PCB Tech - About the offset and improvement of PCB daughter board

1. Offset analysis of PCB daughter board

Before the total pressing of the PCB daughter board, there are two ways to stack it. One is to fix the daughter board with the edge of the mother board by using rivets. Naturally, there is no need to worry too much about the alignment. What will be analyzed here is the alignment of another stacking method, that is, the case where the daughter board is located in the middle of the mother board and cannot be pre-fixed with rivets.

The usual method is to perform appropriate size compensation for the shape of the daughter board and the slot processing of the mother board, so that the processed daughter board can fit into the mother board slot of the same shape without too much gap. However, compared to the problem of size compensation, placing a daughter board of similar size in the motherboard slot usually causes excessive offset.

In order to avoid excessive deviation of the PCB daughter board during the pressing process, and at the same time to meet the uniform and sufficient filling of the glue, the gap spacing of the daughter board is often set at 0.15mm. In the actual board making process, after measuring the gap between the mother and daughter boards after the offset, there are not many cases where the measured and statistical distance is as small as 0.05mm, that is, the daughter boards are offset by 0.1mm on a certain side.

This not only has a greater impact on the conduction between subsequent layers, but also the insufficient amount of glue for too small spacing will also affect the bonding force of the daughter-mother board. It can be seen that the bonding method of the daughter-mother board without obvious positioning needs to be further improved.

Up to 0.1mm on one side. This not only has a greater impact on the conduction between subsequent layers, but also the insufficient amount of glue for too small spacing will also affect the bonding force of the daughter-mother board. It can be seen that the bonding method of the daughter-mother board without obvious positioning needs to be further improved.

2. PCB offset improvement

In fact, PCB manufacturers have tried to add concave and convex grooves to the edge of the board for positioning [1], as shown in Figure 3 below. The advantage of this design is that it can improve the positioning accuracy between the mother and daughter boards, but this design still has insufficient glue filling at the corners, and there is a hidden danger of delamination and bursting.

The positioning of the daughter-mother board still requires a corner design. Obviously, the problem of insufficient glue filling at the corners cannot be avoided, but this situation can be reduced by the new design to reduce the insufficient glue filling area at the corners, thereby reducing the corners Risk of delamination and explosion. As shown in Figure 4 above, the rounded corners of the daughter-mother board are designed. It seems that the shape of the daughter-mother board has not changed significantly. In fact, the radius of the rounded corners at the corners has been adjusted in the design. In this design, the corners of the mother board are rounded with a larger radius, and the corners of the daughter board are rounded with a smaller radius. There are two advantages of this design. One is that the rounded corner design can make the flow glue have a certain buffer and drainage effect at the corners, so that the corners can be filled with glue more fully; the other is the design of rounded corners. After the glue flow occurs, it is not easy to cause serious corner deviation, which plays a role in positioning. At the same time, the existence of rounded corners prevents the daughter board and the mother board from completely contacting the edges, and the width of the gap can be reserved as much as possible. Fill the glue well.

After adopting the design of the rounded corners of the daughter-mother board, the experiment is to set a 0.2mm PCB pad at the same position of the daughter-mother board, and then drill a 0.2mm through hole. The offset of the daughter-mother board is usually controlled within 2 mils. At the same time, the fillet width at the corners can be kept at about 3mil, and its cross-section fills fully.