Elimination of process rules in print defects

Most solder paste printing defects have no direct relationship with the printing process. If these defects occur during the assembly process, the PCB specification, template design, and solder paste composition should be re-examined. By paying attention to these factors, most defects can be avoided. There are seven major defects to note: missing prints, uneven printing, solder paste slump, solder balls, smear, offset, and incomplete cleaning.

Material issues

Flattened tin/lead solders often have missing pads on their pads, called "halo" effects. Many companies are moving from tin/lead to gold, silver, and copper solders because gold, silver, and copper solders form a flat coating. Excessive pressure on the rubber squeegee makes this defect even worse. Excessive pressure and use of a soft rubber squeegee can also cause another defect: uneven printing. The squeegee will be trapped in the opening of the stencil and the paste will be squeezed out. At this time, the amount of solder paste passing through the pattern will be uneven.

Solder paste slumped due to the use of inappropriate materials and improper environmental conditions. Higher room temperatures can cause solder paste to slump, especially when fine gap printing is performed on high-width printed boards. Although the use of high-metal pastes can reduce slumps, correcting the material specifications and obeying supplier's advice can eliminate this defect. However, after the printing is completed, the printed solder paste should be tested to measure its degree of spreading.

Another drawback is solder balls, which are usually caused by poor solder paste quality. When printed boards are reflowed, it is ideal to form one large solder ball per solder joint. If there are many small solder balls, it is obviously that the quality of the solder paste is not high, or it may not be stored, aged, or used improperly. Solder ball testing ensures that solder paste does not fail.

Manual processing

After printing, if the circuit board is improperly handled by hand, the solder paste will be contaminated. It may be caused by the contact of some objects with the blot, or it may be caused by the contact of the template with the blot after separation. In addition to human factors, the latter may also be caused by improper print gaps or out of control separation. Remember that moving the stencil over the pad can cause solder paste fouling. For uneven tin/lead pads, the template will move over the pad and cause the solder paste to be inconsistent. The use of gold-plated nickel instead of tin/lead solder improves the surface coating and facilitates template positioning. Another alternative is to add a protective layer on copper. This method is becoming more and more popular and reduces costs.

If the printed board cannot be aligned, it will cause a printing offset. In fact, this is a common defect when the printed board is positioned with edges. This is also evident on large printed boards with mounting holes for identification marks. The use of optical PCB recognition can correct this accidental defect.

Solder paste on poorly printed boards needs to be cleaned and reprinted. If the solder paste is not cleaned thoroughly, it will remain in the through hole and melt during reflow. Solder paste that has not been cleaned around the solder mask opening will appear after reflow soldering. A better method is to monitor the amount of solder paste washed out and use a pen to make a hard-to-erase mark on the edge of the printed board so that if there is a problem in the final inspection, these printed boards can be found.

Process specifications

The following specifications are a standard for in house quality control and are necessary to operate the ISO 9000 quality system. This specification serves as a basis for training operators and establishes a reference for the process. It defines the stencil printing process.

1. Check whether the template design is the same as that listed in the kitting list. Before fixing the stencil to the printing press, check that the stencil is clean, that the opening is blocked, and that the surface of the foil is not damaged.

2. Fix PCB to workbench and check for alignment. After the PCB is fixed, lift the table so that the PCB is in contact with the surface of the stencil. Then align the PCB image with the template image and adjust the print gap as specified in the engineering table.

3. Install the press with a clean, flat rubber squeegee and adjust the drop stroke and the squeegee pressure. Every day must use fresh solder paste. Be sure to check whether the solder paste has exceeded the service life. If it exceeds, notify the operator. If defects are detected after printing, the excess solder paste must be scraped off with an unused old scraper, and then the PCB is cleaned and reprinted in the cleaning system. All this work should be completed before the batch printing.

4. After the operating parameters are programmed into the product route card, a PCB should be printed first, then the print quality is checked and recorded using the test standard parameters. Only after the print quality is satisfactory can the sign be passed on the card. When printing PCBs in bulk, follow this procedure and manually or automatically detect each print.

5. After printing, remove the scraper and wash it thoroughly. Check the template for damage and store it. The printer should also be thoroughly cleaned.

6. Sign the test card to indicate that this batch of printing has been completed.