What should we pay attention to in PCB screen printing design?

Screen printing on PCB usually includes component screen printing and tag number, board name, version number, anti-static identification, bar code screen printing, company logo, and other identification. The screen printing material is non-conductive ink, and the commonly used screen printing ink is white. There are also yellow, black, and red inks, which are used only under particular circumstances.

1、 Silk screen design requirements:

The ratio of word height to word width of silk screen characters is generally required to be ≥ 6:1. There are three common font sizes: among them, when the board density is relatively large, 4/25mil characters (No. 1) are commonly used; For normal density, 5/30mil characters (No. 2) are recommended; When the board is relatively loose, 6/45mil characters (No. 3) are recommended; Generally, the thickness of surface base copper also has corresponding requirements for screen printing width: base copper < 1oz: limit value ≥ 4mil, optimization value ≥ 6mil; When the base copper thickness is 1oz, 5/30mil characters are preferred; For base copper 2oz, 6/45mil characters are preferred.

2、 Requirements for adding PCB screen printing:

1. Placement position: Generally speaking, when placing the silk screen of resistors, capacitors, tubes, and other devices, do not use four directions, which will lead to tiredness when looking at the silkscreen during debugging, maintenance, and welding (the board needs to turn several directions).

2. Try not to punch the through-hole on the silkscreen

3. Do not press the silkscreen on high-speed signal lines (such as clock lines): for high-speed signal lines at the top or bottom, because such signal lines can be regarded as microstrip lines.

4. The reading direction of silk screen printing should be consistent with the use direction. The reading direction of silk screen printing should be consistent with the use direction of the chip, mainly to reduce the probability of reverse welding during welding.

5. The pin number should be clearly marked on the silk screen:

6. Screen printing of special packages: for special packages such as BGA and QFN, the size of screen printing should be completely consistent with the size of the chip

7. Silkscreen of mounting hole: the silk screen of screws is added near the mounting hole, and the length and a total number of screws are marked at the same time, which is convenient for installation.

8. The ambiguity of silk screen printing: the most commonly used RS232, many people will mark RX and TX, but PC terminal also has RX and TX, when to use cross line and when not to cross the line? This leads to the ambiguity of silk screen printing, which makes people foolishly confused.

3、 Precautions for silk screen printing design

a) The line width of screen printing on the board is ≥ 4mil, and the line width of screen printing on components should be avoided to be 0.

b) Distance between silk screen and pad: the silk screen shall not cover the welding points on the board, such as the through-holes of patch device pads and plug-ins. The silkscreen is an insulating material, which will lead to poor welding once the pads are installed; Do not cover the test points, mark points, etc. on the board; It is usually required to maintain a distance of 6mil.

c) The spacing between screen prints: maintain 6mil, and it is acceptable to have overlap between screen prints. Once the overlap causes unrecognizability, it needs to be adjusted.

d) Direction of screen printing: the arrangement of screen printing strings should follow the principle that the strings are from left to right or from bottom to top when facing up.

e) Placement of device tag numbers: device tag numbers should correspond to devices one by one, and the order cannot be reversed or changed; When the device density is relatively high, the tag number can be placed in other places with space on the board by means of leading out or symbol marking.

f) The components' polarity identification and "1" pin identification should be placed correctly and clearly.

g) When exporting labels or symbol labels, the added silk screen and characters should be placed on the silk screen layer of board geometry.

The screen printing of the added board name and version number is also placed on the screen printing layer of board geometry.

h) The device tag number cannot be placed inside the device or outside the board frame.

i) When the board density is relatively high, and there is really no space to place the tag number, you can discuss with the customer not to place the tag number, but you need to issue the assembly drawing to facilitate the guidance of the device installation and inspection.

j) When the customer requires to write copper characters on the top and bottom, the line copper width: how base copper: the character width is more than 8mil, and the height is more than 45mil; 1oz base copper: character width more than 10mil; The height is more than 50mil. At the same time, it is necessary to do a good job of resistance welding and window opening, so that the copper characters on the produced board are relatively bright.

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