PCB production process nine: PCB production process step 7 | silk screen
Review of the previous issue→PCB production process, the eighth issue丨outer layer circuit. In the ninth issue, we came to the step of silk screen printing. We will analyze each process step of silk screen printing, discuss the purpose and main content.
Screen printing process introduction
The purpose of silk screen printing process
1. It is mainly used for the protective layer of the outer circuit to ensure the insulation, shield and solder resistance of the PCB.
2. Make character logos.
Silk screen printing
Solder Mask Concept
Solder resist, commonly known as "green oil", in order to facilitate visual inspection, more green pigments that are helpful to the eyes are added to the main paint. In fact, solder resist paint has yellow, white, black and other colors besides green.
Purpose of solder mask
①Solder protection: leave the through holes and pads to be soldered on the board, cover all the lines and copper surfaces, prevent short circuits caused by wave soldering, and save the amount of solder.
②Shield: Prevent moisture and various electrolytes from oxidizing the circuit and endangering electrical performance, and prevent external mechanical damage to maintain good insulation of the board surface.
③Insulation: Since the board is getting thinner and the line width is getting thinner, the insulation problem between conductors is becoming more and more prominent, which also increases the importance of the insulation performance of solder resist paint.
Solder mask process flow chart
Introduction to pretreatment
Purpose: To remove surface oxides, increase the roughness of the board surface, and strengthen the adhesion of ink on the board surface.
Main raw materials: volcanic ash.
printing
Purpose: Use the screen to print the ink on the board, as shown in the figure below:
Main raw materials: ink
Commonly used printing methods:
Screen Printing
Curtain Coating
Spray Coating
Roller Coating
pre-baked
Purpose: To drive away the solvent in the ink, so that the ink is partially hardened and will not stick to the negative film during exposure.
Production points:
①The setting of temperature and time must refer to the conditions provided by the supplier. The pre-baking conditions for double-sided printing and single-sided printing are different.
②The selection of the oven should pay attention to the ventilation and filtration system to prevent foreign matter from sticking.
③When setting the temperature and time, there must be an alarm. When the time is up, it must be taken out immediately, otherwise over curing will cause endless development.
Introduction to Exposure and Development
exposure:
①Purpose: image transfer
②Main equipment: exposure machine
③Key points of the process:
A Cleaning of exposure machine
B energy selection
C vacuum control
development
① Purpose: To remove unpolymerized photosensitive ink with a potassium carbonate solution with a concentration of 1%.
②Main production materials: Potassium carbonate
Printing introduction
Printed characters:
①Purpose: Facilitate maintenance and identification
②Principle: screen printing method
③Main production materials: text ink
Curing introduction
Curing purposes:
The epoxy resin in the ink is completely hardened by high temperature baking.