Research on improvement of hole position accuracy by Coated Entry Board

1.Introduction

Customers have higher and higher requirements on the drilling accuracy of PCB board, which requires us to meet customers' requirements through technical improvement. We can improve the drilling accuracy by changing the drilling entry board.

At present, the most widely used drilling entry in PCB industry is Aluminum Entry Sheet, which is aluminum foil with thickness between 0.13mm and 0.2mm. Customers chooses the corresponding thickness according to its own product characteristics. The Aluminum Entry Board is used to cover the top of PCB board when drilling, and its main function is to improve hole accuracy, heat dissipation and prevent burr.

2. Introduction of New PCB Entry Board

(1) New PCB entry is coated on the surface of aluminum foil, so it is generally referred to as Coated Aluminum Sheet in the industry, as shown in figure 1

The main function of the Coated Entry:

To facilitate drilling and prevent slipping; 

Reduce drill bit wear; Lubricate and clean the drill bit;

Lower the bit temperature.

(2) Surface coating material, generally composed of special resin, resin shall have the following characteristics: 

lubricity: Lubrication characteristics; 

Thermal fusibility: Low melting point, can quickly volatilize in high temperature; 

Water soluble: Easy to dissolve water, coating does not adhere to the hole wall; 

Support: High hardness is required to prevent needle drift in the hole.

(3) Advantages of Coated Entry for PCB processing:

Using the same conditions after the processing of the entry hole location test accuracy shows that the CPK of the coated Entry is much higher than that of the Ordinary Aluminum sheet.

The hole wall section of the entry processed under the same conditions was sliced after electroplating. The hole wall concavity of the coated cover plate was greatly improved compared with that of the Ordinary Aluminum Sheet.

3.Mechanism analysis of Coated Entry to improve drilling accuracy

3.1 When the drill bit touches the Coated Entry, the coating on the Coated Entry can stabilize the drill needle at the moment when the bit touches the plate surface, which can reduce the deviation. After being cut by the drill bit, it is converted into dust and absorbed.

In figure 2, a C manufacturer in China compared the drilling and drilling process by high-speed camera, and shot the drilling process of Ordinary Aluminum Sheet and Coated Aluminum Sheet respectively. It can be clearly seen from the playback video that when using Coated Aluminum Sheet, the bit is coated and its casing effect makes the bit vertically drill down.

3.2 The drill bit enters the plate

The coating material has the characteristics of low solubility and strong heat dissipation. When the bit touches the plate surface, the heat generated is transmitted through the aluminum board and absorbed by the coating in large amount, thus reducing the temperature in the hole of the bit and plate, as shown in figure 3.

Figure 4 shows a C manufacturer in China tracking and testing the needle temperature during drilling with the thermal imaging temperature measurement technology of infrared thermography. The red line is the needle temperature of Ordinary Aluminum Sheet, and the black represents the needle temperature of Coated Aluminum Sheet.

It is clear from figure 4 that the bit temperature with Coated Aluminum Sheet is much lower than the bit temperature with Ordinary Aluminum Sheet. The lower the temperature of the bit, the more rigid it is and the less likely it is to bend. Therefore, the use of Coated Aluminum Sheet improves the rigidity of the bit during the drilling process, thus improving the drilling accuracy.

3.3 Chip Removal

The coating on the surface of the coating will become gaseous volatilization after preheating, which speeds up the discharge of powder chips in the process of volatilization, thus increasing the discharge of chips in the process of drilling. Thus, the drilling resistance is reduced and the drilling precision is improved.

In figure 5, compared with the bit residues after PTFE plate is processed by using Ordinary Aluminum Sheet and Coated Aluminum Sheet, it can be clearly seen from figure 5 that the bit residues are significantly less after using Coated Aluminum Sheet.

4 Summary

The Coated Entry Sheet can improve the drilling accuracy, especially in the processing of some plate with high thickness to diameter ratio. However, as a new type of entry board, the purchase cost is several times that of the Ordinary Aluminum Sheet. The comprehensive use cost of the entry sheet can be reduced by improving the service life of the drill bit, increasing the grinding time and increasing the number of drilling laminates.