[Abstract] With the rapid development of the electronics industry, printed circuit boards, which are important original parts of electronic products, have also developed rapidly. As the auxiliary auxiliary material for the printed circuit board, the cover plate and the pad play a key role in optimizing the quality of the hole, reducing the tool wear, improving the life of the bit and improving the processing efficiency, especially the high-end PCB cover plate. The dependence is growing. In this paper, the research on PCB micro-hole drilling cover pad, its development status at home and abroad and the micro-hole drilling process of the cover pad are reviewed.
1. Printed circuit board micro hole drilling cover and pad
The general structure when drilling micro-holes of printed circuit boards: that is, the uppermost layer is the cover plate, the middle is the PCB board, and the lower layer is the backing plate. In the modern printed circuit board processing process, in order to ensure the high drilling quality and drilling efficiency of the plate, the use and correct matching of the cover plate and the pad are particularly important. At present, the commonly used cover plates in the industry include: a variety of aluminum sheets, composite aluminum cover sheets, phenolic cover sheets, epoxy glass cloth sheets and cold-punched sheets; common pads include: high-density wood mats, phenolic wood mats Plate, melamine wood mat, phenolic paper mat, high-density fiberboard and aluminum foil composite wood mat.
When drilling micro-holes of printed circuit boards, the pads used should meet the following requirements: To have proper surface hardness, it can reduce the burrs on the hole surface and avoid the wear of the drill bit due to excessive hardness; the resin is not included in the composition to avoid Produce drilling; have a large thermal conductivity to quickly conduct drilling heat to reduce the drilling temperature; have sufficient elasticity to reduce the resistance during drilling, so that the drill can be accurately positioned to improve hole accuracy, At the same time, the stiffness must be ensured to avoid the vibration of the drill. Therefore, choosing the right cover is critical to the drilling process.
At the same time, the role of the pad is also crucial. In the PCB drilling process, the pad used is located below the plate to be machined to protect the plate and the drill press, drill through the plate and improve the quality of the micropores. In order to ensure the quality of the hole processing, the backing plate should have good flatness and excellent dimensional tolerance, easy to cut, the surface is required to be hard and flat, the inner hole surface or the drill bit at high temperature, and the drilling should be fine and powdery, easy to discharge. Chips.
In the process of micro-hole drilling of printed circuit boards, selecting suitable cover pads and drilling with various printed circuit boards can not only improve the drilling quality, improve the yield, but also prolong the life of the drill bit and reduce the production cost. Therefore, the development of a new type of cover pad with superior performance and reasonable cost to meet the needs of high-end PCB drilling has become an important research goal of the manufacturer of the cover plate.
Second, the development status of the cover and the pad at home and abroad
The PCB drilling pad is almost the same as the PCB. During the drilling process, the cover and the pad are added under the PCB to protect the PCB and improve the hole accuracy. The phenolic resin cover plate was just used, which can meet the requirements of conventional drilling in a certain degree. For the reason of the easy occurrence of drilling, epoxy fiberglass board was once used in the PCB industry, but it can not completely solve the problem of drilling, so it can not meet the production requirements, and the cost of epoxy fiberglass board is also It is higher, so its history is not very long and it is abandoned.
In the early 1990s, due to the low price, high stability, low drilling and relatively good heat resistance of wood fiberboard, it was used as a drilling pad. In the mid-to-late 1990s, the type of cover pad used in PCB drilling was also selected to use another type of metal with a better thermal conductivity. At the beginning, ordinary soft aluminum was used. The thermal conductivity of aluminum is much larger than that of resin. The maximum temperature of the drill can be reduced from 2 000 ° C to more than 1 000 ° C, but its material is too soft, which is easy to cause scratches, which causes the drill to slip. There are abnormalities such as poor hole position accuracy and broken needles. As a result, alloy aluminum cover plates with better processing properties have emerged and become one of the varieties of cover plates that are still in common use today. At the beginning of the 21st century, as the aperture diameter gradually decreased (below 0.3 mm) and the requirements for drilling quality increased, the manufacturers of cover and pad began to develop and improve the original phenolic resin paper pad products. , density, surface hardness, flatness, thickness uniformity, material, etc. are all improved. At the same time, in order to solve the problem that the surface of the alloy aluminum cover plate is too hard and easily cause slippage, a lubricated aluminum piece has been developed.
Depending on the composition of the material, the cover is often divided into four categories: lubricated aluminum, plain aluminum, phenolic paper cover and epoxy glass cover. The bolsters are mainly divided into three categories: wooden slabs, wood slabs, and phenolic paper slabs. At present, the domestic use of the cover plate is mainly aluminum, which accounts for 82.2%; the use of the pad is mainly wood pulp board, which accounts for 59.1%.
With the development of high-end, functional and specialization of PCB technology, the cover pad is used as an auxiliary material for drilling of printed circuit boards, and is also becoming more refined, diversified and functionalized. The type and quality of the cover pad plays a crucial role in the hole quality, processing efficiency, yield, bit life, and final reliability of the printed circuit board.
Third, research on micro-hole drilling technology of cover and pad
The evaluation criteria of the hole quality are hole position accuracy, burr, nail head, hole wall roughness, hole wall contamination and tear. There are many factors that affect the quality of PCB hole processing, including drill press performance, drill pin, board material, and type and thickness of the cover plate, as well as drilling process parameters and drilling environment. Different PCBs can cause different problems during PCB drilling, so it is necessary to study the drilling defects of each PCB in order to find a suitable cover pad for matching. At present, some scholars have conducted some explorations on the drilling problems of different types of PCB boards.
(i) In terms of rigid printed circuit boards.
Due to the high hardness of the rigid printed circuit board material, the drilling temperature during the drilling process and the wear of the drill bit are serious, which is a key problem of the current rigid board.
(ii) In the drilling of flexible boards.
When the flexible plate is drilled, it will cause problems such as difficult discharge of drill cuttings and nail heads, and the influence of these problems on the quality of the drill hole; Wang Sihua et al.
The key factor that affects the quality of the flexible board drilling is the matching of auxiliary materials.
This is followed by the type of copper foil. Flexible circuit boards have been drilled due to poor material rigidity
The nail head problem is very easy to occur in the process, and the cover plate and the back plate of such plate drilling can be as far as possible
Choose a phenolic type. Zhang Xin analyzed micro-hole drilling and drilling of flexible printed circuit boards
Research on cutting quality, etc.
(3) In the aspect of rigid-flexible plates.
The steel plate is made of FR4 material, and the soft plate is made of PI or PET. Since the difference between the two materials may cause problems such as bonding and different shrinkage ratio, the stability of the product is difficult to grasp.
At present, there are few research literatures on the optimal combination of the selected cover pads when drilling different PCBs. The domestic research is still in its infancy, and only a few companies have conducted some research for the actual needs of production. And explore.
The preparation and properties of the coated aluminum substrate for PCB drilling were analyzed. It was pointed out that the heat absorption and water solubility of the rubberized aluminum plate were good, so that the drilling temperature could be lowered.
Based on the above research, at present, the specific effects of the pad in the PCB drilling process are rarely involved; there are few researches on the combination and matching of different PCB drilling pads; in addition, the drilling characteristics of different PCB plates are not yet available. For comparative analysis, these require further research.