to whether they are correct.

2) Steel mesh thickness: generally measured with a micrometer, the tolerance is generally 0.005mm, if there are steps, the steps also need to be measured synchronously.

3) Flatness of steel mesh: Place the steel mesh on the horizontal inspection machine platform and use a plug gauge to measure. The flatness of the four sides of the steel frame should be less than 1mm.

4) Whether the steel mesh has scratches, deformation of the opening, burrs, and clear mark points, and the number is generally 3.

5) Whether there are any missed openings. If the company has a steel mesh inspection machine, the steel mesh inspection machine can be used for inspection and comparison. If not, you need to take the steel mesh file given by the steel mesh factory and compare it with your Gerber file. In addition, you need to take the film and your steel mesh in kind, and you can also compare the film with your PCB.

6) For 0201, 0402, BGA, FinePitch ICs, you also need to measure whether the actual opening size is consistent with the size you require. Generally, the deviation is required to be within 0.03mm.

7) Steel mesh tension: Generally, it is required to have 4 corners + 1 center point, the tension of each point is greater than 30N/mm, and the tension difference between any two points is less than 10.

Regarding the opening of steel mesh, many factories have their own specifications. Steel mesh factories generally have corresponding specifications, and IPC-7525 has written the relevant requirements clearly.

The thickness of the steel mesh is generally determined by the PITCH of the fine-pitch components on the board or the smallest chip component. In addition, for these fine-pitch components or the smallest chip components, you need to decide the size and method of the opening, which is generally done by shrinking, extending, bridging, etc., but remember to calculate the opening area ratio and width-to-thickness ratio, which is very important, especially the opening area ratio, which directly determines the release of solder paste. After confirmation, you can communicate your ideas with the steel mesh factory. For some common devices, in fact, many times the steel mesh factory will open the holes for you according to their existing specifications, and generally there will be no problems. Of course, if you have a better idea, you can also make a request to the steel mesh factory, but for now, many engineers generally do not specify it. However, there is a rule that should be used from beginning to end, which is the area ratio and width-to-thickness ratio of the opening mentioned above. Of course, there are many other details that you need to pay attention to, such as the geometric shape of the opening and the treatment of the hole wall. However, these are generally handled by default by the steel mesh factory. If you have special requirements, you can communicate with the steel mesh factory.

In addition to the normal material pad openings, there are some special positions. For example, when opening the through hole on the ground pad, remember to avoid it to prevent the tin liquid from flowing to the back through the through hole, resulting in failure to print and PCBA scrapping. In addition, OSP boards generally require that holes be opened at the copper leakage position to prevent oxidation at the position without holes in the later stage. If the oxidized position is the test point, it may cause your test pass rate to drop seriously. Another thing is the opening of the screw hole position. Friends who have experience in computer assembly can take a look. Your computer motherboard screw hole is generally printed with solder paste, because many consumer electronics are OSP boards. The screw hole is printed with solder paste, so the grounding effect will be better. If there is no printing, the copper foil will be oxidized in the later stage, and the contact effect between the screw and the PCBA board screw hole (ground) will be relatively poor.

Steel mesh hole opening is a work that combines theory and practice, and you may need to verify it many times to form a set of applicable methods.