How to set the conditions for acceptable solder paste printing graphics?

Fine-pitch components (e.g. QFN, BGA, etc.) have extremely high requirements for solder paste uniformity, thickness and edge definition due to their small pin spacing. Any minor printing defects can lead to poor soldering, such as bridging, open circuits or empty soldering. Therefore, these special requirements must be fully taken into account when setting acceptable conditions.

Acceptable conditions for solder paste printing graphics are set as follows:

Reject and Acceptable Conditions for Solder Paste Printed Graphics are shown in Tables 1-1 and 1-2 and are based on the fact that the widespread use of solder paste inspection technology has made the amount of solder paste measurable. Due to the different impact of the amount of solder paste on different packages, it is not possible to have a uniform standard, must be based on the use of the package, pad size, stencil openings, etc. Consideration, and here only provides a way of thinking for reference.

Based on the control factors and the regular inspection items of the SPI, we establish the acceptable conditions for the solder printing pattern in the following 3 dimensions:

l Positional deviation, which is expressed as a percentage of the coordinate deviation in the SPI;

l Solder Paste Volume, which is expressed as a percentage of the volume of solder paste to the opening volume of the stencil in the SPI;

l Transferred Area, which is expressed as a percentage of the deposited area to the opening area of the stencil in the SPI;

Comprehensive Consideration

When setting these acceptable conditions, factors such as the type of package used, pad size, stencil design, and the accuracy of the SPI inspection technique must be considered. Different products and processes may require different condition settings. In addition, as technology continues to advance and processes are optimised, these conditions may need to be adjusted accordingly. Finally, by collecting and analysing SPI inspection data on a regular basis, problems in the solder paste printing process can be identified in a timely manner and corrective measures can be taken accordingly to ensure the stability and reliability of solder paste printing quality. At the same time, these data can also provide valuable references for process improvement and optimisation.