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

Introduction

With the trend of miniaturization, high integration and high performance of modern electronic products, the hole density of PCB board is more and more big, the hole diameter is smaller and smaller, the reaction in the drilling process is the hole to line ( conductor ) distance is smaller and smaller, which requires higher and higher drilling hole precision. At present, the industry mainly from the drilling tool, plate, cover plate, drilling machine and production parameters to improve the hole location accuracy. The cover plate is one of the most important factors that affect the accuracy of hole position.

At present, there have been many kinds of coated aluminum sheet cover plate which can improve the hole location accuracy. compared with the ordinary aluminum sheet, it has better drilling, lubrication, heat dissipation, can effectively improve the hole location accuracy, reduce the broken needle, inhibit the hole thickness, etc. However, there are few reports on the mechanism of improving hole location accuracy of coated aluminum sheet. in this paper, the mechanism of improving hole location accuracy of coated aluminum sheet is analyzed from the slip and heat dissipation model of drilling tool, and the performance advantages and evaluation points of coated aluminum sheet are explained in detail. It provides a theoretical reference for the related technicians who produce and use coated aluminum sheets.

Mechanism analysis of improving hole location precision of coated aluminum sheet

Brief introduction of coated aluminum sheet

Coated aluminum sheet is the surface of the aluminum foil coated with a layer of surface coating, commonly known in the industry as coated aluminum sheet or lubricating aluminum sheet.

The coated aluminum sheet is mainly beneficial to drilling, preventing slipping, reducing wear of drill bit, lubricating and cleaning drill bit, reducing temperature of drill bit, etc. This is mainly depend on its surface coating material, generally consist of special resin, and has certain lubricity, water solubility, support and low heat fusion.

The advantages of coated aluminum sheet on PCB processing are as follows: ( 1 ) the hole location test precision of coated aluminum sheet is lower than that of ordinary aluminum sheet. ( 2 ) the roughness of the hole wall of the coated aluminum sheet cover plate is greatly reduced compared with that of the ordinary aluminum sheet when the plate processed under the same conditions is sliced in the hole wall after electroplating.

Theoretical analysis of drilling slip

In that mechanical drill of the printed circuit board, when the drill bit drill down to contact the coated aluminum sheet cover plate, the resin coating on the cov plate can stabilize the drill pin at the moment when the drill bit contacts the board surface, thereby reducing deflection. In order to better explain the mechanism and influencing factors, this paper analyzes the stress state of the balance state of the drill tip from the basic principle of deviation in drilling, especially the instantaneous slip of the drilling needle.

The basic principle diagram of drilling deviation is shown in fig. 2.2. at the moment when the drill bit goes down and contacts the board surface, the drill bit is subjected to the component force in the horizontal direction ( x / y direction ) due to the fact that the vertical center line of the main shaft is not completely vertical to the horizontal center line of the production board, and the dynamic deviation of the main shaft is applied, thus forming the instantaneous slip of the drill bit. From the analysis of the force on the drill tip, it is found that the drilling slip σ 0 is related to the ability parameter of the cover plate to prevent the drilling tool from sliding at the moment of drilling, and this ability parameter is defined as the generalized friction coefficient u of the cover plate. The larger the u, the greater the ability of the cover plate to prevent the drill bit from slipping into the drilling moment, the more stable the drill bit, and the lower the slip amount σ 0 at the drilling moment, so as to improve the drilling vacancy accuracy.

Wherein, h represents the production plate thickness, x0 represents the theoretical position of the hole in the file, x ˊ represents the position of the actual positioning of the spindle, x represents the actual position of the drill tip at the moment of drilling down, σ represents the reverse drilling hole deviation, Δ σ positive and negative hole difference, θ represents the included angle between the central axis in the process of drilling down and the normal of the plate plane, l represents the length of the drilling blade, σ 0 + run - out represents the comprehensive deviation value between the bending of the drilling blade and the dynamic deviation of the spindle. θ 0 represents the angle between the center plane of the production plate and the x / y plane, θ 1 represents the offset angle caused by the dynamic run - out of the spindle, θ 1 ˊ represents the offset angle caused by the drill tip slip + dynamic run - out, and σ 0 represents the amount of slip of the drill tip at the moment the drill bit is drilled into the cover plate.

Theoretical model analysis of heat dissipation

In PCB drilling, the high-speed operation of the drilling tool cutting PCB generates a lot of heat, if not discharged in time will make the drilling tool has been in a high temperature state, resulting in a decrease in the rigidity of the drilling tool, thus reducing the drilling hole location accuracy and hole wall quality. One of the most important factors of using high pure aluminum sheet as drilling cover plate in the industry is to take away the surface heat of the circuit board through the high thermal conductivity of metal aluminum and reduce the temperature. The heat transfer is a complex process. in order to better explain the advantages of coated aluminum sheet in high precision drilling and its main influencing factors, the heat transfer process is simplified to contact heat transfer model and cover plate heat transfer model.

