Why Is Silk Screen Printing So Important for PCBA Processing and Assembly?

Have you ever wondered how vision and robotic systems in surface mount SMT placement machines (SMT) can place electronic components exactly where they are needed on tiny printed circuit boards, with very tight tolerances?
The goals of SMT vision systems are varied: matching the circuit pattern of the artwork to the top or bottom of the manufactured printed circuit board (PCB), determining the position of the PCB in the x, y, and z planes, and locating the exact location on the circuit to place a specific component. With the help of an indexing (positioning) system, the PCB is positioned according to program parameters, however, this positioning is not precise enough for components placement. The silkscreen provides reference points for the imaging system.

What Is Silkscreen?

Although it sounds like a financial term, “fiducial” refers to a fixed reference base for comparison purposes, the fiducial is an embedded object or surface mark used as a directional measure or reference point for indexing and imaging systems.
Silkscreens are used in several industries and applications such as physics, 3D computer graphics, optics, and photography. They are used in geophysical flights to reference measurements, in augmented reality media to locate where to activate a composite image or game board, and in medical imaging by correlating body scans with body positions.
In automated electronic assembly applications, silkscreens are alignment marks used in indexing systems to orient PCBs in the correct direction and skew. Precise insertion of components into circuits using screen-printing vision systems. Silkscreens, also known as “circuit pattern recognition marks,” are a critical aspect of the entire automated manufacturing process through testing and packaging.
The importance of silkscreen placement cannot be underestimated because if at any point in the assembly process, the exact location of the board or circuit is not measured correctly, an automation “train wreck” will result!

How Does Silkscreen Placement Affect Component Insertion?

Precise silkscreen placement ensures your PCB is oriented in the correct direction and aligned as close to perfect as possible. SMT placement machines use cameras to locate the silkscreen and then adjust the position of the component based on the exact location of the board.

The downside of robotic assembly arms is that they won’t complain to you about how long they work.
PCBs are fed to SMT machines for assembly from a conveyor belt. Each PCB is held or clamped in a slightly different orientation than the others, and this slight variation is enough to ruin the board unless it is recalibrated. To this end, two or three fiducials are marked on the PCB to allow the placement robot to accurately determine the orientation of the board.
By comparing the actual position of the silk screen relative to the board floor plan stored in the machine’s memory, the vision system can reliably calculate the “offset” to which the part must be moved to accurately place it.

How Is the Silk Screen Set on the Printed Circuit Board?

The silkscreen is usually made by leaving a circular area of the printed circuit board uncovered by a solder mask. Within this circular area, the copper plating underneath is exposed. Good image recognition requires nothing around the silkscreen, so care must be taken to have a “matte” background around the silk screen during assembly. Alternatively, the fiducials can be covered with a clear solder mask, as the vision system can still detect the copper circle.
From the first two fiducials (placed diagonally from each other), the machine can recognize where the PCB is located by its X and Y positions, as well as the tilt of the PCB in the conveyor clamp. Finally, a third fiducial can help the machine compensate for any shrinkage or stretching of the PCB.
If components are placed on a double-sided PCB design, then fiducials must be placed on both sides of the PCB. Multiple silkscreen placements for PCB positioning are called “global fiducials,” components that require a high degree of positioning accuracy may have a single “local fiducial” placed near that component location to provide a higher degree of precision.

What Happens If Silkscreen Is Not Used?

If you are manually assembling single or prototype boards, then not using silkscreen is fine. However, if you plan to go into automated production, then you will find that SMT placement machines will be an option, as their operation requires reference points on the PCB to align them correctly. Additionally, manual assembly results in a higher percentage of boards failing testing and quality standards, and a lot of time will be spent repairing imperfect hand soldering.

Silkscreen Placement Tips

Here are some tips for placing silkscreen on a printed circuit board:
1. The optimal size of a silkscreen should be between 1 mm and 3 mm. Aim for a matte “buffer” area around the mark that is roughly the same size as the diameter of the mark.
2. The silkscreen must be free of solder masks unless the mask is transparent.
3. For global fiducials, there should be at least 2 marks, preferably 3, to ensure accuracy in component placement. Marks should be located at opposite ends of each other and as far apart as possible on the board. If you have 3 silkscreens, place them in a triangle and keep them as wide apart as possible on the board.
4. If there is not enough space at the edge of the board, a global silkscreen is necessary.
5. The distance from the silkscreen to the edge of the board should be 0.3 inches, not including the buffer zone for the silkscreen.
6. For local fiducials, place at least two silkscreens diagonally on the outside edges around a specific component. Local silkscreens can also be utilized to correct placement errors on a component.
7. The placement of silkscreens can make the difference between a smooth assembly line and one that constantly needs adjustments. Even if a supplier tells you that they have upgraded their machine to work without the marks, it is wise to always incorporate silkscreens into your printed circuit board design. You may need these marks during testing or when rework may be necessary in the future.