Application of coated aluminum sheet in high precision drilling of printed circuit board

Application of coated aluminum sheet in high precision drilling of printed circuit board

With the development of modern electronic products increasingly to miniaturization, Gao Jicheng, high performance trends, PCB hole density increasing, the aperture is more and more small, the reaction in the drilling process is the hole to the line (conductor) distance is more and more small, the drilling hole position accuracy requirements more and more high. At present, the precision of hole position is improved mainly from the aspects of drilling tool, cushion plate, cover plate, drilling machine and production parameters. The cover plate is one of the most important factors that affect the accuracy of the hole position.

At present, there are many kinds of coated aluminum cover plates that can improve the accuracy of holes at home and abroad. Compared with ordinary aluminum sheets, the coated aluminum sheet covers have better guiding, lubricating and cooling effects, which can effectively improve the accuracy of hole location, reduce the broken pin and inhibit Kong Cu. But the coated aluminum sheet to improve the reported hole position accuracy of the mechanism, the drill drill instantaneous slip and heat dissipation model of coated aluminum sheet can improve the accuracy of hole position mechanism of in-depth analysis, detailed answers to the performance advantages and assessment of coated aluminum sheet. It provides a theoretical reference for the related technical personnel for the production and use of coated aluminum sheets.

Analysis of the mechanism of the precision of the lifting hole position of the coated aluminum sheet

Brief introduction of coated aluminum sheet

The coated aluminum sheet is a surface coating on the surface of aluminum foil. In the industry, it is usually called coated aluminum or lubricating aluminum.

The coated aluminum sheet is mainly conducive to drilling, preventing skidding, reducing the wear of the drills, lubricating and cleaning the drills, reducing the temperature of the drill and so on. This mainly depends on the surface coating materials, general special resin composition, and has certain lubricity, water solubility, and low melt support.

The advantages of coated aluminum sheet of PCB board processing is mainly reflected in the following points: 1. Using the same processing conditions after the plate hole position accuracy test results, flat coated aluminum sheet. The deviation of hole position greatly compared with the ordinary aluminum sheet is reduced; the use of the same conditions after processing the plate in the hole wall sections after plating. The hole wall plate coated aluminum sheet is rough compared with ordinary aluminum sheet decreased greatly generous.

Theoretical analysis of slip slip

In printed circuit board drilling machine, drill bit contact coated aluminum plate, resin coated cover can contact surface in the drill drill to stabilize the moment, reduce the deviation. In order to better explain the mechanism and influence factors, from the basic principle of deviation in drilling, especially the drill hole of the sliding moment, stress analysis of drill point equilibrium.

The basic principle of drilling deviation diagram, drill down contact surface of the moment, because of its spindle vertical center line and production in the horizontal center line is not completely vertical, and dynamic displacement under the horizontal direction to drill spindle (X/Y direction) of the component, so as to form a point moment slip. From the stress analysis of the drill drill and cover slip sigma 0 stop drill ability parameters of drill moment slip, this definition of the ability parameter for the generalized friction coefficient of u cover. The larger the U, the greater the ability to prevent slipping from drilling. The more stable the drill bit is, the less the instantaneous slip is 0, which will improve the accuracy of borehole vacancy.

Among them, H said the production of plate thickness, X0 said the theoretical position of the holes in the X file, 'said actual spindle position, X said the actual position of the drill drill drill point moment, said negative borehole deviation sigma, delta sigma theta and hole position difference, the axis angle in the process of drilling and drilling tool in the normal to the plane, L said. 0+run-out said the drill blade long, comprehensive deviation drill spindle dynamic edge bending and offset value. The angle theta 0 production in the center plane and the X/Y plane, theta 1 run-out spindle caused by dynamic offset angle theta 1 'offset drill + dynamic run-out leads to the slip angle, 0 sigma said slip drill drill drill plate moment.

