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PCB/PCBA knowledge
DFM (Design for Manufacturability) of PCB silkscreen
Table of Contents

Functions of PCB silkscreen

PCB silkscreen
PCB silkscreen

PCB silkscreen can be seen on most PCB boards, but what are the functions of PCB silkscreen?
Here are some main functions of PCB silkscreen:

1. There are countless kinds of electronic components, but how to distinguish what electronic components are attached to the PCB pad? In fact, we judge what electronic components should be pasted at each position through the silkscreen on the PCB board;

2. SMT assembles the patch through silkscreen, which is convenient for the factory to find the bit number of each component when patching;

3. PCB silkscreen is convenient for finding the corresponding position of each component during subsequent maintenance;

4. The silkscreens on the PCB are not only the identification of components, but also the product name, manufacturer logo, UL mark, identification codes such as production cycle, etc.

DFM design of PCB silkscreen

Engineers will consider the power integrity and signal integrity of the product during the layout process, but the manufacturability of silkscreen is easily overlooked. For example, the silkscreens cover the SMD pad, which brings inconvenience to the PCB continuity test and component soldering. If the silkscreen design is too small, it will cause difficulty in screen printing, but if it is too large, the silkscreen will overlap each other, making it difficult to distinguish, etc.

PCB silkscreen design
PCB silkscreen design

1.The distance between the silkscreen and the pad
The silkscreen needs to have a distance of 3-6Mil from the solder mask window pad, because it will have deviations during production.

2. Silkscreen line width
The line width of silkscreen, here refers to only the line width, not the width of the entire silkscreen. The screen printing character line width is the width of the lower ink on the screen printing. If the line width is small, the ink will not fall on the screen, and the characters will not appear in the screen printing.

3. Linear white oil block
In the case that a whole piece of silk screen is composed of lines, if the line width is not enough, it seems to be a large piece. In fact, due to the insufficient line width, the light with a small line width cannot be drawn during light painting, which will cause a whole piece of silk screen to be missed.

4. Silkscreen spacing
There are two reasons for the silkscreens to be blurred. One is that the silkscreen line is too thick, and the other is that the distance between the silk screen is too close.

5.Silkscreen height
The silkscreen height is the height of the entire silkscreen, and the minimum silkscreen height limit is 25mil. If the height of the designed silkscreens is less than 25mil, the silkscreens are not clear, and it may be a whole block of ink after silk screen printing.

6. Silkscreen marks are not clear
When designing QR codes and barcodes on the board, we must pay attention to the production capacity. If there is a small gap in the graphic, the silkscreen will be blurred. The printed QR code and the bar code will become a whole block and cannot be scanned and recognized.

Production process of PCB silkscreen

The color of PCB silkscreen is generally white, but also black and yellow. PCB silkscreen color needs to match the color of the solder mask. For example, if the solder mask ink is black, green, and blue, then white PCB silkscreen ink is used.

PCB screen color
PCB screen color

1 PCB silkscreen production capacity
The production capacity of PCB silkscreen printing is also related to the copper thickness. The thicker the copper thickness, the higher the requirements for silkscreens. Because there will be a height difference between the copper foil and the substrate where the copper foil intersects, and the unevenness will cause the silk screen to be blurred.

  • a. When the base copper thickness is 12, 18um, the minimum silkscreen line width is 4.5mil, and the minimum silkscreen height is 25mil;
  • b. When the base copper thickness is 35um, the minimum silkscreen line width is 5mil, and the minimum silkscreen height is 30mil;
  • c. When the base copper thickness is 70um, the minimum silkscreen line width is 6mil, and the minimum silkscreen height is 45mil;
  • d. When the silkscreen is a negative film effect (negative silkscreen), the silkscreen line width is ≥ 8mil, and the minimum silkscreen ink width is>5mil

2 Silkscreen processing on the pad
The silkscreens need to be removed on the pad, otherwise it will affect the solderability of the board. When moving silkscreens, it should be noted that the polarity of the silkscreen box cannot be changed when modifying or moving silkscreens, otherwise the polarity of components will be reversed.

3 Avoid missing silkscreens
When the silkscreens are on a large tin surface (or gold surface), under the premise of keeping the silkscreen identification clear, move the silkscreens as much as possible without cutting the silkscreens, or allow the silkscreens to be on the tin surface (or gold surface), and the process is sprayed tin or (immersion gold) first Postprinted silkscreens.

4 Silkscreen forward and reverse processing
The display effect of the top-level silkscreens is straight, and the display effect of the bottom-level silkscreens is reverse. If the conditions are not met, it needs to be mirrored. For example, the silkscreens at the bottom layer are straight silkscreens, but if they are not mirrored, they will be inverted silkscreens, which makes the silkscreens on the board look ugly.

5 Negative film (negative word) processing
When the PCB silkscreen in the design file is a negative film effect, the silkscreen ink cannot be applied to the pad or enter the hole. The single side of the white oil block for solder mask should be more than 10mil, and the single side of the white oil block for via hole cover oil should be 4mil. The silkscreen spacing needs to be>5mil, and the line width needs to be>8mil.

6 Add logo and identification code
When the user places an order and needs to add an identification code for PCB making, it is necessary to clearly add those logos, and even specify the location where the logo is added, so as not to conflict with the PCB factory. The added identification codes include period, logo, UL code, flame retardant grade, anti-static mark, environmental protection mark, etc.

you asked we answer

Silkscreen is a layer of ink traces used to identify components, test points, parts of the PCB, warning symbols, logos and marks etc.

The color is typically white, although other colors are also available.

  1. Component reference designators.
  2. Part outlines or electronic symbols.
  3. Polarity markings.
  4. Probe points.
  5. Pin numbers.
  6. Switch settings.
  7. Board information, including name, part number, and revision.
  8. Manufacturing information.

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