Study of optimal laser parameters for cutting QFN packages by Taguchi's matrix method

ARTICLE IN PRESS

Optics & Laser Technology 39 (2007) 786–795 www.elsevier.com/locate/optlastec

Study of optimal laser parameters for cutting QFN packages by Taguchi’s matrix method Chen-Hao Lia, Ming-Jong Tsaia,, Ciann-Dong Yangb a

Graduate Institute of Engineering, National Taiwan University of Science and Technology, Taiwan b Department of Aeronautics and Astronautics, National Cheng Kung University, Taiwan Received 22 July 2005; received in revised form 26 January 2006; accepted 12 February 2006 Available online 19 April 2006

Abstract This paper reports the study of optimal laser parameters for cutting QFN (Quad Flat No-lead) packages by using a diode pumped solid-state laser system (DPSSL). The QFN cutting path includes two different materials, which are the encapsulated epoxy and a copper lead frame substrate. The Taguchi’s experimental method with orthogonal array of L9(34) is employed to obtain optimal combinatorial parameters. A quantified mechanism was proposed for examining the laser cutting quality of a QFN package. The influences of the various factors such as laser current, laser frequency, and cutting speed on the laser cutting quality is also examined. From the experimental results, the factors on the cutting quality in the order of decreasing significance are found to be (a) laser frequency, (b) cutting speed, and (c) laser driving current. The optimal parameters were obtained at the laser frequency of 2 kHz, the cutting speed of 2 mm/s, and the driving current of 29 A. Besides identifying this sequence of dominance, matrix experiment also determines the best level for each control factor. The verification experiment confirms that the application of laser cutting technology to QFN is very successfully by using the optimal laser parameters predicted from matrix experiments. r 2006 Elsevier Ltd. All rights reserved. Keywords: Laser cutting; QFN package; Taguchi

1. Introduction The Quad Flat Non-lead (QFN) package is one of the main semiconductor packaging technologies. It consists of a plastic encapsulated package with a copper lead frame substrate. This packaging technology, which has many advantages, such as less size and weight, good electrical performance and high speed and frequency, has been applied widely in many products [1]. In cutting QFN ICs, the conventional technology adopts a diamond saw that can cut with high speed along a cutting line under the aid of cooling liquid. After the cutting process, a single QFN IC is then obtained. However, the technology makes the QFN ICs breakable and easy to crack. Nowadays lasers are increasingly used in manufacturing and automotive industries to obtain high-speed producCorresponding author. AC101, No. 43, Keelung Road, Section 4, Taipei 10672, Taiwan. Tel.: +886 2 27376286; fax: +886 2 27376799. E-mail address: [email protected] (M.-J. Tsai).

0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.02.005

tion, such as laser cutting, welding, marking, etc. There are many control parameters affecting the laser cutting quality, including the effect of laser cutting speed on surface temperature [2], on surface quality [3], and on surface roughness [4], and the effect of laser frequencies on heat affected zone (HAZ) [5]. Several studies [6–9] on how laser control parameters affect the cutting quality were reported. However, its application to QFN packages has not been considered in the literature before. In this paper, the feasibility and advantages of using laser technology in cutting a QFN package will be demonstrated. A diode pumped solid-state laser system (DPSSL) and Taguchi’s experimental method were used to achieve optimal laser cutting quality. The Taguchi’s matrix method is an experimental solution for the robust-performance problem, which offers an efficient matrix-iteration scheme to obtain optimal and robust-quality parameter setting. Many successful applications of Taguchi Methods have been reported to improve several processes and product reliability and quality

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

787

Fig. 1. The cutting path and dimensions of a 5  5 QFN package.

[10–12]. In this paper, a new application of Taguchi’s matrix method is employed to the determination of laser parameters for cutting a QFN package.

