Bridge Inspection in Solder Paste Printing Process (II)

Solder paste inspection is actually a solder paste printing image processing technique that separates the solder paste pattern from the background image to create a new “only paste pattern” image. This image containing only the solder paste pattern will be analyzed in the subsequent process to determine important information such as the amount, location, and deposition of the paste.

Solder paste inspection methods include applying single and multiple thresholds directly to captured images, image subtraction techniques, and structural layering based techniques. Derivatives include the use of UV-dye-enhanced solder pastes, laser profiles, or other methods that also provide 3D images, as well as X-ray technology. In fact, any method that can properly separate the solder paste pattern from the background image can be used as a follow-up analysis—such as the detection of a bridge. Figures 2 and 3 show examples of solder paste inspection based on structural delamination. Figure 3 includes a 3D image generated based on 2D structure probability data - here, the higher the height, the higher the probability that solder paste will appear there.

Bridges and other printing defects

In solder paste printing, the term “bridge” is often used to describe defects that are scattered across the adjacent pads. At some critical dimensions, the bridging solder paste may not be retracted in subsequent reflows, causing short circuits or other related defects in the final assembly. Not all bridge-like defects have the amount and geometry that adversely affect a given process. Conversely, for a process that does not always connect adjacent pads together to form a well-defined bridge, gap defects are obvious.

The bridge is not the only printing defect. Excessive solder paste, poor printing patterns, and poor alignment can also increase the probability of similar defects, especially "short circuits," which may occur at these locations during later assembly processes. Although subsequent processes play an important role in determining the quality of the final PCB assembly, however, post-print inspections and accurate evaluation of bridging and other defects will provide very direct feedback on proper printing process control.

Evaluating bridging, bridging, and other printing defects may be a subjective task with few clear definitions and limitations, and the device can only analyze the exact features. There is a need for reliable methods to rank and determine the importance of printing defects relative to the process to provide meaningful output and define actual useful process boundaries. Bridges, bridges, and excessive solder paste - these are subjective descriptions of print defects, and no further technical evaluation. None of these can accurately predict whether bridging-related defects will occur in later processes. In fact, there is no measurement method that can accurately predict this, they may be applied separately or simultaneously to show the process trends, and at what locations the probability of such defects will increase.