Failure Caused by the PCB Printing Material (2)

5 Pinholes and Blisters

Pinholes are the most vexing problem in screen printing. The printed circuit board needs to be processed in the next process after screen printing, such as etching, and pinholes are absolutely not allowed here. There are various reasons for pinholes, and the problem of pinholes is one of the most important inspection items in quality management.

5.1 Dust and debris on the screen cause pinholes.

(1) When the screen plate is coated with photoresist, dust is mixed in and attached to the screen plate to form pinholes. Carefully check the screen, and find that the pinholes are repaired in time.

(2) During the screen printing process, there is dust or debris attached to the screen plate to block the mesh holes, resulting in pinholes after screen printing. To do this, wipe the screen.

5.2 The surface of the printed board is contaminated.

(1) When the surface of the printed board is cleaned, there is a cleaning agent remaining on the surface to form a thin film, which causes the printing material to not infiltrate, resulting in “ink injection” and pinholes.

(2) Do not wear gloves when handling the printed board, and directly touch the cable with your hands. The grease and sweat on your hands will contaminate the surface of the printed board, causing “ink injection” to cause pinholes.

5.3 There are bubbles in the printing material.

In order to adjust the viscosity of the printing material, add a solvent for stirring, and the printing material produces bubbles. At this time, it can be left for a period of time, and most of the bubbles will disappear naturally. However, due to the viscoelasticity of the printing material, some bubbles will not disappear naturally. It will disappear when screen printing, but the air bubbles in some printing materials must use a defoamer to disappear. There are fast and slow defoaming agents, and the addition amount of the two is generally 0.1 to 1%. If the amount is too much, not only will it fail to defoaming, but it will become a foaming agent.

6 The screen printed wires are zigzag.

The screen-printed wires are jagged. The reason for this phenomenon is that when the screen is made, the mask is too thin, and the screen image has gaps. To avoid this phenomenon, the plate-making mask should have appropriate thickness and elasticity. For images with high size requirements, high-mesh polyester filaments should be used. This phenomenon is very easy to occur if the wire is perpendicular to the direction of movement of the scraper. In addition, when stretching the mesh, the wire direction of the mesh and the mesh frame form a certain angle, and the optimal angle is 22.5°.

Countermeasure:

A. To produce high-quality halftone images, the edges of the halftone wires must be sharp.

B. The printed board has better adsorption to the printing material.

C.The thixotropy of printing material is better.

D. The distance between the screen and the printed board and the angle of the blade edge of the squeegee should be appropriate.

E. Screen tension, screen spacing and printing pressure should be appropriate.

7 There is a screen pattern on the screen printing pattern.

There are screen patterns on the screen printing graphics, mainly because the screen mesh selected is too low, the thixotropy of the printing material is not good, and the drying and curing speed of the printing material is too fast. To this end, a high-mesh screen can be used to make the screen, and the thixotropic property is good, and the slow-drying material is used.

8 Spots

The screen printing graphics appear speckled and affect the smooth screen printing. The reasons for this phenomenon are:

8.1 Screen printing speed and drying of printing material are too slow.

8.2 The solid content of the printing material is too low.

8.3 The thixotropy of the printing material is too large.

8.4 Influence of static electricity.

8.5 The pigment in the printing material is not uniformly dispersed, and the pigment particles attract and cohere to each other, causing color spots.

Countermeasure:

(1) Improve the fluidity of the printing material.
(2) Use a solvent with fast volatilization speed.
(3) Select high-viscosity printing material.
(4) Minimize the influence of static electricity as much as possible, and use a static elimination device.

9 The color of screen printing graphics is uneven.

There are various reasons for the uneven color of screen printing. Only in terms of printing material, the colorant in the printing material is unevenly dispersed, the fineness of the printing material is large, and the screen printing ink bleeds. To prevent such failures, filter the printing material before screen printing, and thoroughly stir the printing material.