Analysis of theoretical model of heat dissipation

In the drilling of PCB, drilling tool runs at high speed and PCB generates a lot of heat. If not discharged in time, the drilling tool will remain in a high temperature state, leading to the decrease of drill tool rigidity, so as to reduce the accuracy of hole location and the quality of hole wall. Usually, the high-purity aluminum sheet is used as the cover for drilling. An important factor is that the heat on the surface of the circuit board is taken away and the temperature is reduced by the high thermal conductivity of metal aluminum. The heat transfer is a complex process. In order to better explain the advantages and main influencing factors of the heat dissipation of the coated aluminum sheet in the high-precision borehole, the heat dissipation process is simplified to the contact heat dissipation model and the cover heat dissipation model.

Contact heat dissipation model

When the aluminum sheet is exposed to the PCB plate, the heat dissipation between the two is mainly through the heat conduction mode of the micro convex body. Qc, Qg and Qr are used to represent the contact heat dissipation between the contact surfaces, the heat conduction of the gap, the heat radiation, and the total heat dissipation Qj of the contact surface is three.

Qj=Qc+Qg+Qr (2-1)

The two solid surfaces that contact occur, because the contact surface is not completely smooth, so there are actually only contacts at some points, and most of the interfaces between them are filled with interstitial gases. The thermal conductivity of the gap gas is poor. When the heat passes through the contact interface, the heat flow will flow to the contact micro convex body. Therefore, improving the contact area of the micro convex body is an effective way to improve the contact heat dissipation. In production, we mainly take the control of the flatness of the lower surface of the aluminum cover plate and improve the tightness of the contact between the aluminum surface and the PCB surface.

Two the tightness of solid contact is closely related to the assembly pressure between the planes. When the pressure is low, only a few small convex bodies are deformed and the actual contact area between the contact surfaces. 

The formula 2-3, 2-4, 2-5 and the formula of quality and area are replaced by 2-2.

In the formula, M1 and M2 are the equivalent mass of aluminum lamination and coating respectively, and T0, T1 and T2 are air, aluminum lamination and coating temperature respectively. C1 and C2 are the specific heat capacity of aluminum slice and coating layer respectively. Epsilon 1, epsilon 2 respectively aluminum coating layer and the radiating surface of generalized emissivity, A equivalent cooling area, sigma Stefan - porsman constant, h as phase change material transformation potential, p 1 and P 2 respectively, density of aluminum sheet layer and a coating layer, R for D1, the equivalent radius. D2 are the thickness of aluminum sheet layer and a coating layer, T1 and T2 respectively in the heat time aluminum layer and the coating layer transfer.

According to the difference between the coated aluminum sheet and the ordinary aluminum sheet and the different types of aluminum sheets of different models of different manufacturers, the classification is discussed as follows:

Comparison of ordinary aluminum sheet and coated aluminum sheet

For ordinary aluminum film without resin coating, therefore, type 2-6 is simplified as

Comparison of 2-6 and 2-7 shows that when the heat Q (i.e. using the same parameters of drill with a circuit board), coated aluminum sheet equivalent radius r, temperature T, time T than ordinary aluminum sheet to be small, so the coated aluminum sheet of the cooling performance is better than the ordinary aluminum sheet, smaller area, it can effectively reduce the temperature of the drilling tool;

Comparison of different coating types of aluminum sheet

At present, the biggest difference between different types of coated aluminum panels is the resin coating, rather than the aluminum layer. Therefore, it can be considered that the ratio of the total heat absorbed by the aluminum layer to the total heat is k, which is 2-6.

When the heat Q, equivalent radius r, time t and coating density, thickness of D, C, H phase specific heat potential, reflectivity epsilon negative correlation. Considering the actual situation of production and application of coating, D, P, e can enhance the amount is not large, it is recommended to use a specific heat capacity and phase transformation potential of the coating material in the manufacture of coated aluminum film.

Experimental test and discussion

Effect of different aluminum sheet cover plates on the quality of drilling holes

experimental condition

Two kinds of coated aluminum sheets, A, B and common aluminum C, were selected as drilling covers, drilling under certain conditions, drilling tool wear, hole wall quality and hole location accuracy.

The influence of different aluminum sheets on the winding of the drill knife

The drilled 5000 holes after the drill blade parts were observed using 3D stereo microscope, comparison of various aluminum sheet for wrapping suppression. It is shown that the inhibition ability of coated aluminum sheet on drilling tool winding is better than that of ordinary aluminum sheet, mainly due to the lubrication effect of coating layer. The better the lubricity, the stronger the inhibition ability of aluminum plate cover to drill wire wrapping.

Influence of different aluminum sheet on the roughness of hole wall

The holes after drilling with different aluminum sheets are metallized, and the 5000hits region is sliced to measure the diameter of the hole and the wick. The hole shape and hole thickness of the common aluminum sheet is larger than that of the coated aluminum sheet, which indicates that the wear of the drill knife is serious.

Influence of different aluminum sheet on the accuracy of hole position

The accuracy of every 1000 holes is tested. The accuracy of the hole position of the coated aluminum sheet is increased by more than 0.5mil. There is also a certain difference between the different coated aluminum sheets. The hole deviation of coated aluminum sheet A is lower than that of coated aluminum sheet B, and the difference increases with the increase of drilling number. When the number of drilling holes reaches 5000, the difference is 0.32mil. 

Influence of different hole spacing on the accuracy of hole position

Through the analysis of the heat dissipation model, it is known that the specific heat capacity and latent heat of the coating material are the important factors that influence the accuracy of borehole accuracy, that is, the radius of the aluminum sheet cover with different coating materials has different influence radius on drilling temperature. Therefore, the holes between different holes are used to drill and test the accuracy of the hole position. The accuracy of hole position increases with the hole spacing, but the increasing trend gradually decreased, when the hole spacing reaches a certain value, the accuracy of hole position is basically unchanged, but the three kinds of aluminum plate spacing corresponding value is sorted by size: C>A>B, temperature effect of ordinary aluminum area (radius R is reflected in the model the largest coated aluminum sheet), A, coated aluminum sheet B minimum.

In order to further verify its relationship with the specific heat capacity and latent heat of phase change, the coating material was tested by DSC. The coating A has an obvious endothermic peak at about 80 C, mainly because of the phase change heat absorption of coating, while the coating B has no obvious endothermic peak, but its heat flux is always higher than that of A. It shows that the specific heat capacity of coating B is higher than that of coated A, so its temperature action area is smaller. Generally speaking, the specific heat capacity of the resin is much larger than that of the metal, and the high purity aluminum does not produce phase transformation at 200 degrees. So the temperature range of the ordinary aluminum sheet is higher than that of the coated aluminum sheet.

Summary

The test results show that coated aluminum sheet is superior to ordinary aluminum sheet in controlling drilling tool winding, inhibiting hole thickness and hole position accuracy, and this superiority is mainly determined by the ability of coating resin to prevent slipping, drilling and drilling, such as slipping ability, specific heat capacity and latent heat of phase change.

conclusion

The coating type aluminum sheet cover plate can improve the accuracy of hole position and the quality of the hole wall, especially when processing some high thickness diameter ratio PCB, the effect of improving the accuracy of hole position is obvious. In the manufacturing of the cover plate, the coating materials with high specific heat capacity and high latent heat of phase change should be selected, and the bonding strength with the aluminum sheet and the generalized friction coefficient of the surface should be guaranteed. In the use of the cover plate, different sizes of coated aluminum sheets should be selected for different drilling requirements. However, the purchase cost is ordinary plate coating aluminum sheet several times, it is generally only used circuit board in high precision and high quality drilling requirements, of course, also can improve the service life of the drill, grinding time, enhance the number of laminated plate increases the drilling to reduce the integrated cost of coating plate.