Introduction:

In the PCB manufacturing industry, a solder mask is applied to the surface of printed circuit boards (PCBs) to ensure insulation, prevent oxidation, and improve the aesthetic appearance of the PCB. This layer is typically applied to areas that do not require soldering. With the rapid growth of the electronics industry, solder mask technology has advanced significantly.

The solder mask between SMD pads is called a solder bridge, which helps prevent unintended bridging during soldering. As PCB wiring density increases, the distance between SMT solder pads becomes smaller, and the width of the solder mask bridges decreases. This has made soldering pretreatment increasingly important.

Test Method:

For this test, the same type of polished Copper Clad Laminate (CCL) was subjected to different pretreatment methods, including needle brushing, non-woven cloth grinding, volcanic ash treatment, and sandblasting. After these pretreatments, the solder mask process was applied. A solder bridge test film was used during soldering exposure for alignment. After development and curing, the PCBs were compared and tested based on the different pretreatment methods.

• The tension test of 3M tape was conducted on solder bridges of various widths (0.05mm, 0.075mm, 0.1mm, and 0.125mm) in PCBs manufactured using different pretreatment methods.
• PCBs treated with different pretreatment methods were subjected to various surface treatments, including lead-free HASL, immersion gold, and immersion tin. These treated PCBs were then tested using a 3M tape tensile test, and their solder masks were analyzed for degradation after sectioning.

Analysis of Test Results:

Figure 1 presents an SEM image of the copper layer after different pretreatment methods. The analysis shows that for boards treated with needle brushing and non-woven cloth, the copper surface becomes roughened by the brush wheel, leaving directional wear marks. This roughening alters the copper’s appearance and has a slight restorative effect on surface defects, though some minor trenching is observed. In the case of volcanic ash treatment, the abrasive material is sprayed onto a grinding roller to roughen the surface, improving adhesion between the solder mask and copper while effectively removing the oxide layer. Sandblasting, which uses high-pressure powder spraying (such as alumina), also roughens the copper surface, increasing the contact area and binding strength between the solder mask and copper layer.

Analysis of Solder Mask Results:

Figure 2 illustrates the effects of solder mask application on polished CCL after various pretreatment methods and development. Figure 3 shows the solder bridge effects on CCL after soldering and curing, while Figure 4 presents a cross-sectional view of the solder bridge after curing.

Analysis of Lead-free HASL Results:

Figure 5 shows the result after applying lead-free HASL, and Figure 6 provides a cross-sectional view of the solder bridge after lead-free HASL treatment.

Analysis of Immersion Gold:

The following images depict the results of a PCB after immersion gold treatment:

• Figure 7 shows the effect of immersion gold on the PCB after soldering.
• Figure 8 presents a cross-sectional view of the soldering bridge after immersion gold.

Analysis of Immersion Tin:

The following figures show different solder bridge sizes on the soldered PCB after immersion tin treatment:

• Figure 9 illustrates the solder bridge (0.075mm) after immersion tin.
• Figure 10 shows the solder bridge (0.1mm) after immersion tin.
• Figure 11 depicts the solder bridge (0.125mm) after immersion tin.

Conclusion:

Based on the three pretreatment methods analyzed, the results show the following:

• The copper surface treated with volcanic ash (b) and sandblasting (c) are uniformly rough.
• The copper surface treated with a needle brush and non-woven cloth (a) shows slight wear and minor furrows.

From the tests conducted, it was found that:

• The PCB treated with volcanic ash exhibited the least solder mask erosion, and no solder bridges larger than 0.1mm were found to peel off during immersion tin treatment.
• The PCB treated with a needle brush and non-woven fabric showed significantly less solder mask erosion compared to sandblasted PCBs, with no solder bridges larger than 0.125mm peeling off during immersion tin treatment.
• The sandblasted PCB experienced the most solder mask erosion, and solder bridges smaller than 0.125mm were observed to peel off during immersion tin treatment.

In conclusion, volcanic ash is the most effective pretreatment method, followed by the needle brush and non-woven fabric method. Sandblasting is the least effective as a pretreatment.

If you have any questions about PCBs or PCBA, please contact me at info@wellcircuits.com.