Solder Paste Printing

Everything About Solder Paste Printing

1. What Is Solder Paste Printing?

Solder Paste printing is placing pads on the PCB through the holes of the steel plate (Stencil) to contact the paste.

2. Components Of Solder Paste

• Activator (ACTIVATION): the composition mainly plays a role in removing the PCB copper film pad surface and parts of the role of oxidation material soldering. It is also able to reduce the surface tension of tin and lead.
• Thixotropic agent (THIXOTROPIC): During the printing process, there will be trailing, sticking, and other phenomena. This component is mainly to adjust the viscosity of the solder paste and printing performance to prevent the above undesirable phenomena.
• Resin (RESINS): This component plays a vital role in fixing the parts. Its function is mainly to increase the adhesion of the solder paste but also to protect and prevent re-oxidation of the PCB after soldering.
• Solvent (SOLVENT): The component is the solvent of the solder component in the mixing process of the solder paste to regulate the uniformity of the life of the solder paste that has a particular impact.

3. Pre-printing Inspection Of Solder Paste

• Check the correctness of the PCB board to be printed.
• Check whether the surface of the PCB is complete and free of defects and dirt.
• Check whether the stencil is consistent with the PCB and whether its tension meets the printing requirements.
• Check whether the stencil is blocked. If the phenomenon of blocked holes needs to wipe on the stencil with dust-free paper dipped in alcohol and blow-dry with an air gun. Air guns need to maintain a distance of 3-5CM from the stencil.
• Check whether the use of solder paste is correct. According to the “storage and use of solder paste” use, note: pay attention to the return temperature, stirring time, lead-free and leaded distinction.

Cautions：

• If the use of stencil printing, solder paste viscosity should be (20) 0 ~ 14OOPa s. Technicians should take out the best density of 900Pa s from the refrigerator at room temperature to recover at least two hours out of the fridge within 24 hours after use; Technicians should open newly opened solder paste on the lid of the can to write down the date and the name of the user.
• Solder paste stirring: Solder paste should be thoroughly mixed with a solder paste mixer or manually before use.
• Check whether the PCB is used wrong and there is no bad. Read the PCB product certificate of conformity. If the PCB manufacturing WHP period is more significant than six months, the PCB should be dried in advance (drying at 125°C for four)

4. SMT Solder Paste Printing Process Requirements

• The primary printing defects are more miniature tin, even tin, pulling tip, shift, leakage of printing, more tin, collapse, PCB board dirty, etc.
• Solder paste printing thickness for the stencil is 0.02mm ~ +0.04mm.
• To ensure that the post-furnace welding effect is without defects.

5. Factors Affecting The Printing Of Solder Paste:

Solder paste printing is the basis for good or bad circuit board soldering, where the location of the paste and the amount of tin is more critical. Poor solder paste printing often results in solder short circuit (solder short), empty solder, and other problems. However, to print the solder paste well, we must also consider the following factors.

• Squeegee type (Squeegee): Technicnas should do the solder paste printing based on the characteristics of different solder paste or red glue to choose the appropriate squeegee. Currently used in solder paste printing, the squeegee is stainless steel.
• Squeegee angle: squeegee scraping angle of the paste.
• Squeegee pressure: the pressure of the squeegee will affect the amount of paste (volume). In principle, if other conditions remain unchanged, the greater the pressure of the squeegee, the less the volume of solder paste will be. Because the pressure is enormous, equal to the gap between the steel plate and the board compressed.
• Scraper speed: the speed of the scraper will directly affect the shape of the paste printing and paste volume and directly affect the quality of solder. Technicians will set the general scraper speed at 20 ~ 80mm/s. In principle, the speed of the scraper must match the viscosity of the paste. The better the fluidity of the paste, the faster the scraper speed should be; otherwise, it is easy to bleed.

6. Solder Paste Printing Process

The process flow of printing solder paste includes the following steps: Preparation before printing adjusts the printing process’s operating parameters: printing solder paste, then printing quality inspection, later cleaning, and end.

• Adjust the stencil and use alcohol to clean the printing table and stencil. Adjust the height of the stencil fixture so that the stencil and platform are tight. Move the stencil, coarse alignment, fixed stencil, fine adjustment with the platform, fine adjustment to each hole, and absolute pad alignment.
• Have a clean, good squeegee ready to be mounted on the assembly machine.
• With a solder paste stirring knife,  add solder paste to the stencil. The first time to add solder paste height at about 1CM, width 1.5-2CM, length depending on the size of the PCB, both sides than the printing area of about 3CM long. It should not be too long or too short; later, every two hours, to add solder paste, the amount of tin is about 100G.
• In PCB printing, the printing of the first 5PCS board requires full inspection. After the printing quality is OK, notify IPQC for the first inspection to confirm that the printing quality is not abnormal, notify the production line operator to start production.
• During the normal printing process, the operator should check the printing effect every half hour to see if there is more miniature tin, even tin, pulling the tip, shift, leakage, and other harmful phenomena. And focus on the printing effect of overly dense components such as “BGA, QFP, SOP, the row of plugs,” etc.
• For every 5 PCS printing, technicians need to clean a stencil if the PCB board has a pin too dense components “BGA, QFP, SOP, row of plugs” to increase the frequency of cleaning every 3 PCS cleaning.
• If bad printing is found in consecutive 3 PCS printing, the production process will notify the technician for debugging; cleaning the bad printing PCB board. When cleaning poorly printed PCBs, do not use complex objects to directly scrape the surface of the PCB to prevent scratching the PCB surface lines. PCB with gold fingers, technicians should avoid gold fingers, repeatedly wipe with dust-free paper with a bit of alcohol, blow dry with a wind gun, check under a magnifying glass, no residual solder paste for OK.
• The standard printing process regularly checks the overflow of solder paste to collect.