PCB Tech - PCB process Shield frame design and production considerations

PCB process Shield frame design and production considerations

When I first came into contact with the shielding frame and shielding can, both design and production suffered a lot. Because the circuit board manufacturer is also the first time to make the shielding frame, so the quality has always been Unable to meet the requirements, and often occurrences of poor welding caused by the deformation of the shielding frame, after a period of continuous design changes and improvements, only then gradually figured out how to design, manufacture, and package the shielding frame so that it is no longer flat For the problem of Coplanarity, you can also use the nozzle of SMT smoothly to print.

I forgot to explain that the function of the shielding frame is to shield electromagnetic interference. It has two purposes. One is to prevent external noise interference. It is generally used on the board of unlimited communication, especially the signal of Big Brother. The other is to avoid itself. The generated electromagnetic interference affects other electronic equipment (EMC) or causes a burden on the human body.

Material selection of shielding frame
At the beginning, we chose SPTE 0.3mm tinplate, which is a kind of tinned steel plate on both sides, which is generally used on cans, but this material is relatively hard, and the manufacturer says that it is difficult to achieve a flatness of 0.1mm (Coplanrity) It is required that because the thickness of the steel plate is 0.127mm, it is hoped that the flatness of the shield frame can be made within 0.1mm.

Later, we switched to C7521R, commonly known as nickel silver, which is a copper-nickel alloy, copper (66%), manganese (0.5%), nickel (19.5%), and a small amount of zinc: silver-white color, high toughness, and Ductility, certain strength, stability, fatigue resistance, corrosion resistance, and stamping.

Basically, C5721R is easier to reprocess and level than SPTE, so the manufacturer strongly recommends this material. The important thing is that 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 meet factory production, especially to make it compatible with SMT automatic production. Therefore, the design has to consider how to prevent the shielding frame from being deformed due to stress release due to the passage of time after stamping, forming and leveling.

In order to comply with the automatic production of SMT, a 4mm diameter area must be designed on the shield frame to be near the center of gravity, so that the suction nozzle of the SMT machine can take and place the shield frame. It is easy to deviate, and there is also a risk of throwing parts when rotating, which will reduce the utilization rate of the 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 at the corners, which will be released over time.

When the situation permits, you can also consider adding a long strip of openings at the edge of the shield frame to facilitate the leveling operation during the secondary processing. Otherwise, as the size of the shield 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 weaker. If manual operation is required, it will be bent and deformed due to accidental collision with these places.

Roll packaging of shielding frame
SMT engineering usually likes tape-on-reel (tape-on-reel), not pallet material (Tray), as far as I know is that pallet material is more difficult to control, and must use hard pallets, PVC soft pallets are easy Distorting the shielding frame makes it difficult to control the accuracy of the pickup.

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 easier the shielding frame is to be bent and cause poor flatness, and the larger the radius The inner radius of the shielding frame of the shielding frame must be larger.

In addition, pay attention to whether the tape-and-reel packaging is packaged by the stamping manufacturer itself. If the municipal party committee is externally packaged, it will be difficult to clarify the responsibility when the product has a problem.