When most hardware engineers contact PCB layout tools, PADS, Allgero, Protel and other tools from the beginning, they will inevitably contact the packaging and manufacturing of components. Whether it’s SMD components, QFP, QFN, BGA, PGA, SOT23, TO263, MOIC and other chips, or passive components such as 0402, 0603, 0805, etc., all have to touch the package. Some are taken from the standard library, and some are made by ourselves. If you have seen many other people’s layout designs, you will find that the same components have many subtle differences. These differences also more or less affect the quality of the patch and the visual inspection effect, and more directly affect the service life of electronic products.
The topic to be discussed today is “packaging of components”. The manufacturing process of standard package is not described here. Here, the common problems are discussed, and it will benefit engineers and friends, so as to improve the quality of SMT patches.
1, silk screen printing. Screen printing may not seem important and has no actual electrical characteristics, but it contains a lot of information. For two-pin or three-pin devices of different sizes, screen printing can ensure the correct placement of the patch. For diodes, tantalum capacitors, ESD and other polar devices, it is the only reference for visual inspection. For chip components, the position of one foot can be confirmed by screen printing. And for BGA or QFN components, screen printing can visually detect whether the placement of the chip is correct. It is very helpful to judge the system failure. Besides, screen printing can also reflect a lot of valuable information, such as company, board number, hardware settings, voltage, version and so on. Therefore, complete screen printing information has a great influence on the quality of patches.
2. Pad, pad size has always been a very tangled problem for engineers and friends. If the pad is made too large, it will make routing more difficult. If it is too small, it will lead to poor contact. Generally, there will be layout guidance for complex chips. This is the first choice for layout reference. Generally speaking, for BGA or other chips, the pad size is 80% of the ball diameter.
3. Heat dissipation pads, heat dissipation is an increasingly important issue for power devices, whether it is small mobile products with high range or large communication equipment. The heat dissipation of power is always the design center in hardware design. From the point of view of layout, the top and bottom layers below the power devices often have a heat dissipation pad, and some vias are regularly arranged on the top to connect them. The heat dissipation path is chip-> chip package ->top layer heat dissipation pad-> via hole ->bottom layer heat dissipation pad-> air.
4. Connector, connector. For components that often need to be plugged and unplugged, such as earphone interface, board edge keys, HDMI interface, etc. Often, the components will fall off after several times of plugging and unplugging, which will directly affect the user experience. This is because the positioning pad is too small, or the positioning pad falls off, resulting in the component falling off. We suggest that engineers can manually enlarge the positioning pad and directly modify the solder mask layer and paste mask layer. It is necessary to directly add a through hole on the pad to increase the stress on the pad.
5. Solder Mask and Paste Mask layers. For component packaging, these two layers will often deal with each other. Solder mask controls PCB green oil to open windows, while Paste Mask is mainly responsible for opening windows of steel mesh. Manual modification of these two layers is often encountered in some complex designs. Often good use will get twice the result with half the effort.
6. In addition, we often see devices that are also 0402 or 0603. Due to different packages, some packages are made without allowance, which leads to insufficient solder paste when mounting. During the drop test or user’s use, the components are easy to fall off, which directly affects the product quality. Therefore, if conditions permit, make as much allowance as possible.
