Laser decapsulation of molding compound from wafer level chip size package for solder reflowing

ARTICLE IN PRESS

Materials Science in Semiconductor Processing 8 (2005) 502–510

Laser decapsulation of molding compound from wafer level chip size package for solder reflowing H. Qiua, H.Y. Zhengb,, X.C. Wangb, G.C. Limb a Hewlett-Packard Singapore, 1150 Depot Road, Singapore 109673 Singapore Institute of Manufacturing Technology, Nanyang Drive 71, Singapore 638075, Singapore

b

Available online 18 November 2004

Abstract A potential laser etching method has been investigated and implemented in decapsulating wafer level chip size package (WLCSP). A chemically and physically clean surface could be obtained and lead to successful solder reflowing. Firstly, fast and cost-saving transfer molding was suggested to replace underfill dispensing in encapsulation. An industrial-grade KrF excimer laser was utilized to remove the compressed molding compound (epoxy resin and silica fillers) on Au/Cu or Pb–Sn/Cu bumps. An oblique angle of incidence was found effective in achieving a filler-free and chemically clean surface. Explosion forces from epoxy underneath the fillers, together with the transient thermal expansion of whole substrate, were attributed to the higher filler removal efficiency than at normal incidence. In addition, etching rates at angular irradiation were found more balanced between the surrounding areas and the bump. Helium, in comparison with oxygen, helped to reduce polymer molecules left over the bump top. In higher laser fluence or oxygen environment, carbonyl groups with stronger bonds to metal surface (bump) were found on bump top so that success rate of solder reflowing was greatly lowered. A success rate higher than 80% was achieved at an optimal laser fluence and scanning strategy, with inert gas assistance. r 2004 Elsevier Ltd. All rights reserved. Keywords: Laser cleaning; Laser ablation; Epoxy resin; Molding compound; Wafer level chip size package

1. Introduction Today, area array and surface mount package have become common due to their increased use in portable consumer products of high volume. Bare chip or wafer level chip size package (WLCSP), with less than 0.8 mm pitch, has become the mainstream for these products. A method of forming a WLCSP is to first form conductive bumps on the pads. Then form a layer of insulation on the interconnect surface of the wafer, such that the Corresponding author. Tel.: +65-67938504; fax: +65-

67922779. E-mail address: [email protected] (H.Y. Zheng).

bumps are submerged in the layer of insulation, and then remove a portion of the layer of insulation to expose the upper portions of the bumps. Accurate dispensing of encapsulant underfill has become a key component in the reliable manufacture of CSP chip devices. The use of underfill encapsulant beneath the flip-chip die is necessary to increase reliability significantly by reducing the strain on the solder bumps during thermal cycling. It is imposed by coefficient of thermal expansion (CTE) differences between the die and the substrate [1]. Then a subsequent etching process, such as plasma etching, can be applied to remove a portion of the layer of epoxy and expose the upper portions of the bumps. Following by the Pb/Sn solder ball/paste reflowing onto all the bumps, the long interconnections

1369-8001/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2004.08.001

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with the bumps are formed as a bridge between PCB and package. However, one of the primary concerns was then relatively slow process of the underfill dispensing and curing process. In contrast, transfer molding has been used for a long time in high-volume, low-cost encapsulation and is therefore well established as the process of choice for applications such as memory devices and logic devices [2]. High temperature and pressure liquefies epoxy resin, mixes with silica ball fillers (10 mm diameter) and forces through a mold chase over the die and die frame and into the cavity on the frame where the die was placed earlier. When mold compound (mixture of epoxy and fillers) is used to form the layer of insulation, plasma etching cannot be used to remove a portion of the layer of insulation as plasma does not remove mold compound efficiently (neither the epoxy nor the large size fillers). Hence, applying plasma to mold compound will adversely affect the reliability of the resultant WLCSP. Then instead of plasma etching, a mechanical grinding process is employed to remove a portion of the layer of insulation along with upper portions of the bumps, thus exposing the ground surface of the bumps. The layer of insulation can comprise epoxy or mold compound. When mold compound is used, the layer of insulation is molded on the interconnect surface of the semiconductor wafer. A disadvantage of the grinding process is that the bumps are flattened to become level with the layer of insulation. This is not desirable because when solder is subsequently disposed on the bumps, the solder can only adhere to the exposed ground surface of the bumps. In contrast, it is desirable to have the bump protrude beyond the surface of the layer of insulation, so that the solder can adhere to the exposed top and side surfaces of the bumps, which results in more reliable solder joints. Another disadvantage of the grinding process is the risk of over-grinding, due to the relatively poor accuracy of the grinding process. When the resultant thickness of the layer of insulation becomes too thin to provide good insulation, the reliability of the WLCSP can be adversely affected. An alternative method of forming a WLCSP is to employ laser etching where a laser beam is scanned on the layer of insulation and a portion of the layer of insulation is removed by ablation. In contrast with plasma etching, laser etching is a dry process and is capable of ablating mold compound selectively, i.e. wherever the laser beam is directed. A lot of work has been done on laser ablation of polymer [3–7]. Laser cleaning of surface contaminants from polymer to oxides has also been extensively studied on various surface, such as Si, InP, metal, and stone [8–10]. Actually, laser etching of molding compound has been reported before, in which the fine whiskers are exposed from epoxy on Cu lead surface for failure analyses by KrF excimer laser ablation [11]. Some C, Sb, O and Si elements on the

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whisker surfaces were detectable by auger electron spectroscopy. Nevertheless, such a surface is not clean enough for a successful solder reflowing in next step. In this paper, as a first attempt, laser removal of mold compound applied in WLCSP was investigated in order for a fast, dry, cost-saving packaging technology. Parameters such as laser fluence, incident angle and assisted gases were studied. Bump surface was characterized by FTIR, XPS and SEM. And solder ball reflowing is carried out finally for those bumps. 2. Experimental The schematic of a WLCSP encapsulated by epoxy molding is shown in Fig. 1. The fillers of silica ball in epoxy are about f25 mm in average. Bumps could have a top layer of Au with 5 mm thickness, or 97Pb–Sn solder with 30 mm thickness for feasibility enhancement of soldering. A KrF excimer laser (Lambda Physik LPX220i) with a 248 nm wavelength and 23 ns pulse duration was used to etch the WLCSP. The laser operated at an 80 Hz repetition rate and fluence from 0.2 to 5 J cm
2. The chip size is 10 mm  10 mm and the beam spot size is 3 mm2. The chip is put up-side-down by a post holder so that the ablation product will not fall back to the chip. Together with a nozzle and an extractor set aside, bumps are protected from pollution by debris to the most. Both oxygen and helium were tested. The laser beam was focused at an angle from 01 to 801 onto the surface. When put in an appropriated position, the package traveled in X and Y direction so that laser scanning would cover some square area. XPS measurements were performed with a VG Scientific ESCALAB MK II system. Al/Mg K-a radiation from a twin node was used. FTIR was carried out in Excalibur/S UMA500 Fourier Transform Infrared Spectrometer (BioRad). All reflow are done in reflow oven Heller 1800.

3. Results and discussion 3.1. Filler cleaning Firstly, we decapsulate the chips with Au top layers by laser in 01 incidence angle. In Fig. 2, a mirror is put in Fillers Bump

500 um

250 um

Top layer Epoxy resin

10 ~ 30 um 110 um

Barrier layer Si Wafer

Fig. 1. A schematic showing the cross-section of the WLCSP encapsulated in epoxy resin.

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position 1. After a number of laser pulses, a lot of silica fillers are still left over the bump, as shown in Fig. 3 from (a) to (e). The surrounding copper circuits embedded in epoxy have already been exposed. The predominant adhesion force for tiny particles smaller than 50 mm on a dry surface is Van der Waals force [12]. Z Excimer Laser

Stage & Package Sample

X Y Gas Nozzle

Extractor θ Objective Lens

Position 1 Mirror

Position 2

Fig. 2. Experimental setup of laser etching of WLCSP.

In laser dry cleaning, an absorptive substrate is much more preferred than the absorptive particles. The strong thermo-elastic forces caused by rapid thermal expansion of substrate eject particles away. Here, however, chemical bonds between particle surface and epoxy resin formed in compression molding are dominant and much stronger. It has long been recognized that by deposition of a thin film onto the contaminated surface (often liquid like water, alcohol, etc.) the cleaning efficiency is greatly enhanced (laser steam cleaning) [13,14]. The explosion force originated from the film or film–substrate interface is even stronger. Exactly, steam laser cleaning shares some similarities with our laser removal of fillers in epoxy resin. The detonation explosion of epoxy, in proportion with the epoxy absorption, acceptation cross-section upon the laser beam and laser fluence, act as an important impact at removing fillers. Some fillers are embedded partly in gold layer (5 mm) or even deeper in Pb–Sn layer

Fig 3. Laser decapsulation effect of Au/Cu bump at 01 incident angle after (a) 800, (b) 900, (c) 1000, (d) (e) 1100 Pulses. Laser frequency, 80 Hz; energy density, 2 J cm
2.

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Fig 4. Laser decapsulation effect of Au/Cu bump at 751 incident angle after (a) 800, (b) 900, (c) 1000, (d) (e) 1100 Pulses. Laser frequency, 80 Hz; energy density, 1.3 J cm
2.

(30 mm) on Cu bump. Melting occurred due to high laser energy. When the epoxy resin becomes thinner and thinner, much of laser energy goes through and is absorbed and reflected by the Au or Pb–Sn layer. The total reflectivity and absorption change with epoxy thickness and optical properties, like an antireflection (AR) coating. After the fillers are embedded in melt Au or Pb–Sn layers, it is even harder to remove them by laser, as shown in Fig. 3(d). A rough bump top adds difficulties in solder ball reflowing. In laser dry cleaning, Zheng et al. found that, as the incident angle increase to 151, the cleaning efficiency of f2.5 mm silica balls on Si wafer by KrF Excimer laser had fallen down to 0 [15]. A highest near-field intensity distribution under the transparent silica particle is found at normal incidence due to focusing effect. In the previous studies [8,16], it has been discovered that an oblique incident angle increases the cleaning efficiency of particle removal from metal surface. It is suggested that the nontransparent particle blocked the laser beam and there was an enhanced absorption (larger acceptation section) at glanced angle between particle–Cu interface. In transfer molding, an epoxy resin layer often is plated on the silica fillers previously to enhance the mixture with epoxy, thus the transmittance of silica fillers for UV laser etching is quite low. With an oblique angle of incidence, the epoxy concentrated underneath fillers preferentially enlarges the cross-section for intercepting the slanted laser beam. As the epoxy becomes thinner and thinner, there is an optimal slanted angle (/range)

Thermal Expansion Force

Laser beam

Explosion Force Filler Epoxy resin

Filler

Bump Fig. 5. Illustration of explosion force by oblique laser beam irradiation from the underneath pushing/rolling fillers from right to left hand.

which guarantees an increased absorption depth by top layer while less arrival of laser beam to the underneath layer material (Au). The laser removal of epoxy/silica particles from Au/ Cu bump in an incidence angle is shown in Fig. 4. The stage scanned from left to right while the laser beam stayed unchanged and irradiated from right to the left. This moving strategy prevented redeposition and serious pollution of debris from the ongoing ablation area to the cleaned area. As illustrated in Fig. 5, since explosion pressure comes from underneath at one side, top epoxy/ particles are pushed/rolled down to the left hand of bump where they accumulate. A ‘‘wind blowing’’ effect from right to the left was observed in Fig. 4. In Fig. 6, the cross-sections of Pb–Sn bump after laser etching

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incident angle from 621 to 701 was found optimal for both Au/Cu and Pb–Sn/Cu bumps. For the Au/Cu package, there also existed a lower optimal angle range from 551 to 621. This may be due to the thinner Au layer with shallowly embedded fillers, which are easier to be removed as compared to those deeply embedded in the Pb–Sn layer. Therefore, the angle requirement for removing the fillers on Au/Cu bumps is less critical allowing a larger angle range. However, as the optimal angle is relevant to the occurrence of largest absorption of p polarization by epoxy resin and metal (Au, Sn, Pb), the cleaning efficiency became lower again when the angle exceeded the optimal angle. 3.2. Laser removal of epoxy resin The wettability of bump in soldering is related to not only the particles, but also the cleanliness of the surface. In excimer laser ablation of polyimide, it was found that, C content increased in the ablated areas while O content and N content decreased [5,19]. Oxygen plasma has been known as the most aggressive in cleaning molding compound on packaging substrate due to its combined sputtering action and oxidizing environment compared Silica particles left on bump

showed that the stop end (left) of laser etching was thicker than the start point (right). The absorption of epoxy resin is 100% at 248 nm [17]. However, absorption decreases as the epoxy layer becomes thinner on bump. Consequently, the temperature increase at these positions becomes even low in consideration of the loss by reflective surface of Au, Pb–Sn, or Cu, whereas at the surrounding epoxy resin the laser is still fully absorptive. An unbalanced etching rate was resulted, as shown in Fig. 3. Since a p-polarized laser beam was used in experiment, the absorption coefficient of Au or Pb–Sn increased from 0.62 at normal incidence, to larger than 0.82 at a slant angle, e.g. 55751 [18]. Exactly, at a slanted angle the etching rate on bumps is more comparable to the surrounding areas. The surrounding embedded circuits are less likely exposed then, as shown in Fig. 4. Fig. 7 shows the cleaning efficiency of silica particles from Au/Cu and Pb–Sn/Cu bumps at an incidence angle. Occurrences of clean bump (no particles) at various incident angles were noticed. An oblique

35 Laser cleaning of Au/Cu bump

30 25 20 15 10 5 0 -10

0

10

Silica particles left on bump

(a)

35

Fig 6. Cross-section of (a) Pb–Sn/Cu bump before decapsulation; (b) laser-decapsulated bump at 751 incident angle after 1100 Pulses. Laser frequency, 80 Hz; energy density, 1.3 J/cm2.

Laser cleaning of SnPb/Cu bump

30 25 20 15 10 5 0 -10

(b)

20 30 40 50 60 70 80 90 Incidence angle (°)

0

10

20 30 40 50 60 70 80 90 Incidence angle (°)

Fig. 7. Silica particles left over (a) Au/Cu bump; (b) Pb–Sn/Cu bump versus laser incidence angle after 500 pulses of laser ablation at 5 J cm
2.

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to other gases like Ar, and Ar/H2 [20]. The chemical structure of epoxy resin is shown in Fig. 8. In order that the carbon were fully ‘‘burned’’ on the surface, oxygen blowing at the bump was firstly tried in laser etching. However, the solder reflowing had quite low success rate, as shown in Table 1. For a nanosecond UV laser ablation of polymer, bond breaking at excited electronic states and photo dissociation dominated in low energy. Its photon energy (5 eV) is strong enough to break most covalent bonds including C–C, O–O, H–H, C–H, and O–H in polymer. UV lamp irradiation has been carried out on epoxy resin before. A broadening of carbonyl band at 1710 cm
1 and the appearance of hydroxyls band around 3450 cm
1 in the infrared spectra was observed, owing to a strong oxidation of methylene groups(–CH) [21,22]. The forces adhering the fillers to epoxy (/bump) are proportional to the filler size. Moreover, larger fillers are probably compressed deeper than smaller ones. In the first step of our scanning strategy, high laser fluence (1.3 J cm
2) is preferred to remove all particles, especially those with big sizes. Then, low laser fluence (0.1 J cm
2) was used to clean those remained epoxy from the bump with repeated scannings. A thermal expansion force from substrate merely is not strong enough to overcome the chemical bonds. Strong heat and plasma should be avoided on the bump. Otherwise, particles of high dynamical energy in the plasma scattered back to the hot surface so that an adhesive layer would be formed most probably. In this second step, polymer contaminants were removed physically mainly by rapid thermal expansion of substrate accompanied with UV photon decomposition. In laser ablation of polymer, a modification of chemical and physical properties by laser below the surface has been observed and analyzed which hindered

CH2 - CH2 - CH - O -

CH2 -CCH2

- O - CH2 - CH - CH2 O

O

Fig. 8. Chemical structure of the epoxy resin.

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the further ablation [4,23,24]. Oxygen in the environment actually reacted easily with surface. Laser-induced plasma also experienced oxidation before some of them back-scattered to the bump surface. Exactly, after with strong fluence or oxygen assistance, an ultra-thin glassylike film was often observed adhering strongly to the golden tops which cannot be removed with any further laser cleaning strategies. That always resulted in a failure of solder reflowing. To study the role of gas and fluence in the laser cleaning, the chemical nature of the laser-etched bump was studied by FTIR and XPS. