PCB Technical - PCBA shielding frame design and production?

When contacting the shielding frame and shielding can, both design and production have suffered a lot. Because the PCB manufacturer we are looking for is also the first time to make shielding frames and shielding cans, the quality is high. It has not been able to meet the requirements of SMT, and the deformation of the shielding frame often results in poor welding and other situations.

After a period of exploration and continuous design changes and improvements, I gradually discovered how to design, manufacture, and package the shielding frame so that it no longer has the problem of flatness (Coplanarity), and can also use the suction of SMT smoothly. Nozzle comes to order.

I forgot to explain that the function of this "shielding frame" is to shield electromagnetic interference (EMI). It has two purposes. One is to prevent external noise interference. It is generally used on wireless communication boards, especially mobile phone signals. ; The second is to avoid the electromagnetic interference generated by itself from affecting other electronic equipment (EMC) or causing a burden on the human body.

Material selection of shielding frame

At the beginning, the material of the shielding frame we chose was SPTE 0.3mm "tinplate", which is commonly known as "foreign white iron", which is the kind of steel plate that is tinned on both sides, usually used on cans, but this material is relatively hard, the manufacturer said It is difficult to meet the 0.1mm flatness (Coplanrity) requirement, because the thickness of the SMT steel plate is sometimes 0.10mm and 0.127mm, so it is hoped that the flatness of the shielding frame can be made within 0.1mm, of course, local thickened steel plates can also be used. To increase the amount of tin, it is still hoped that the ability of the shielding frame itself can be improved so that the production yield can be improved.

Later we switched to C7521R, commonly known as "white copper", which is a copper-nickel alloy with copper (66%), manganese (0.5%), nickel (19.5%), and a small amount of zinc. Its color is silvery white and tough High, ductile, with certain strength, good stability, fatigue resistance and corrosion resistance are also very good, and importantly, it is also suitable for stamping processing.

Basically, C5721R is easier to reprocess and level than SPTE, so manufacturers strongly recommend this material. More importantly, you can also eat tin, but the price is a little bit more expensive.

Design of shielding frame

In addition to the functionality, the design of the shielding frame must also consider whether it can be produced in the PCBA factory, especially to make it compatible with the automatic production of SMT. Therefore, the design has to consider how to prevent the shielding frame from deforming due to the stress release caused by the passage of time after the stamping, forming and leveling.

In order to comply with the automatic production of SMT, a 4mm diameter area must be designed on the shielding frame as the center of gravity of the entire shielding frame, so that the suction nozzle of the SMT machine can take and place the shielding frame. If the position of the center of gravity shifts too much, SMT The shielding frame of the machine is prone to skew when it is taken and placed, and there is also a risk of throwing parts when it is rotated, which will reduce the utilization rate of the SMT machine.

In addition, in order to strengthen the structure of the shielding frame, generally we will increase the material at the corners of the shielding frame, because the stress is likely to remain in the corners and will be released after a long time.

When the situation permits, you can also consider adding a long strip of openings at the edge of the shielding frame to facilitate the leveling operation during the secondary processing. Otherwise, as the size of the shielding frame is designed larger, the secondary processing will be more difficult. Leveling, but this must be matched with the packaging, because the structure of the gap will be more fragile. If manual operation is required, all will be accidentally bumped into these places, and there will be a chance to bend and deform, so you have to deal with it carefully.

Roll packaging of shielding frame

If you give the PCB engineer a choice, 11 out of 10 will choose "tape-on-reel" packaging instead of "Tray" packaging. As far as Shenzhen Honglijie understands, it is pallet packaging. It is more difficult to control the quality of printed parts, and hard pallets must be used. PVC soft pallets tend to distort the shielding frame, so it is not easy to control the precision when picking up the parts.

If you want to pack the shielding frame into a tape, you must pay attention to the radius of the tape, especially the inner radius, because the smaller the radius at the innermost circle, the more likely the shielding frame is to be bent and bend and cause poor flatness. The larger the shielding frame, the larger the inner radius required.

In addition, pay attention to whether the tape and reel packaging is packaged by the stamping manufacturer itself. If it is outsourced packaging, then it will be difficult to clarify the responsibility when there is a problem with the PCB product.