Drilling characteristics of entry board and the influence on ...hkpca.org/uploadfileMgnt/02_201871119205.pdfThe process of PCB micro-hole drilling can be divided into four stages:

20 Technical Paper

Journal of the HKPCA / 2018 / Spring / Issue No. 67

Drilling characteristics of entry board andthe influence on PCB micro drilling process

Xin Huang , Lijuan Zheng , Chengyong Wang , Linfang Wang , Dantian Lin , Bingmiao Liao , Lunqiang ZhangGuangdong University of Technology

Patent Examination Cooperation Center of The Patent OfficeShenzhen Newccess Industrial Co., Ltd.

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Abstract

1 INTRODUCTION

In this paper, the drilling characteristics of aluminum (Al) entry

boards, resin coated Al entry boards, and phenolic resin entry

boards were investigated. The chip removal, chip morphology,

thrust force, and drilling temperature were measured and

analyzed. The cutting characteristics among those materials

were compared. In addition, the influence of alloy materials on

thrust force of entry boards was analyzed. The results

demonstrated four efficacy of entry boards, including chip

removal, heat dissipation, buffering, and location. The function

of location and buffering were mainly reflected in the beginning

of drilling. Effect of chip removal and heat dissipation were

related to material of entry board. The resin coated Al entry

board performed better than aluminum entry board in buffer

and chip removal. Moreover the influence of coating resin and

its thickness on drilling process of entry board was obvious.

Chips of aluminum entry board were easy to wrapping the drill

bit than that of resin coated Al entry board. Thermal

deformation of coating resin can increase the curl of aluminum

chip, thus contributing to the chip removal. The effect of alloy

material on thrust force was significant. The increase of coating

resin thickness can enhance the buffer effect on thrust force in

some degree. The heat dissipation of MVC resin coated Al entry

board in PCB drilling was the best.

Printed Circuit Board (PCB) has been widely utilized in the high-

frequency, high-speed, and high-density integrated circuit. Thus,

the material of PCB became increasingly complicated. However,

obtaining high quality ultra-micro-hole is still an urgent problem

to be solved. Entry boards, which can improve the quality of

PCB drilling and the life of the drill bit, is one of the auxiliary

materials in PCB drilling. However, the drilling chip and thrust

force can lead to the vibration and even broken of drill bit. In

addition, the drilling temperature may affect the drilling process

and give rise to entry burrs of PCB.

Currently phenolic resin entry board, aluminum entry board, and

resin coated Al entry board were common entry boards.

Phenolic resin entry board is made of wood fiber or paper

substrate with phenolic (or modified phenolic) resin. The surface

has excellent flatness, but low thermal conductivity. For Al entry

board, the main ingredient is aluminum, contains small amounts

of Mg, Si, Mn or other elements. The thickness is about

0.1mm-0.3mm after oxidized, polishing, and other processing.

Resin coated Al entry board is coated with the foil surface of

one or several layers of resin, then the superstructure is softer

than substructure, such as micro via coating (MVC) aluminum

entry board and lubricating aluminum (LE) entry board.

Drilling was considered as a three-dimensional cutting process

which involved complex cutting mechanism due to complex

cutting edge geometry of drill bit [1]. The use of entry board in

PCB drilling was considerable and widespread. It was studied

that entry board mainly play five roles: (1) protecting the surface

of the PCB from the press foot; (2) guiding the drill bit and

improving the precision of hole position; (3) cushioning effect

and reducing drill break; (4) improving heat dissipation; (5)

reducing the entrance burrs of PCB [2]. However, the drilling

characteristics and mechanism of these entry board were rarely

known. Studies have shown that size and shape of the

generated chips during drilling operation had a great influence

on the surface roughness, drilling torque, and thrust force [3],[4].

