The non-wettable nozzle makes the solder flow in one direction. At the same drag soldering speed, the contact time with the substrate is longer, and there is more energy to force the solder into the through hole. Therefore, non-wettable nozzles exhibit better through hole filling.

 

 

 

Compared with through-hole filling, dipping Sn is a better substrate surface coating. The wettability is slightly better than Cu OSP. Since the circuit board is reflowed twice before soldering, there may be some small oxide layers on the copper, which will affect the wetting speed.

The diameter of the through hole is also important. The substrate has different through hole diameters for different pitches. For 1.27 and 1.50 mm pitches, 0.80 mm through-hole diameter through-hole filling works best. For 2.00 mm pitch, the preferred through hole diameter is 1.10 mm.

 

All components are square leads. When the pitch is 1.27, 1.50 and 2.00 mm, the dimensions are 0.35, 0.40 and 0.50, respectively.

 

Experimental design-bridging

 

In addition to good via filling, avoiding solder joint bridging is also crucial. Although bridging can be reduced by a smaller protruding length on the component, a tool to remove the bridging is required to eliminate this defect. Since the user has no influence on the protruding length, and the cost of cutting all components is very high, the equipment must deal with longer leads. Experience has shown that all parts with a spacing of less than 1.75 mm require an SDC device. The SDC device blows hot air where the lead leaves the liquid solder.

 

In the experiment, there are some additional parameters that affect the bridging. The purpose of designing the solder pads is to eliminate bridging, but in some lead-free applications, this benefit has not been proven.

 

Using the data from the experimental design, the bridging can also be checked. The result is shown in Figure 6. The higher the score, the better. 360 points means there is no bridge.

picture

 

The data shows that there are fewer bridges with non-wettable nozzles. The drag welding speed is preferably 3 mm/s. The inclined conveyor belt is beneficial to the filling of the vias, but has a certain negative effect on the bridging. This can be improved by changing the SDC angle and optimizing the SDC angle to achieve plane welding.

 

The effect of the copper layer is significant. Obviously, the 10-layer copper layer structure absorbs a lot of heat from the soldering area, so that the solder solidifies faster, thereby forming a bridge.

 

In addition, we also did an experiment to study the effects of stealing solder pads and different fluxes. The pad diameter is another factor being studied. Since SDC is a powerful tool to eliminate bridging, the effect of wire protruding length is not significant. If SDC is not used, the protruding part of the wire must be short to avoid bridging.

 

For very fine pitches, the pad diameter is critical. The pitch is 1.27 mm, the outer diameter of the pad is 1.00 mm, and the defects are the lowest. The larger the diameter, the more sensitive to bridging. For the 2.00 mm pitch, there is no significant difference in the outer diameter of the pad between 1.50 and 1.70 mm. The pad outer diameters of 1.80 and 1.90 mm are more sensitive to bridging.

 

The experimental design shows that at higher soldering speeds, stealing solder pads can reduce bridging. At lower soldering speeds, the solder will discharge by itself.

 

Stealing the solder pad makes the substrate hold the solder longer. The wettability nozzle is designed to peel the solder from the substrate and avoid bridging. In some lead-free solder alloy applications, the combination of wettability nozzles and solder pads is not very good. Experiments confirmed this.