The introduction

In spite of the rapid development of PCB technology, many PCB manufacturers focus on the production of HDI board, rigid flex board, backplane and other difficult board parts, but there are still some PCBS with relatively simple circuit, very small unit size and complex shape in the existing market, and the minimum size of some PCBS is even as small as 3-4mm. Therefore, the unit size of class plates is too small, and positioning holes cannot be designed during front-end design. It is easy to produce plate edge convex points (as shown in FIG. 1) by using external positioning method, vacuum PCB during processing, uncontrollable shape tolerance, low production efficiency and other problems. In this paper, the manufacture of ultra-small size PCB is studied and experimented deeply, the shape processing method is optimized, and the result is twice the result with half the effort in actual production process.

Discussion on the shape design of high precision and small size PCB

1. Status Analysis

The choice of shape machining mode is related to the shape tolerance control, shape machining cost, shape machining efficiency and so on. At present, the common shape processing methods are milling shape and die.

1.1 milling shape

Generally speaking, the appearance quality of the plate processed by milling shape is good, and the dimensional accuracy is high. However, due to the small size of the plate, the dimensional accuracy of the milling shape is difficult to control. When milling shape, due to gong inside the arc, gong Angle within the limitation of size and groove width, the choice of cutter size has great limitations, most of the time can only choose 1.2 mm and 1.0 mm, 0.8 mm or even milling cutter for processing, because of the cutting tool is too small, the feeding speed limits, lead to the production efficiency is low, and the manufacturing cost is relatively high, so only suitable for small amount, Simple appearance, no complex internal gongs PCB appearance processing.

1.2 die

In the process of large quantities of small size PCB, the impact of low production efficiency is far higher than the impact of contour milling cost, in this case, the only way to adopt the die. At the same time, for the inner gongs in PCB, some customers require to be processed into right angles, and it is difficult to meet the requirements by drilling and milling, especially for those PCB with higher requirements of shape tolerance and shape consistency, it is more necessary to adopt the stamping mode. Using the die forming process alone will increase the manufacturing cost.

2 Experimental Design

Based on our production experience of this kind of PCB, we have carried out in-depth research and experiments from the aspects of milling shape processing, stamping die, V-cut and so on. The specific experimental plan is shown in Table 1 below:

Discussion on the shape design of high precision and small size PCB

3. Experimental Process

3.1 Scheme 1 —- contour of gong machine milling

This kind of small-size PCB is mostly without internal positioning, which requires additional positioning holes in the unit (FIG. 2). When the end of the three side of the gongs, the last side of the gongs, there are open areas around the board, so that the cutter point can not be stressed, the finished product as a whole with the direction of the milling cutter offset, so that the finished product in the shape of the cutter point obvious convex point. Because all sides have been milling into a suspended state, there is no support, thus increasing the probability of bumps and burrs. In order to avoid this quality anomaly, it is necessary to optimize the gong belt by milling the plate twice, milling part of each unit first to ensure that there are still connection bits after processing to connect the overall profile file (FIG. 3).

Discussion on the shape design of high precision and small size PCB

Influence of gong machining experiment on convex point: the above two kinds of gong belt were processed, 10 pieces of finished plate were randomly selected under each condition, and convex point was measured using quadratic element. The convex point size of the finished plate processed by the original gong belt is large and needs manual processing. The convex point can be effectively avoided by using the optimized machining gongs. 0.1mm, meet the quality requirements (see Table 2), the appearance is shown in Figure 4, 5.

Discussion on the shape design of high precision and small size PCB

3.2 Plan 2 —- Fine engraving machine milling shape

As the carving equipment cannot be suspended during the processing, the gong belt in Figure 3 cannot be applied. According to the production of the gong belt in Figure 2, due to the small processing size, in order to prevent the finished plate from being vacuumed away during the processing, it is necessary to turn off the vacuuming during the processing, and use the plate ash to fix it, so as to minimize the generation of convex points.

Effect of fine carving processing experiment on convex point: the convex point size can be reduced by processing according to the above processing method. The convex point size is shown in Table 3. The convex point can not meet the quality requirements, so it needs manual processing. The appearance is shown in Figure 6:

Discussion on the shape design of high precision and small size PCB

3.3 Scheme 3 —- Laser shape effect verification

Select products with online external dimensions of 1*3mm for testing, make laser profile files along the external lines, according to the parameters in Table 4, turn off the vacuuming (to prevent the plate from being sucked away during processing), and conduct double-sided laser profile.

Discussion on the shape design of high precision and small size PCB

Results: the shape of laser processing on board without bumps products, processing size can meet the requirements, but laser after the shape of the finished product for laser carbon black surface pollution, and this kind of pollution because of the size is too small, cannot use plasma cleaning, use alcohol to clean cannot effectively handle (see figure 7), such processing results can meet customer requirements.

3.4 Scheme 4 —- Effect verification of die

Die processing ensures the precision of the size and shape of the stamping parts, and there is no convex point (as shown in FIG. 8). However, in the process of machining, it is easy to produce abnormal corner compression injury (as shown in FIG. 9). Such abnormal defects are not acceptable.

Discussion on the shape design of high precision and small size PCB

3.5 summary

Discussion on the shape design of high precision and small size PCB

4. Conclusion

This paper aims at the problems in the high-precision and small-size PCB gongs with the shape precision tolerance of +/-0.1mm. As long as reasonable design is made in the process of engineering data and proper processing mode is selected according to PCB materials and customer needs, many problems will be solved easily.