Long chips were easy to wrap the drill bit, while small-well

broken chips can be readily ejected from the drilled hole. Batzer

et al. [5] had reported that seven types of chips were observed

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during dry drilling of cast aluminum alloys, including conical,

fan-shaped, chisel-edge, amorphous, needle, impacted, and

dust-like chips. Long continuous chips were produced during

machining of aluminum alloys. Zheng et al. [6] used micro drills

with a diameter of 0.1mm to drill the PCB (FR4). She found that

drilling of aluminum entry board demonstrated the characteristic

of normal metal cutting, and the aluminum chip was prone to

entangle around the drill bit, which may damage the drill bit and

hole wall. Other studies considered that accumulated heat can

diffused rapidly through aluminum chip due to its excellent

thermal conductivity [7]. Thrust force was one of the significant

parameters in drilling process. A lot of work has been carried

out on the thrust force of PCB. Qu et al. [8] used 0.15-0.3 mm

carbide drill bits drilling three different kinds of double-sided

copper clad laminate with spindle speed of 80 -200 krpm. He

noted that the thrust force had close relationship with the

material and structure of PCB. Wang [9] investigated the thrust

force when drilling PCB with 0.3mm diameter tungsten carbide

drill bit. The result indicated thrust force increased sharply when

the drill came into contact with the PCB. The thrust force of

copper foil was the largest in PCB drilling. However, little

research on thrust force of entry board and its influence on PCB

drilling process has been done.

Based on previous studies about drilling process of aluminum

alloys and PCB. This paper analyzed the drilling characteristics

of entry board, and the influence mechanism of entry board on

drilling process of PCB. It is significant to the selection of entry

board and the development of PCB processing.

The PCB drilling processes were performed on a high speed

drilling machine with a spindle speed of up to 250 krpm, which

was designed by Guangdong University of Technology and

manufactured by Dong Sheng ltd. [10],[11] (shown in Fig. 1).

The work piece was typical lead-free and six layers compatible

FR-4 (S1170) made by Guangdong Shengyi Technology Co.,

Ltd.. The FR-4 board was a copper-clad laminate sheet, in

2 EXPERIMENTAL METHODS

which the epoxy resin was reinforced by E-glass fiber and one

or two faces were covered with a copper foil. The thickness of

the PCB is 0.8mm, the structure and physical performance of

FR-4 ere shown in Fig. 2 and Table I respectively. All of the No.

A129UCSN cemented carbide micro-drills (6-9 % Co and

91-94 % WC. Vickers hardness 1,750-2150 HV30) were made

by Shenzhen Jinzhou Precision Technology Corporation and

the diameters were 0.25 mm. The basic parameters of the

micro-drills were shown in Table II. The drilling conditions were

consistent as shown in Table III.

Four types of entry boards were used: Al-base entry board AL-

Cu-140, MVC-coated Al-base entry board MVC/Al-Cu-140-60,

LE-coated Al-base entry board LE/Al-Mn-80-30, and phenolic

resin entry board LC-110. The melamine backing board with

thickness of 2.5mm was used. All of the entry boards and

backing boards were made by Shenzhen Newccess Industrial

Co. Ltd.. The performances of entry boards were shown in

Table IV.

Fig. 1 PCB drilling system

Fig. 2 The structure of FR-4

22 Technical Paper


The measurement of thrust force and drilling temperature were

consistent with previous experiment research [6],[10]-[13]. High

Table I Physical performances of S1170 FR-4

Table II Basic parameters of micro drill

Table III Basic parameters of micro drill

speed camera was used to photograph the drilling processes

of four kinds of entry boards. Acquisition of the thrust force was

made using a KISTLER 9257B three-phase quartz force

measuring instrument. The drilling temperature was monitored

by a TVS-500SX infrared thermal imager. Chips from the entry

board micro-drilling were collected and analyzed by HITACHI S-

4300 SEM, and the composition of chip were analyzed by

Energy Disperse Spectroscopy (ESD). The experimental data in

this paper were all repeated for 3 times.

The process of PCB micro-hole drilling can be divided into four

stages: drilling of the entry board, then the copper foil and

resin/glass fiber alternately, and finally the backing board [6].

Effect of four entry boards on chip removal, thrust force, and

drilling temperature of PCB were studied in this work.

Chip formation during the drilling was caused by multiple

cutting edges of varied rake angles and cutting speeds. As

mentioned earlier, drilling the aluminum entry board

demonstrated the characteristics of normal metal cutting and

formed continuous chips. The first generated aluminum chip

was pushed upward by the subsequent chip and curled. As the

3.1 Chip removal

3 RESULTS AND DISCUSSIONS

Table IV Performances of entry boards

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drilling continued, the aluminum chip was continuously pushed

upward to the end of helical flute by the cutting action of the

drill [6].

Fig. 4 shows the chip removal of Al-Cu-140, MVC/Al-Cu-140-

60, LE/Al-Mn-80-30, and LC-110. The aluminum chip

occasionally entangled the drill along the helical flute of the drill

(Fig. 4 mark A). This phenomenon is caused by the plastic

Fig. 4 Chip removal of different entry boards

(D=0.25 mm, =3 m/min, n=185 krpm, =15 m/min)� �� r

(a) Al-Cu-140

(b) MVC/Al-Cu-140-60

(c) LE/Al-Mn-80-30

(d) LC-110

deformation of aluminum which has been mentioned by Zheng

[6]. However, chip removal of four kinds of entry boards was

quite different. Chip of MVC/Al-Cu-140-60 and LE/Al-Mn-80-30

were thrown away along the direction of 30 and 45

respectively, which may facilitate the chip evacuation (Fig. 4b, c).

Compared to the Chip removal of two resin coated Al entry

board. Significant chip wrapping occurred in the later drilling of

MVC/Al-Cu-140-60 (Fig. 4 mark B), but not occurred in the

LE/Al-Mn-80-30 drilling. Chip wrapping can be found obviously

in the early drilling of LC-110 (Fig. 4 mark C), and then blocked

in the flute (Fig. 4 mark D). It was believed that the chip removal

of entry board is greatly related to the material. Chip removal of

resin coated Al entry boards were better than that of others,

and it can promote the chip evacuation of PCB.

Chips can rotate with the drill and fracture once the chip

exceed a critical strength. Thus, the study of chip morphology

was needed to identify the conditions which promote better

chip evacuation. Moreover, the understanding of chip curling

and breaking will help develop better tools for composite

materials [14].

Bartezer [5] et al. studied all seven types of Al-alloy chips, and

pointed out that small well-broken chips were desirable. Larger

chips can not move easily through the flutes, which increased

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3.2 Chip morphology

Fig. 5 Schematic of chip formation in drilling [1]

24 Technical Paper


torque requirements and perhaps causing drill breakage. Chip

of LE/Al-Mn-80-30 (Fig. 6c) is a typical conical chip. Chips of

Al-Cu-140 (Fig. 6a, e, f) and MVC/Al-Cu-140-60 (Fig. 6b, g, h)

were all long ribbon chips and were obviously longer than chips

of LE/Al-Mn-80-30. LC-110 is the typical horizontal coiled

helical chip (Fig. 6d). Conical chips were predominant at all

drilling tests [1]. In terms of chip morphology, the most

conductive was LE/Al-Mn-80-3, and other three kinds of chip

were likely to impede the chip discharge due to the long strip.

Moreover, compared the thickness and coated resin between

three Al-based entry board, thickness of Al-Cu-140 and

MVC/Al-Cu-140-60 is 140 m and thicker than that ofµ

LE/Al-Mn-80-30. Resin thickness of MVC/Al-Cu-140-60 is

60 m which is thicker than 30 m of LE/Al-Mn-80-30. Fig. 6a-c

shows slightly spiral and some adhesion in the end of

MVC/Al-Cu-140-60 chip, but not obvious in Al-Cu-140 long

stripe chip. The adhesion of MVC/Al-Cu-140-60 chip were

mainly made of carbon, oxygen, and a small amount of

aluminum (Fig. 7c, d), which indicated the composition was

resin. The results showed that chip morphology of aluminum

entry board varies with the thickness of aluminum and coating

resins. Thicker aluminum resulted in the longer chip. Thin

coated resin was easily thermal to distorted and curled in the

early drilling and may benefit the breakage of aluminum chip.

Cracks were found in the enlarged view of top surface of Al-Cu-

140 chip (Fig. 6 mark A), but still far away from the fracture

strength of the material. These micro cracks spread out and

reach to the cracks of free surface, and propagating toward the

tool tip [1]. And he put forwards a 3D view of drilling chip

formation, in which free and back surfaces were formed along

the cutting edge, while top surface was generated by the

µ µ

Fig. 6 Chip morphology (a)(e)(f) Al-Cu-14; (b)(g)(h) MVC/Al-Cu-140-60;

(c) LE/Al-Mn-80-30; (d) LC-110


Fig. 7 Energy spectrum of chips

(a)(b) Al-Cu-140 (c)(d) MVC / Al-Cu-140-60


25Technical Paper

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out-most point of the cutting edge [1] (Fig. 5). Fig. 7 (mark B)

shows the back surface of Al-Cu-140 chip. The back surface

was smooth while free surface was rough. Comparing the

energy spectrum from free surface with back face of Al-Cu-140

chip, the atomic percentage of oxygen decreased noticeably

(Fig. 7a, b), which indicated a Oxidation reaction. Trent and

Wright [15] believed this phenomenon was caused by the

uneven flow of chip (uneven chip flow pattern) on the blunt or

rounded edge. Burrs were found in the chip of MVC/Al-Cu-140-

60 (Fig. 7 mark C). This may due to the top surface was

subjected to shear forces in the chips formation process. And

the film teared and the burrs formed as the chip separated from

the workpiece.

Thrust force of PCB with four different kinds of entry board was

compared, and the effects of different materials on the thrust

force were studied in this work. Typical thrust force of glass

fiber reinforced plastic composites increased linearly [16]. While

slope of thrust force decreased after use entry board. Fig. 8

shows the thrust force increased with the drilling continued.

Thrust force of different entry boards varied. Thrust force of

LC-110 increased rapidly at the begin of drilling, but the thrust

force was the smallest. There was 2 ms buffer region apparent

in MVC/Al-Cu-140-60 and the maximum thrust force appeared

when 4.9 ms. Growth slope of Al-Cu-140 was similar with

MVC/Al-Cu-140-60. And they were both less than that of

LC-110. Besides, there was a reaction force in Al-Cu-140

drilling (Fig. 8 mark A). The maximum thrust force appeared

when 3.85 ms. Different from three entry boards above, there

was neither sharp rise nor buffer area in the growth of

LE/Al-Mn-80-30 thrust force. The maximum thrust force

appeared when 1.85 ms and it was the largest thrust force

among the four kinds of entry boards.

It was found that the peak thrust force of the entrance was the

largest in PCB drilling [17]. Thus, the entry board displayed

buffer action as delaying the increase of thrust force. The best

buffer effect occurred in MVC/Al-Cu-140-60 drilling. However,

3.3 Thrust force

the buffer effect was not obvious in LE/Al-Mn-80-30 drilling as

they were both resin coated Al entry board. It is attributed to

the thickness of coated resin. The resin thickness of

MVC/Al-Cu-140-60 was thicker than that of LE/Al-Mn-80-30.

Thrust force of several aluminum boards with different alloy

elements materials were compared in Fig. 9. Maximum thrust

force occurred in the drilling of LE/Al-Mn-80-30. This may

related to the based material of aluminum. Al-Cu-140 and

MVC/Al-Cu-140-60 are made up of pure aluminum with high

aluminum content of more than 99.5%, while the aluminum

content of LE/Al-Mn-80-30 is about 97% and the tensile

strength is large (as shown in Table IV). Thus the thrust force of

LE/Al-Mn-80-30 was larger than other entry boards. In other

words, the alloy element of the aluminum entry board may

affect the thrust force.

