Bridge Inspection in Solder Paste Printing Process (3)

When a solder paste pattern that crosses or is near, or exceeds, a preset boundary is defined relatively well, it indicates that a bridging or bridging occurs. Various types of bridged graphics must be measured to determine the specific shape and the probability of related defects that will result in subsequent assembly processes. The same data can be used to monitor minor trends and help to effectively control the printing process.

One of the most important characteristics is that the solder paste pattern crosses or extends across the gap. A typical bridge pattern will span the entire gap, but it will only span, and it is not certain that there will be related defects in subsequent processes. Some situations less than the width of the gap may be as troublesome or no problem. Therefore, this feature must be taken into account in order to measure the effect of defects on the process.

Although the solder paste pattern along the bridging direction may be very narrow or “virtual” or the bridge-like geometry is poor, the probability of bridging somewhere may be higher during reflow. The ground is only related to the amount of solder paste. As the area covered by the solder paste increases, the probability of causing related defects such as bridging increases when there are enough crossing patterns across the gap.

The "narrowest" point in the bridging direction - the weakest connection - indicates that the solder paste may or may tend to be pulled back in the back flow. If the cross-section along the bridge is narrow enough, the probability of the solder paste breaking and pulling back from there is greater than if the paste is still relatively (or severely) wide at the narrowest point. Because the pattern width of the solder paste increases, if there is enough solder paste through the gap, and the total amount of solder paste that forms the bridge is sufficient to maintain the bridging in the reflow, the probability of bridging-related defects will increase. .

Important gaps in solder paste area and bridges

Even the detection of geometric shapes we have developed a unique gap detection analysis method that reliably measures the area of ​​solder paste in the gap during the SMT assembly process, as well as meaningful bridge-like geometries and solder paste crossovers Graphic area. The total amount of solder paste in the gap and the bridge-like effective pattern area across the gap are used together to determine whether the solder paste will cause the probability of a bridge-related defect in the subsequent assembly process.

When the total amount of solder paste exceeds the preset boundary, a defect “inside-gap paste pattern” is usually generated regardless of the shape and position within the gap. The true bridge does not need to be confirmed, and it does not require further defect descriptions to be identified. This situation reflects poor print quality, poor alignment, bridging, or all of these defects, as well as the possibility of bridging related defects at this location during subsequent assembly processes, at least the probability of a short circuit occurring will increase. When bridges and bridge-like spans exceed preset boundaries, certain "bridge" defects will occur.

Simple user-defined bridge detection parameters

We have reduced the need for user-defined input parameters, as follows:

1, the largest gap within the solder paste area - defined as a percentage of the total gap area;

2, low, medium and high sensitivity - for bridge detection;

3, the maximum span - for bridge-like shape across the gap, defined by the percentage of gap width.

The lateral span is identified by the choice of sensitivity, which is a potential, very valuable measurement process that occurs for the actual bridge. The operator selects low, medium, and high bridge detection sensitivities to define a bridge that results from a minimum of sufficient solder paste scattering or width. In related terminology, the purpose is to measure the equivalent span of some bridge-like shapes. These bridge-like shapes should at least be considered obvious. The "low" setting requires that the bridging shape has more critical solder paste volume, "bridging capability", or apparent "bridging geometry." The "high" setting measures a very clear and small number of well-connected solder paste patterns. With the bridge sensitivity set to "high" and the span limit set to the corresponding "low," a greater number of shorts and bridge-like "solder paste" patterns will be detected.

When the maximum "internal solder paste" and "span" (potential bridging capability) are set individually. Mathematically, in the bridging shape reported in the same gap, the total gap in the solder paste area will not be greater than the maximum "span area." The span limit becomes a limitation of the amount of solder paste in the actual gap, which makes the user interface more simplistic and optional.

Minimum user-defined detection parameters

1, low, medium and high sensitivity - for bridge detection;

2, the largest span, or "bridge potential capabilities" - for bridge-like shape, but also the actual maximum clearance within the solder paste area.