Contact heat dissipation model

When the aluminum sheet cover plate is in contact with the PCB surface, the contact heat dissipation between the aluminum sheet cover plate and the PCB surface is mainly conducted by the heat conduction mode of the microprotrusion body, as shown in fig. 2.1. Qc, qg and QR are used to represent the contact heat dissipation, gap gas conduction heat dissipation and heat radiation heat dissipation of the microprotrusion between the contact surfaces respectively, and the total heat dissipation amount QJ through the contact surfaces is the sum of the three:

Qj = QC + qg + QR ( 2 - 1 )

Since the contact surfaces of the two solid surfaces where the contact occurs are not completely smooth, the contact actually occurs only at certain points, and most of the area between the interfaces is filled with interstitial gas. 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 for transmission. Therefore, improving the contact area of the microprotrusion body is an effective way to improve the contact heat dissipation. in production, the flatness of the lower surface of the aluminum sheet cover plate and the tightness of the contact between the aluminum sheet and the upper surface of the PCB are mainly controlled.

The tightness of the contact between the two solids is closely related to the assembly pressure between the planes. When the pressure is small, only a few microprotrusions between the contact surfaces are deformed, the actual contact area is small, and with the increase of the pressure, the microprotrusions which are in contact with each other before are extruded to generate deformation, so that the contact area is increased, the two contact surfaces are made closer, the air layer is thinner, more new microprotrusions are contacted, and the actual contact area is increased. Previous studies have shown that even when the contact pressure between two solid surfaces reaches 10 MPa, the actual contact area accounts for only 1 % ~ 2 % of the nominal area.

Contact heat conduction between two solid contact surfaces is a complex problem influenced by many factors, such as thermal properties, mechanical properties, rough surface morphology, surface roughness and load, temperature, medium and environment. The effect of the surface roughness of the cover plate and the assembly pressure in the drilling hole on the cooling of the drilling hole is explained by a simple contact cooling model only from the application point of view.

Cover plate heat dissipation model

The heat emitted by the aluminum sheet cover plate mainly consists of three parts: first, the cover plate heats up and absorbs heat; Secondly, the cover plate is dispersed into the surrounding environment through heat radiation; Third, the heat absorbed by the phase change of the coating layer, the heat balance equation is as follows:

Wherein, q is the heat emitted by the cover plate, i.e. the contact heat dissipation quantity QC of the microprotrusion body of the PCB to the cover plate in the contact heat dissipation model, which can be regarded as a constant when the drilling parameters and the plate are determined.

Q1 is the heat absorbed by the heating of the cover plate:

Q2 is the amount of heat radiation, including aluminum sheet layer, coating layer respectively to the surrounding environment heat radiation amount, then:

Q3 is the heat absorbed by the phase change of the coating phase change material:

Replace formula 2 - 3, 2 - 4, 2 - 5 and formula of mass and area with formula of 2 - 2 to obtain:

Wherein m1 and m2 are that equivalent MAS of the aluminum sheet layer and the coat layer respectively, t0, t1 and T2 are the temperatures of the air, the aluminum sheet layer and the coat layer respectively, and C1 and C2 are the specific heat capacities of the aluminum sheet layer and the coating layer respectively. ε 1 and ε 2 are respectively that generalize emissivity of the radiating surface of the aluminum sheet lay and the coating layer, a is the equivalent radiating area, σ is the Stefan - Porsman constant, h is the phase-change potential of the phase-change material, ρ 1 and ρ 2 are the density of the aluminum sheet lay and the coating layer respectively, r is the equivalent radius, D1 and D2 are the thicknesses of the aluminum sheet layer and the coating layer respectively, and t1 and T2 are the acting time of heat transfer in the aluminum sheet layer and the coating layer respectively.

According to the different types of coated aluminum sheet and ordinary aluminum sheet and different manufacturers of different types of coated aluminum sheet, its classification is discussed as follows:

Comparison between ordinary aluminum sheet and coat aluminum sheet

There is no resin coat layer for ordinary aluminum sheet, so that formula 2 - 6 is simplify to

Compared with formulas 2 - 6 and 2 - 7, when the heat q is certain ( i.e., the same circuit board is drilled with the same parameters ), the equivalent radius r, the temperature t and the action time t of the coated aluminum sheet are smaller than those of the ordinary aluminum sheet, so that the heat dissipation efficiency of the coated aluminum sheet is better than that of the ordinary aluminum sheet, the action area is smaller, and the temperature of the drilling tool can be effectively reduced;

Comparison of different coated aluminum sheets

At present, different manufacturers of different types of coated aluminum sheet cover the biggest difference lies in the resin coating layer, rather than the aluminum sheet layer, so it can be considered that the aluminum sheet layer absorption, radiation of heat accounted for the proportion of the total heat k is certain, 2 - 6 can be simplified as follows

It can be seen from formula 2 - 8 that when the heat q is constant, the equivalent radius r and the action time t are negatively correlated with the coating density ρ, the thickness d, the specific heat capacity c, the phase change potential h, and the reflectivity ε, respectively. In view of the actual situation and production application, the coating ρ, d, ε can not increase the amount, so in the production of coated aluminum sheet recommended the use of coating material with large specific heat capacity and phase change potential.

Experimental test and discussion

Effect of different aluminum sheets on drilling quality

Experimental condition

Two different manufacturers of coated aluminum sheets a, b and ordinary aluminum sheet c are selected as the cover plate for drilling, drilling is carried out under certain conditions ( such as table 3.1 ), and drilling tool wear, hole wall quality, hole location accuracy, etc. are tested.

Effect of different aluminum sheets on the winding of drill bit

Aft 5000 holes are drill, that cutter body parts are observed by a 3d stereo microscope, and the inhibition performance of various aluminum sheet on filament winding is compared, and the results are shown in fig. 3.1. The figure shows that the coating type aluminum sheet has better inhibition ability to the winding wire of the drilling tool than the ordinary aluminum sheet, mainly related to the lubrication effect of the coating layer, and the better the lubricity, the stronger the inhibition ability of the aluminum sheet cover plate to the winding wire of the drilling tool.

Effect of different aluminum sheets on pore wall roughness

The drilled holes with different aluminum sheets were metallized and sliced in the 5000 hits area to measure the hole thickness and wick. the results are shown in table 3.2 and fig. 3.2. The hole shape and the hole diameter of the ordinary aluminum sheet shown in the figure are larger than those of the coated aluminum sheet, indicating that the drill tool is seriously worn.

Effect of different aluminum sheets on hole location accuracy

The hole location accuracy test was carried out for every 1000 holes. as shown in fig. 3.1, the hole location accuracy of the coated aluminum sheet was increased by more than 0.5 mil compared with that of the ordinary aluminum sheet. However, there are some differences between different coated aluminum sheets. as shown in the figure, the hole deviation of coated aluminum sheet a is lower than that of coated aluminum sheet b, and the difference value increases with the increase of the number of holes. when the number of holes reaches 5000, the difference value is 0.32 mil.

Influence of different hole spacing on hole location accuracy

The specific heat capacity and latent heat of phase change of the coating material are the important factors affecting the drilling hole location precision, that is, the radius of the influence area of different coating materials on the drilling temperature is different. Therefore, different hole spacing is used for drilling and testing the hole location accuracy, and the results are shown in figure 3.2. The hole location precision increases with the increase of the hole spacing, but the increasing trend gradually decreases. when the hole spacing reaches a certain value, the hole location precision basically remains unchanged, and the corresponding spacing values of the three aluminum sheet cover plates are ranked as follows: c > a > b according to the size, indicating that the temperature affected area of the ordinary aluminum sheet ( radius r reflected in the model ) is the largest, the coated aluminum sheet a is the second, and the coated aluminum sheet b is the smallest.

To further verify its relationship with specific heat capacity and latent heat of phase change, the coating material was tested by DSC, and the results are shown in fig. 3.5. As shown in the figure, coating a has an obvious endothermic peak at about 80 ℃, mainly because the coating phase change endothermic, while coating b has no obvious endothermic peak, but its heat flow rate has been higher than a, indicating that the specific heat capacity of coating b is higher than coating a, so its temperature action area is small. In general, the specific heat capacity of the resin is much larger than that of the metal, and the high pure aluminum does not undergo a phase change at 200.d egree. C., so that the temperature affected area of the ordinary aluminum sheet is higher than that of the coated aluminum sheet.

Summary

The test results show that the coated aluminum sheet is superior to the ordinary aluminum sheet in inhibiting the winding of the drilling tool, inhibiting the hole diameter and controlling the hole position precision, and the superior degree mainly depends on the parameters of the coating resin, such as the ability of preventing the drilling tool from slipping at the moment of drilling, the specific heat capacity and the latent heat of phase change, etc.

Conclusion

The coated aluminum sheet cover plate can improve the hole location precision and the hole wall quality, especially when processing some circuit boards with high thickness-diameter ratio, the hole location precision is improved obviously. In the manufacture of the cover plate, the coating material with high specific heat capacity and latent heat of phase change should be selected, and the adhesion between the coating material and aluminum sheet and the surface generalized friction coefficient should be ensured. In the use of the cover plate, different specifications of coated aluminum sheets shall be selected for different drilling requirements. However, at present, the purchase cost of the coating cover plate is several times of that of the ordinary aluminum sheet, so the coating cover plate is generally only applied to circuit boards with high precision and high quality drilling requirements, and the comprehensive use cost of using the coating cover plate can be reduced by increasing the service life of a drill bit, increasing the grinding times, increasing the number of drilling stacks, and the like.