Fig. 8 Drilling thrust force of common used entry boards


Fig. 9 Influence of alloy elements on entry boards thrust force


26 Technical Paper


3.4 Drilling Temperature

Drilling temperature was mainly from the friction heat between

the tool and workpiece. The friction heat diffused mainly

through chips, tools, workpiece, and other cooling device.

Therefore, the drilling temperature was closely related to the

material properties, tool wear, ambient temperature, and drilling

conditions. Minimizing the generation of heat and increasing the

heat diffusion path were the two most effective way to control

the drilling temperature.

Fig. 10a shows the drilling temperature of entry board. The

temperature was very low just about 30 C to 40 C. And the

temperature of four commonly used entry board was a little

different. The drilling temperature of MVC / Al-Cu-140-60 was

O O

slightly higher than that of Al-Cu-140. However, the

temperature obviously decreased in PCB drilling with entry

board used, and three kinds of Al entry board performance

more prominent (Fig. 10b). It was attributed to the excellent

thermal conductivity property of Al. The accumulated heat in

continued drilling of the PCB can be diffused rapidly, this was

also confirmed by Wang in the research of drilling temperature

of CFRP/Aluminum stacks [18]. However, heat dissipation effect

of three kinds of Al entry board varied. LE / Al-Mn-80-30

performed more excellent, MVC / Al-Cu-140-80 was worse

than Al-Cu-140. The maximum temperature of MVC / Al-Cu-

140-80 reached 170.7 C. This is depended on the thickness

and species of coated resin. The resin of LE / Al-Mn-80-30 was

soluble. The resin melt as the drilling temperature reached a

certain value. Moreover, as the above studies of chip

morphology of MVC / Al-Cu-140-80, there was resin adhesion

on the chip, which is easy to impede the chip removal and

resulted in higher drilling temperature.

1) The chip removal was greatly related to the material

properties and the coated resin of entry board. Thicker

aluminum generated longer chips which were easy to wrap the

drill bit. Coated resin was beneficial to chip discharge. The

thickness of coated resin influence the chip morphology. Thin

coated resin was easily thermal to distort and curl in the drilling

process, while thicker coated resin was easily to condense into

lumps. In terms of improving the chip removal, LE/Al-Mn-80-30

was the best choice in four kinds of commonly used entry

board.

2) Entry board displayed buffer action. It can delay the increase

of thrust force. Alloy elements had a greater impact on the

thrust force of aluminum entry board. Higher hardness and

content of the alloy may give rise to greater thrust force. Al-Cu-

80 showed the minimum thrust force. In concern of decreasing

the thrust force and preventing drill breakage, MVC aluminum

entry board was more suitable.

O

4 CONCLUSIONS

(a) Drilling temperature of four kinds of entry boards

(b) Drilling temperature of PCB with different entry boards

Fig. 10 Drilling temperature of entry boards and PCB


27Technical Paper

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3) Entry board showed the function of heat diffusion. Although

drilling temperature of different kind entry board has little

difference. However, heat diffusion of entry boards performed

well in continued drilling of PCB, especially the LE entry board.

Thinker coated resin showed better heat diffusion effect. In

terms of promote the heat dissipation of PCB, LE entry board

with thinker resin is available.

This research is funded by the National Science Foundation for

Young Scientists of China (Grant No. 51405090), Major

Scientific Research Projects of Guangdong Province (Grant

No.2014KZDXM030), and Innovation and Better University

Project of Education Department of Guangdong Province

(Grant No.15ZK0208).

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Documents

Drilling characteristics of entry board and the influence on ...hkpca.org/uploadfileMgnt/02_201871119205.pdfThe process of PCB micro-hole drilling can be divided into four stages: