HDI Circuit Board Inspection

X-ray inspection

Based on experience, X-rays are not necessarily mandatory for BGA assembly. However, it is certainly a good tool to have on hand, and it should be recommended for CSP assembly. X-rays are very good for checking solder shorts, but less effective for finding solder opens. Low-cost x-ray machines can only look down and are adequate for inspection of solder shorts. An X-ray machine that can tilt the object under inspection is better open to inspection.

glue inspection

Glue dispensing is another complex process that tends to deviate from the desired result. As with solder paste printing, a clearly defined and properly executed process monitoring strategy is required to keep the process under control. Manual inspection of the dot diameter is recommended. Results were recorded using a range control chart (X-bar R chart).

Before and after a dispensing cycle, it is a good idea to drop at least two isolated dots of glue on the board to represent the diameter of each dot. This allows the operator to compare the quality of the glue dots during the glue cycle. These dots can also be used to measure the dot diameter. Glue spot inspection tools are relatively inexpensive, and basically there are portable or benchtop measuring microscopes. It is not known if there is any automated equipment specifically designed for glue spot inspection. Some automated optical inspection (AOI) machines can be adapted to accomplish this task, but may be overkill.

Confirmation of the first-article. Companies typically perform a detailed inspection of the first board that comes off the assembly line to confirm the machine's settings. This method is slow, passive and inaccurate. It is common to see a complex board containing at least 1000 components, many with no markings (values, part numbers, etc.). This makes inspection difficult. Verifying machine settings (components, machine parameters, etc.) is a positive approach. AOI can be effectively used for inspection of the first board. Some hardware and software vendors also provide feeder setting confirmation software.

Coordinating the validation of machine settings is the ideal role for a process monitor who leads the machine operator through the line to validate the process with the help of a checklist. In addition to verifying the feeder settings, the process monitor should carefully inspect the first two boards using available tools. After reflow soldering, process monitors should perform a quick but detailed inspection of critical components (fine-pitch components, BGAs, polar capacitors, etc.). Meanwhile, the production line continues to assemble the boards. To reduce downtime, the line should be full of boards prior to reflow while the process monitor inspects the first two boards after reflow. This can be a little risky, but you can gain confidence in doing so by verifying the machine settings.