Study on Fabrication Process of 2.0mm Small Pitch Multi-Grafted Rigid-flex PCB

With the development of miniaturization and three-dimensional assembly of electronic products, Rigid-flexible boards have developed rapidly in recent years. At the same time, in order to cope with the more stringent miniaturization trend, some products have tried to compress the deflection length to within 2.0mm to save installation space. For such small-pitch rigid-flex board, the cover film cannot be connected in series after opening the window, which greatly affects the filming efficiency, and also faces a large processing bottleneck in the shape uncovering, which cannot meet the mass production requirements, so how to Optimizing the process and maximizing the potential of existing equipment is critical.

This paper introduces a method of bonding the secondary laser processing small-pitch cover film, and adopts the pre-milling and uncovering process and subsequent special process control to improve the difficulty of covering the film due to the too small width of the flexible region. And the problem of inefficiency in manual uncovering has laid a technical foundation for mass production.

 

Analysis of designing and processing difficulties of small pitch multi-graft Rigid-flex PCB

This is a difficult rigid-flex PCB. As an example of a product structure diagram in Figure 1, we can see that 8 branches are connected to the periphery of the hard plate in the middle area, and the lengths of the flexible areas of the 8 branches are It is 1.575mm, which means that the film spacing and the uncovering distance are less than 2mm.

 

 Small pitch rigid-flex plate diagram

Figure 1 Small pitch rigid-flex plate diagram

 

For complex small-pitch multi-graft design, the cover film cannot be connected in series after partial window opening, that is, it cannot be aligned through the edge positioning holes of the board, and can only be adhered one by one by hand, as shown in Figure 2 .

manual alignment film effect chart

Figure 2 manual alignment film effect chart

 

The manual method has poor alignment accuracy and low efficiency, which cannot meet the mass production requirements. In addition, since the width of the flexible region is too small, it is difficult to manually remove the cover after the shape, which is disadvantageous for mass production, as shown in Figure 3.

Uncovering the area renderingsUncovering the area renderings

Figure 3 Uncovering the area renderings

 

Small pitch multi-graft rigid plate processing solution

 

1. Product design optimization

Here, an ordinary four-layer rigid-flex plate is taken as an example to optimize the processing flow, and the laminated structure is designed as shown in Figure 4 .

 

 Four-layer rigid-flex plate composite structure design

Figure 4 Four-layer rigid-flex plate composite structure design

 

Firstly, the second laser file is added to the cover film after the first laser window opening, and the protective copper wire is set at the corresponding position of the soft board, and the protection copper wire is located in the contour milling groove area, so that there is no residue after the CNC processing; Multi-grafting rigid plate, optimize the original shape and uncovering process to pre-milling groove before pressing, that is, the top and bottom rigid plates open the window before pressing, and directly expose the flexible area after pressing, without molding Uncover it.

 

 

 

2. Process optimization

(1). Secondary laser cover film bonding and pre-milling cover

For the optimization of the cover film processing file, the secondary processing is mainly to split the original single processing into secondary processing, as shown in Figure 5 below.

 

Graphic comparison before and after overlay film processing optimization

           Figure 5: Graphic comparison before and after overlay film processing optimization

 

The first laser burned the cover film, only the two sides of the rigid-flex joint near the hard-plate pattern were processed, and then the rivets were aligned and fixed by a soldering iron at each of the small-piece cover films, as shown in Fig. 6.

 

 Cover film pre-fixing

Figure 6 Cover film pre-fixing

 

The second laser burns the cover film, cuts the other two joints at the rigid-flex joint, then tears off the excess cover film, and retains the small cover film of the flexible area fixed by double-sided preheating, and then quickly presses the cover film normally. The specific effect diagram is shown in Figure 7 below.

 

Fast processing effect after laser processing cover film bonding

Figure 7: Fast processing effect after laser processing cover film bonding

 

The subsequent pre-milling groove is used to make the milling groove file, and the top and bottom rigid plates are pre-milled (opening the window before pressing), as shown in Figure 8 and Figure 9 below.

 

Schematic diagram of the milling groove processing file

Figure 8 Schematic diagram of the milling groove processing file

 

Opening the window before the rigid board is laminated

Figure 9 Opening the window before the rigid board is laminated

 

 

(2) Special protection measures

In addition, it should be noted that when copper is used, it is not possible to remove the slag with potassium permanganate containing a strong base to prevent the exposed coating film from being corroded or destroyed by the strong alkali syrup, that is, plasma degumming after drilling, and degreasing when immersing copper. Lower the cylinder and then produce it according to the normal process.

 

There is also a follow-up soldering control to control the rework, allowing re-washing before curing (using the automatic film-removing machine to remove the solder mask). Under no circumstances should the immersion high-temperature film-removing tank be used to retreat and rework.

 

(3) Product effects

The final effect of the product production is shown in Figure 10 below. The surface of the flexible area is glossy, there is no trace of chemical water corrosion, and the edge of the rigid-flex joint is smooth. The effect is not different from the normal shape and the uncovering process.

 

Product production effect

Figure 10 Product production effect

 

The heat resistance test was carried out again, and the lead-free tin furnace was passed through the lead furnace at a maximum furnace temperature of 260 ° C for 3 times, and the inner layer of the slice was well connected, and the blasting layer was not delaminated, and the effect is as shown in Fig. 11 and Fig. 12 .

 

 Heat resistance test appearance

Figure 11 Heat resistance test appearance

Heat resistance test slice diagram

Figure 12 Heat resistance test slice diagram

 

After the process test, the secondary laser is used to process the small-distance cover film, and the pre-milling and uncovering process and the special process control of the post-process are used before pressing, which can improve the coverage of the cover film due to the too small width of the flexible region. Difficulties and inefficiency in manual uncovering; the process inspection has no chemical water corrosion in the flexible zone, the appearance of the finished product is intact, and the reliability test has no blasting delamination, which can meet the needs of mass production.

 

 

In this paper, the difficulty of covering and uncovering the cover film of small-pitch multi-grafted rigid-flexed plate is analyzed and improved. The overall quality is controlled by the secondary laser cutting + pre-milling and uncovering process, which lays a technical foundation for mass production. . The improvement method outlined in this article is only for reference and reference by peers. Please correct me if you have any deficiencies.