PCB Tech - What aspects should be considered in PCB layout and design

1 In the PCB layout and design, production requirements

The requirements for plate thickness, copper thickness, process, solder mask/character color, etc. are clear. The above requirements are the basis for making a board, so R&D engineers must write clearly. According to the customers I have contacted, Gree has done a relatively good job. The technical requirements of each document are written very clearly, even if it is usually ours. It is believed that the most normal use of green solder mask ink and white characters are written in the technical requirements, but some customers are free if they can avoid it. If nothing is written, they will be sent to the manufacturer for proofing production, especially some manufacturers have some special requirements. Failure to write it out caused the manufacturer to consult the requirements in this regard the first thing after receiving the email, or some manufacturers did not meet the requirements in the end.

2 Design of drilling

The most direct and biggest problem is the design of the smallest aperture. Generally, the smallest aperture in the board is the aperture of the via. This is directly reflected in the cost. The vias of some boards can obviously be designed as 0.50MM holes, namely Only put 0.30MM, so that the cost will directly rise sharply. PCB manufacturers will increase the price if the cost is high; in addition, there are too many vias, and the vias on some DVDs and digital photo frames are really full of the entire board and can’t move. I have made too many boards in this area for 1000 holes. I think it should be 500-600 holes. Of course, some people will say that more holes are good for the signal conduction of the board and the heat dissipation. I think we need to take one. Balance, while controlling these aspects, it will not lead to increased costs. I can give an example here: our company has a customer who makes DVDs in Shenzhen, and the quantity is very large. This was also the case when we first cooperated. Later, the cost was a big problem for both parties. After communicating with R&D, the aperture of the vias was increased as much as possible, and some vias on the large copper skin were deleted. For example, the heat dissipation holes in the middle of the main IC use 4 3.00MM Holes are replaced. In this way, the cost of drilling is reduced. One square can reduce the drilling cost of tens of dollars, which is a win-win situation for both parties; the other is some slots, such as 1.00MM X 1.20MM For manufacturers, it is really very difficult to make the ultra-short slot. First, it is difficult to control the tolerance. The second drill also comes from the slot, which is not straight, and it is a bit curved. We have done some of these boards before., As a result, the board of a few cents of RMB, due to the unqualified slot hole, deducted 1 US dollar/block, we also communicated with the customer about this problem, and later directly switched to the 1.20MM round hole.

3 Circuit design

For line width and line spacing, open circuit and short circuit, etc., are the most common among manufacturers. Apart from the special ones, for some more conventional boards, I think the line width and line spacing are of course the larger the better. I have seen some documents. Straight line, there must be a few bends in the middle, there are several lines of the same width and size in the same row, and the spacing is different. For example, the spacing is only 0.10MM in some places, and 0.20MM in some places. I think R&D is in When wiring, pay attention to these details; there are also some circuit pads or traces and the distance between the large copper skin is only 0.127MM, which increases the difficulty of the manufacturer to deal with the film. It is best to have the distance between the pad trace and the large copper skin. 0.25MM or more; some traces have a very small safety distance from the periphery or V-CUT, and it is okay for the manufacturer to move them, while others must be done only if R&D design is good, and there are even traces that are not connected to the same network., And some are obviously on the same network, but they are not connected. In the end, the manufacturer communicated with R&D and found that it was a short-circuit and open-circuit, and then revised the data. This situation is not rare. Experienced engineers may be able to see For those with inexperience, only follow the design documents. As a result, either modify the document to re-proof or use a blade to scrape or fly the line. For boards with impedance requirements on the circuit, some R&Ds are not written, and in the end it does not meet the requirements. In addition, the through holes of some boards are designed on the SMD PAD, and the tin leaks when soldering.

4 Design of solder mask

The problems that are more likely to occur in solder mask are some copper skins or traces where copper should be exposed. For example, a solder mask should be opened on the copper skin to facilitate heat dissipation, or copper should be exposed on some high-current traces. Generally, these additional solder masks are placed on the Soldermask layer, but for some R&Ds, a new layer is created., On the mechanical layer, on the forbidden wiring layer, there are all kinds of things, not to mention, without special instructions, it is difficult for people to understand. I think the most ideal is to put the TOP Soldermask or BOTTOMSoldermasK layer is the best, it is the easiest to understand. In addition, it is necessary to explain whether the green oil bridge in the middle of the IC should be retained, and it is best to give an explanation.

5 Character design

The most important aspect of characters is the design requirements of character width and character height. Some boards are not very good in this aspect. The same component even has several character sizes. As a manufacturer, I think it is unsightly. I think I must learn from those motherboard manufacturers., The component characters in rows, the same size, make people look pleasing to the eye. In fact, it’s better to design the characters above 0.80*0.15MM, and the silk-screen printing process is better for manufacturers; in addition, there are some large white oil blocks, such as those on the crystal oscillator, or some power strips. Some manufacturers use white oil caps. Live on the pads, some have to expose the pads, these must also be explained; I have also encountered some wrong silk screen positions, such as swapping the characters of resistors and capacitors, but these errors are still very few; there is also a need Added logos, such as UL logo, ROHS, PB logo, manufacturer's LOGO and serial number.

6 In PCB design, shape design

Nowadays, the boards are rarely rectangular, and they are all irregular, but mainly there are several kinds of line drawing outlines, which make people unable to choose. In addition, in order to improve the utilization rate of the equipment (such as SMT), V-CUT must be made up, but the board pitches are different. Some have pitch and some have no pitch. It is okay for the first factory to make samples and batches. It will be more troublesome to change the supplier later, if the second factory If you don't fight according to the first factory, the steel mesh won't fit. Therefore, under no special circumstances, it is best not to have spacing; in addition, some file designs may draw a small rectangular hole on the outline layer for the slot to be drilled. This situation is more common in files designed by PROTEL software. Relatively speaking, PADS is better. It is easy to be misunderstood by manufacturers to punch out this hole or make it into NPTH attribute if it is placed on the shape layer. For some PTH attributes, it is easy to cause problems.