Width of cutting line (WLine) (e ) Width of HAZ (WHAZ ) (e )

2. QFN packages and laser cutting system Fig. 1 depicts the internal structure, the cutting path, and the microscopic dimensions of QFN packages. A QFN package is a plastic encapsulated lead-frame-based Chip Scale Package with the lead pad on the bottom of the package to provide electrical interconnection with the printed circuit board [1]. Several QFN chips are packaged in an array form so that they need to be separated into an individual QFN IC that can be mounted on different printed-circuit boards for different applications. Because the whole size of a QFN package is small, the width of the tolerable cutting path between packaged ICs is also very restricted. For a 5  5 QFN package the maximum tolerant dimension of the cutting path is 0.26 mm and the thickness is 0.9 mm shown in Fig. 1. The patch is made up of two materials, one is the epoxy with a thickness of 0.9 mm and the other is the copper circuit-pad bonding epoxy for which the dimensions of copper and epoxy are 0.24 and 0.66 mm, respectively. Fig. 2 shows the six main parameters governing the cutting quality, including the width (W ðeÞ HAZ ) of HAZ, the width ðeÞ (W ðeÞ ), and the depth (D ) of the cutting line for epoxy, Line Line ðcÞ ðcÞ and the three counterparts W ðcÞ , HAZ W Line , and DLine for copper circuit-pad bonding epoxy. ðcÞ The equipment for measuring the widths (W ðeÞ HAZ , W HAZ ) ðeÞ ðcÞ of HAZ and the widths (W Line , W Line ) of cutting line is an electron microscope with measurement accuracy of 1 mm. ðcÞ The equipment for measuring the depths (DðeÞ Line , DLine ) of the cutting line is an optical electron microscope with measurement accuracy of 1 mm. Due to the irregularity of the widths of HAZ and cutting line, the values of widths along the cutting trajectory are indeed not constant. In order to obtain an accurate average measurement, a mean

(e ) ) Depth of cutting line ( DLine

CHIP

Heat affect (c ) Depth of cutting line ( DLine )

(c ) Width of HAZ (WHAZ ) (c ) Width of cutting line (WLine )

Fig. 2. Illustration of laser cutting quality for a QFN Package.

Fig. 3. The layout of a laser cutting system for QFN Packages.

value of ten-point measurements is used. The depth measurement for the cutting line is taken at four fixed positions and a mean depth can be obtained. The experimental system shown in Fig. 3 consists of a DPSSL (model: Rofin 100D, wavelength ¼ 1.064 mm), a

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

788

laser cutting head, an X  Y moving table driven by two servo motors. The laser frequency, the driving current, and the cutting speed are three control factors, which can be adjusted through the control panel of the laser system.

Unit llen ength gth

Unit llen ength gth

Power

Power

Single pulse

3. Laser cutting qualities of QFN packages Before we perform the matrix experiments to determine the optimal laser parameters, the feasibility of applying laser system to cut a QFN package is investigated first in this study. The cutting qualities will also be examined. The influences of laser power and cutting speed on the cutting qualities, including the widths and depths of cutting line, and the widths of HAZ, will be evaluated by the aforementioned DPSSL laser and be compared with the theoretical predictions.

(a-1) higher current

Unit length

Unit Un it length length Power

Power

Single pulse High overlap

Low overlap

(b-1) higher frequency

3.1. Available power output from DPSSL system The laser average output power P (Watt) is a major factor affecting the cutting qualities. The available output power of a DPSSL system is a function of the input current A and the laser pulse frequency F. The on-line measurement data of power P for various combinations of A and F are shown in Table 1. A related measurement of power is the single pulse power K (Joule), which is the power contained in a single pulse [13]: P . (1) F As shown in Table 1, either high current or high frequency leads to high average output power and single pulse power [14]. The other two factors of power dominating cutting quality are the number of laser pulses per unit length S and the power per unit length Pp (Joule/mm), defined, respectively, as

(a-2) lower current

(b-2) lower frequency

Unit it length

Unit it length

Power

Power

Single pulse Low overlap

High overlap

(c-1) high cutting speed

(c-2) low cutting speed

K¼

S¼

F ; V

Pp ¼ KS ¼

P , V

Fig. 4. Peak power for single laser pulse and power overlapping for various control factors.

(2)

where V is the cutting speed and S is also known as pulse overlapping, which is an indication of the smoothness and the continuity of laser cutting. Either low cutting speed or Table 1 Output power and single pulse power at various currents and frequencies Current (A)

Frequency (kHz)

Laser average output power (Watt)

Single pulse power (103 Joule)

29 29 29 33 33 33 37 37 37

0.5 2 3.5 0.5 2 3.5 0.5 2 3.5

1.9 8.9 16.7 5.6 13.9 23.3 6.5 18.3 34.6

3.8 4.45 4.77 11.2 6.95 6.66 13 9.15 9.89

Fig. 5. Photographs of power of unit length at different combinations of control factors.

high pulse frequency provides high pulse overlapping, as shown schematically in Fig. 4. The variation of the power per unit length under different combinations of current, frequency, and cutting speed are illustrated in Fig. 5. The

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

above three different power measures, i.e., the average output power P, the single pulse power K, and the power per unit length Pp , affect different aspects of cutting qualities. Because the three types of power factors are functions of the current A, the frequency F, and the cutting speed V, we thus can search for a best combination of A, F, and V to result in the desired cutting quality, which will be considered in the next section. 3.2. The depth of cutting line The theoretical prediction of the depth of a cutting line DLine , which is a function of the average output power P and the cutting speed V can be simplified as the following expression [15]: DLine ¼ k1

P k2 þ pffiffiffiffi , V V

(3)

where k1 and k2 are coefficients determined by the properties of the materials under cutting and the other laser parameters. From the above equation, it is evident that either low cutting speed or high laser average power is helpful in increasing the depth of a cutting line [16]. Fig. 6a shows the measured depths of a cutting line for epoxy ðcÞ (DðeÞ Line ) and for copper compounded epoxy (DLine ) under different settings of average output power P and cutting speed V. The measured results reveal that exploiting higher P and lower V has the effect of increasing the depth of a cutting line. This tendency is consistent with the theoretical prediction in Eq. (3), although it was not derived specifically for cutting a QFN package. 3.3. The width of cutting line A theoretical model for predicting the width of cutting line W Line was developed as [15] " #1=2 k4 P pffiffiffiffi , (4) wLine ¼ k3 ln  V k5 þ k6 = V where k3 ; k4 ; k5 , and k6 are constants relating to the properties of the materials and to the other laser parameters. From the equation, it is evident that either lower cutting speed or higher average output power increases the width [4,15–18]. Fig. 6c shows the experimental results of the widths of cutting lines for epoxy ðcÞ (W ðeÞ Line ) and for copper compounded epoxy (W Line ) using various combinations of average power output p and cutting speed V. Consistent with the prediction of Eq. (4), Fig. 6c reveals that the widths of cutting line is reduced by increasing the cutting speed, since higher cutting speed shortens the thermal contact time and thus narrows the cutting line; whereas higher laser output power P increases the widths of cutting line caused by the higher thermal energy released per unit time.

789

3.4. The width of HAZ Assuming that the laser beam maintains a constant TEM00 mode, the beam intensity distribution in the n direction, which is defined as the direction of the width of the HAZ, can be written as [19] Iðt; vÞ ¼ k7 Pe½ðVtx0 Þ

2 þy2 =k

8

,

(5)

where k7 and k8 are material constants and ðx0 ; yÞ is the coordinate along the direction of n. The beam intensity distribution Iðt; vÞ is a direct indication of the HAZ and can be used to estimate the width of the HAZ. From Eq. (5), it is expected that applying either higher average output power or lower cutting speed will enlarge the beam intensity distribution so as to increase the width of the HAZ [4,19]. This prediction is confirmed by the measured ðcÞ width of the HAZ (W ðeÞ HAZ , W HAZ ) for cutting a QFN package as illustrated in Fig. 6b. It is observed that lower cutting speed and larger average output power widen the width of the HAZ by noting that lower V increases the thermal contact time and larger P increase the thermal input; both have the effect of enlarging the HAZ. From the experimental results shown in Fig. 6, it is noted that the cutting widths and depths of the two constitution components of QFN packages are quite distinct. The ðeÞ cutting widths and depths of the epoxy (W ðeÞ HAZ , W Line , ðeÞ DLine ) are found to be wider and deeper than those of the ðcÞ ðcÞ copper circuit-pad bonding epoxy (W ðcÞ HAZ , W Line , DLine ) by employing the same power per unit length. The main reason is that epoxy has properties of higher heat conductivity, higher heat absorptivity, and lower density than those of the other component. The differences in the material properties result in distinct material constants ki’s in Eqs. (3)–(5), which then give rise to the variance in cutting depth and width.

4. Taguchi experimental design In the last section, we have examined the general tendency of laser cutting qualities for QFN packages. With a given DPSSL cutting system having its inherent constraints in power output and cutting speed, it is interesting in obtaining the best cutting quality. Because the best cutting quality is to be achieved under the on-line operation environment of the given DPSSL cutting system, what we need is an efficient plan for conducting experiments, rather than a mathematical optimization tool. Taguchi’s matrix experimental design is a good guide to fulfill this purpose. It provides an efficient method to reduce the number of experiments and to obtain optimal laser cutting parameters. By utilizing this method, we can quantify the influences of the various control factors on the cutting quality and identify the optimal combination of cutting to yield the best cutting quality.

ARTICLE IN PRESS 790

C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

Fig. 6. Cutting qualities at different combinations of control factors.

4.1. Main control factors According to the previous examination on the cutting quality for QFN packages and to the existing knowledge about the laser cutting technology, it becomes obvious that the current A, the laser pulse frequency F, and the cutting speed V are the three decisive parameters influencing cutting qualities, which are then taken as the main control factors in the matrix experiments to be performed later. To determine the best level for each control factor involves some tradeoffs between contradictory cutting performances. 4.1.1. Current Although the lower current can provide lower peak power for single pulse laser and reduce cutting heat so as to

decrease the widths of a cutting line and HAZ. However, using lower current is difficult to cut off the materials effectively. On the contrary, the higher current can generate high peak power and release more cutting heat and thus increases the cutting line and HAZ widths. Nevertheless, the cutting process may be more efficient by adopting higher current. Fig. 4a shows the relations between different currents and peak power of single pulse. 4.1.2. Frequency Lower frequency usually leads to higher peak power of single pulse and gives higher cutting ability. However, the lower frequency also accompanies low-level pulse overlapping and non-continuous power density within a unit length (see Fig. 4b). Therefore, low-frequency laser cutting tends to produce a non-continuous cutting line and to

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

generate burn spots along the cutting path. On the contrary, higher-frequency cutting has relatively low peak power for single pulse and leads to less cutting ability, but it provides high-level pulse overlapping to yield continuous power density within a unit length. This feature of highfrequency cutting has the advantage of producing continuous cutting path. Fig. 4b shows the peak power and pulse overlapping for low and high laser frequencies. 4.1.3. Cutting speed Low-speed cutting process often accompanies high-level overlapping and continuous power density per unit length and hence tends to cut the material completely. But it produces more heat and wider HAZ and cutting line. On the contrary, high-speed cutting process results in narrow HAZ and cutting line at the risk of incomplete cutting. Fig. 4c shows the pulse overlapping and power density for low and high cutting speeds. 4.2. Preliminary experiments identifying the ranges of control factors From the above discussions, it was found that choosing a control factor too large or too small may cause degradation of cutting qualities. To identify the appropriate range for each control factor, we will conduct two preliminary experiments to determine the acceptable lower bounds and upper bounds. Preliminary experiments are helpful in shortening the allowable ranges of control factor and in reducing the number of matrix experiments. The first experiment, which tests the cutting ability by adopting low power of unit length, employs laser parameters having current of 29 A, frequency of 0.5 kHz and cutting speed of 3.5 mm/s. The second experiment, which tests the cutting ability by adopting high power of unit length, employs laser parameters of 37 A, 3.5 kHz, and 0.5 mm/s. The photographs in Fig. 7 show the cutting results of the two experiments. It can be observed that the first experiment adopting lower power of unit length yields incomplete cutting, while the second experiment adopting higher power of unit length yields over-cutting with excessive widths of cutting line and HAZ. The above two experiments exhibit the opposite limits of cutting qualities and just set the lower and upper bounds

Fig. 7. Photographs showing the cutting qualities for the two preliminary experiments (15  amplification).

791

Table 2 Control factors and their levels Factor

Parameter

Level 1

Level 2

Level 3

A B C

Current (A) Frequency (kHz) Cutting speed (mm/s)

29 0.5 0.5

33 2 2

37 3.5 3.5

Table 3 Orthogonal array of L9(34) Exp

1 2 3 4 5 6 7 8 9

Levels of control factors

Noise factor

A

B

C

e

1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3

1 2 3 2 3 1 3 1 2

1 2 3 3 1 2 2 3 1

for each control factor. It is safe to say that the allowable range for each control factor is between the bounds set up by the two experiments, i.e., current between 29 and 37 A, frequency between 0.5 and 3.5 kHz, and cutting speed between 0.5 and 3.5 mm/s. During the following matrix experiments, we need to specify the levels for every control factor. With the aboveestablished ranges of control factors, we chose the maximum, the minimum, and the mean as the three levels for each control factor. That is to say, the three levels for control factor A (the current) are 29, 33, and 37 A; the three levels for control factor B (the frequency) are 0.5, 2, and 3.5 kHz, and the three levels for control factor C (the cutting speed) are 0.5, 2, and 3.5 mm/s. Table 2 summarizes the control factors and their assigned levels. 4.3. Orthogonal array and ANOVA According to the above chosen number of control factor and their assigned levels, matrix experiments formed from the orthogonal array of L9(34) [20] are the most efficient plan for the experimental design. Table 3 shows the nine experiments involved in the orthogonal array of L9(34) and the accompanying level assignments for each experiment. Each experiment is repeated three times to reduce the influence of the uncontrolled factors (noise factors). The cutting quality for each experiment is quantified by a scoring system that is described in the next section. For each experiment, three quality scores y1 , y2 , and y3 are obtained by repeating the experiment three times. These quality scores are further transformed to the

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

792

signal-to-noise ratio Z (S=N ratio) via the relation: " # 3 1X 1 Z ¼ 10 log , 3 i¼1 y2i

(6)

where the quality score yi with larger-the-better style has been assumed. The overall mean value of Z over the nine experiments becomes Z¯ ¼

9 1X

9

Zi .

W ðeÞ Li ¼

(8) A control factor with the largest effect means that it has the most significant influence on the cutting quality. The analysis of variance (ANOVA) [21] is used to discuss the relative importance of all control factors on the cutting quality and to determine which control factor has the highest effect. Parameters used in ANOVA are calculated by the following equations: !2 9 3 1 X 1X Sm ¼ Zi ; S A ¼ Z2  S m , 9 i¼1 3 i¼1 Ai 9 X

(10a)

(7)

i¼1

The effect of a control factor level is defined as the deviation of its related S=N ratio Z from the mean value Z¯ . For example, when the effect of level A1 is concerned, we note that the control factor A is at level 1 in experiments 1–3. Hence, the average ZA1 and the effect of A are given, respectively, as     ZA1 ¼ 13 Z1 þ Z2 þ Z3 ; Effect of A9max ZA  min ZA .

ST ¼

(1) Smaller widths of HAZ and cutting line are desired and must be transformed into higher scores; oppositely, higher widths are transformed into lower scores. We utilize the following equations to quantify the above principles: h i ðeÞ ðeÞ W ðeÞ W ðeÞ Hi ¼ HAZmax þ W HAZmin  W HAZi 100; i ¼ 1; 2; 3,

Z2i  S m ;

Se ¼ ST 

X

V A ¼ S A =f A ;

i ðeÞ ðeÞ W ðeÞ Linemax þ W Linemin  W Linei 1000; i ¼ 1; 2; 3, (10b)

ðeÞ ðeÞ ðeÞ where W ðeÞ HAZmax (W Linemax ) and W HAZmin (W Linemin ) are the ðeÞ maximum and minimum of W ðeÞ HAZ (W Line ) in the experiðeÞ ments of orthogonal array of L9(34). W ðeÞ Hi and W Li , i ¼ 1,2,3 are the width-quality scores for the epoxy component in QFN package recorded from the three repeated measurements for each experiment listed in Table 3. The scaling factors of 100 and 1000 in Eqs. (10) are employed to balance the contributions between W ðeÞ HAZ ðeÞ and W ðeÞ Line by noting that the magnitude of W HAZi is about ten times larger than that of W ðeÞ Linei . The definitions in Eqs. (10) ensure that the scores increase linearly with decreasing widths of HAZ (W ðeÞ HAZ ) and of cutting line (W ðeÞ Line ). In a similar way, we can define the equations of the width-quality score for the component of copper circuit-pad bonding epoxy in QFN package as h i ðcÞ ðcÞ ðcÞ W ðcÞ ¼ W þ W  W Hi HAZmax HAZmin HAZi 100; i ¼ 1; 2; 3,

(11a)

S controlfactor ,

i¼1

VA FA ¼ ; Ve

h

SA sA ¼  100%, ST

ð9Þ

where S m is the average of squares of sums, SA is the sum of squares related to control factor A, S T is the sum of squares of the variance, S e is the sum of squares of the errors correlated to all control factors, V A is the variance related to factor A and f A is the degree of freedom for factor A, F A is the F-ratio related to control factor A [22] and sA is the percentage contribution related to control factor A. sB and sC can be calculated by a similar way. The computed values for sA , sB , and sC tell us the relative importance of the three control factors in determining the cutting qualities. 4.4. Quantification of cutting quality This step is to evaluate the quality scores yi ’s required in the computation of S=N ration Z in Eq. (6). After the six ðeÞ ðeÞ ðcÞ ðcÞ ðcÞ quantities W ðeÞ HAZ , W Line , DLine , W HAZ , W Line , and DLine are measured, we may quantify the cutting quality by the following scoring principles.

W ðcÞ Li ¼

h

i ðcÞ ðcÞ W ðcÞ þ W  W Linemax Linemin Linei 1000; i ¼ 1; 2; 3. (11b)

(2) Deeper depth is desired and must be transformed into a higher score. Oppositely, shallower cutting is transformed into a lower score. The following equation is utilized to quantify the above principle: DðeÞ Li ¼

DðeÞ Linei DQFN

;

DðcÞ Li ¼

DðcÞ Linei DQFN

;

i ¼ 1; 2; 3,

(12)

where DQFN is the QFN thickness, which is equal to 0.9 mm ðcÞ for epoxy and the present case. DðeÞ Li and DLi , i ¼ 1; 2; 3 are the three repeated measurements of the depths of epoxy and copper circuit-pad bonding epoxy for the each experiment listed in Table 3. Synthesizing the above two scoring principles, we combine Eq. (10), Eq. (11) and Eq. (12) to obtain the total quality score as   ðeÞ ðeÞ ðcÞ ðcÞ yei ¼ DðeÞ yci ¼ DðcÞ Li W H i þ W Li ; Li W Hi þ W Li , i ¼ 1; 2; 3,

ð13Þ

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795 Table 4 Experimental results of orthogonal array of L9(34) Exp no. 1st output

1 2 3 4 5 6 7 8 9

yi ¼

2nd output

ye1

yc1 Mean ye2

89 125 63 69 90 68 46 92 61

55 72 99 112 45 54 45 57 42 66 52 60 24 35 70 81 45 53

1 ye i þ yc i ; 2

yc2

81 45 63 125 103 114 79 55 67 87 51 69 97 51 74 52 30 41 59 23 41 81 59 70 69 47 58

Table 5 S=N responses of control factors

3rd output

Mean ye3

yc3

S=N ratio

Levels

Factor

Mean

91 47 69 127 105 116 74 50 62 93 57 75 86 54 70 65 39 52 73 39 56 88 72 80 81 57 69

i ¼ 1; 2; 3,

793

36.637 41.137 35.672 36.465 36.892 34.046 32.700 37.711 35.510

Level 1 Level 2 Level 3 Zmax Zmin Effect Rank

A

B

C

e

37.815 35.801 35.573 37.815 35.573 2.242 3

35.267 38.58 35.076 38.58 35.076 3.504 1

36.131 37.704 35.088 37.704 35.088 2.616 2

36.612 35.961 36.616 36.616 35.961 0.655 4

The italic numbers denote the best levels.

(14)

where yei and yci are the total cutting-quality scores for epoxy and for copper circuit-pad bonding epoxy, respectively. From Eq. (13), it was found that yei and yci , are formed from the depth-quality score multiplied by the sum of the width-quality scores. yi is the mean score by averaging yei and yci . Each experiment in the matrix experiments listed in Table 3 is repeated three times, and thus produces three quality scores yi , i ¼ 1; 2; 3. Before the matrix experiments, the laser focus length is adjusted to its most appropriate position that provides high power density and gives the best cutting quality. The other factors influencing the cutting quality are adjusted according to the settings listed in Tables 2 and 3. Using the measured yi , the S=N ratio computed from Eq. (6) becomes

 1 1 1 1 þ þ Z ¼ 10 log . (15) 3 y21 y22 y23 The quality scores and the S=N ratios for the night experiments are listed in Table 4. Once the scores for the nine experiments are all determined, we could apply ANOVA to identify the best combination of levels for current, cutting speed, and laser frequency. 5. Experimental results and discussions By using Eq. (8), the S=N ratios for the three levels of each control factor are computed and the results are tabulated in Table 5. The best level for each control factor is the one with the highest S=N ratio. From Table 5, we find that the best current level is A1 ¼ 29 A, the best frequency level is B2 ¼ 2 kHz, and the best cutting speed level is C 2 ¼ 2 mm=s. The experiment adopting the best level combination A1 B2 C 2 happens to be the second experiment listed in Table 3. As indicated in Table 4, the second experiment does have the highest S=N ratio. The S=N ratios for the nine experiments and their related cutting qualities are depicted in Fig. 8. The highest score occurs at the second experiment, which yields complete

Fig. 8. Schematic relations between scores and cutting qualities.

Table 6 Results of ANOVA Factor

Sum of square (S)

Degree of freedom (f)

Variance F-ratio (V) (F)

Contribution (s) (%)

A B C e Total (e)a

9.205 23.290 10.406 2.036 44.937 2.036

2 2 2 2 8 2

4.603 11.645 5.203 1.018 — 1.018

20.484 51.828 23.157 4.531 100 —

a

4.521 11.341 5.111 1 — —

(e) is pooled error.

cutting with narrow widths of HAZ and cutting line. The experiments in the left side of experiment 2 produce incomplete cutting, whereas the experiments in the right side of experiment 2 produce over-cutting with excessive widths of HAZ and cutting line, and even produce burn. The effect of each control factor is computed from the value of Zmax  Zmin , based on which Table 5 shows that factor B (frequency) has the largest effect and thus has the most significant influence on the cutting qualities. The effect of control factor can also be estimated from ANOVA

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

794

by applying the formulas in Eq. (9), and the results are listed in Table 6. It can be seen from Table 6 that the percentage contribution of the control factors to the cutting qualities in the decreasing order is (1) B (51.828%), (2) C (23.157%), (3) A (20.484%). The remaining contribution (4.531%) is due to noise factor e. The comparatively small magnitude of this residue contribution indicates that the cutting quality is mainly dominated by the three control factors, i.e., the laser

frequency (B), the cutting speed (C), and the driving current (A), in the order of decreasing contribution. Fig. 9 is a photograph (20  amplification) showing the laser cutting product of a QFN package by using the best cutting parameters. It is observed that the widths of HAZ and cutting line are, respectively, W ðcÞ HAZ ¼ 0:26 mm and W ðcÞ ¼ 0:012 mm for copper circuit-pad bonding epoxy, Line ðeÞ and W ðeÞ ¼ 0:24 mm and W ¼ 0:038 mm for epoxy. HAZ Line By substituting these measurement data into Eq. (15), we can check that the predicted best cutting parameters do lead to the highest quality score among all the possible combinations of cutting parameters.

Fig. 9. Photograph of cutting product by using optimal cutting parameters determined from matrix experiments.

Fig. 10. Photographs of verification experiments at various frequencies (15  amplification).

Table 7 Verification experiments Exp no.

1 2 3 4 5

Current (A)

29 29 29 29 29

Frequency (kHz)

0.1 0.5 1 2 5

Cutting speed (mm/s)

2 2 2 2 2

1st output

2nd output

S/N ratio

3rd output

ye1

yc1

Mean

ye2

yc2

Mean

ye3

yc3

Mean

37 53 85 116 53

19 29 65 102 37

28 41 75 109 45

69 68 91 124 60

23 30 69 98 46

46 49 80 111 53

58 75 99 123 80

16 39 77 107 54

37 57 88 115 67

31.187 33.726 38.151 40.956 34.690

ARTICLE IN PRESS C.-H. Li et al. / Optics & Laser Technology 39 (2007) 786–795

To verify the predicted best cutting parameters, a series of verification experiments are designed and performed according to Table 7. The purpose of the verification experiments is to investigate the influences of the different frequencies on the cutting quality, while keeping current and frequency at their best levels, 29A and 2 mm/s, respectively. The verification experiments need to demonstrate that the highest score does occur at the frequency of 2 kHz as predicted from the above matrix experiments. Fig. 10 contains a series of photographs showing the cutting product by using five frequencies. Inspecting these experimental results, it was found that applying lower frequencies, such as 0.1, 0.5, and 1 kHz, leads to incomplete cutting, while applying higher frequency, such as 5 kHz, yields over-cutting with excessive widths of HAZ and cutting line. Applying the intermediate frequency at 2 kHz yields the least widths of HAZ and cutting line and in the meantime ensures complete cutting. 6. Conclusion This paper presents a laser cutting technology for QFN (Quad Flat No-lead) packages with the help of Taguchi’s matrix experiments to obtain the optimal cutting quality, which has least widths of HAZ and cutting line and ensures a complete cutting. A quantified mechanism was proposed for examining the laser cutting quality. This is the first application of laser technology for cutting a QFN package. What is learned from this study is that almost 95.47 percentage of laser cutting quality is contributed from only three control factors, namely, laser frequency (51.828%), cutting speed (23.157%), and laser driving current (20.484%). The variance analysis using data obtained from matrix experiments for cutting a QFN package also shows that the optimal cutting parameters are 29 A for the current, 2 kHz for the laser frequency and 2 mm/s for the cutting speed. The use of matrix experiments, as introduced here in terms of the laser cutting of QFN package, helps us to identify the most influential factors and find the optimal values for these factors with a minimum number of experiments. Acknowledgements The authors would like to thank the Gallant Precision Machining Company Limited (Taiwan) for providing experimental materials and devices. The authors also thank Chun-Chao Wang at National Kaohsiung First University of Science and Technology for technique support.

795

References [1] Chen N, Chian K, Her TD, Lai YL, Chen CY. Electrical characterization and structure investigation of quad flat non-lead package for RFIC application. Solid-State Electron 2003;47:315–22. [2] Rajendran N, Pate MB. The effect of laser beam velocity on cut quality and surface temperature. Am Soc Mech Eng, Heat Transfer Div 1988;104:121–7. [3] Neimeyer R, Smith RN, Kaminski DA. Effects of operating parameters on surface quality laser cutting of mild steel. J Eng Ind 1993;115:359–66. [4] Rajaram N, Sheikhahmad J, Cheraghi SH. CO2 laser cut quality of 4130 steel. Int J Mach Tools Manuf 2003;43:351–8. [5] Kaebernick H, Jeromin A, Mathew P. Adaptive control for laser cutting using striation frequency analysis. Ann CIRP—Manuf Technol 1998;47(1):137–40. [6] Olsen FO. Investigations in optimizing the laser cutting process, In: Lasers in materials processing. Conference Proceedings, American Society for Metals, Los Angeles, USA, 1983. p. 64–80. [7] Steen WM, Kamalu JN. Laser cutting. In: Bass M, editor. Laser materials processing, vol. 3. New York: North Holland; 1983. p. 17–111. [8] Hamoudi WK. The effects of speed and processing gas on laser cutting of steel using a 2 kW CO2 laser. Int J Joining Mater 1996;9(1):31–6. [9] Nagarajan R. Parametric study of the effect of laser cutting variables on the cut quality. Master thesis, Wichita State University, Wichita, Kansas, 2000. [10] Phadke MS, Kackar RN, Soeeney DV, Grieco MJ. Off-line quality control in integrated circuit fabrication using experimental design. Bell Syst Tech J 1983;62(5):1273–309. [11] Phadke MS, Dehnad K. Optimization of product and process design for quality and cost. Quality Reliability Eng Int 1988;4(2):105–12. [12] Pan LK, Wang CC, Hsiao YC, Ho KC. Optimization of Nd:YAG laser welding onto magnesium alloy via Taguchi analysis. Opt Laser Technol 2004;37:33–42. [13] Thawari G, Sarin Sundar JK, Sundararajan G, Joshi SV. Influence of process parameters during pulsed Nd:YAG laser cutting of nickelbase superalloys. J Mater Process Technol 2005;170:229–39. [14] Zhipei S, Ruining L, Yong B. Experimental study of high-power sidepumped Nd:YAG laser. Opt Laser Technol 2005;37:163–6. [15] Cenna AA, Mathew P. Analysis and prediction of laser cutting parameters of fibre reinforced plastics (FRP) composite materials. Int J Mach Tools Manuf 2005;42:105–13. [16] Zhou BH, Mahdavian SM. Experimental and theoretical analyses of cutting nonmetallic materials by low power CO2-laser. J Mater Process Technol 2004;146:188–92. [17] Li K, Sheng P. Plane stress model for fracture of ceramics during laser cutting. Int J Mach Tools Manuf 1995;35:1493–506. [18] Sheng P, Cai L- H. Model-based path planning for laser cutting of curved trajectories. Int J Mach Tools Manuf 1996;36:739–54. [19] Sheng PS, Joshi VS. Analysis of heat-affected zone formation for laser cutting of stainless steel. J Mater Process Technol 1995; 53:879–92. [20] Yilbas BB. Laser cutting quality assessment and thermal efficiency analysis. J Mater Process Technol 2004;155–156:2106–15. [21] Taguchi G. Introduction to quality engineering. Tokyo: Asian Productivity Organization; 1990. [22] Fisher RA. Design and analysis of experiments. London: Olive and Boyd; 1925.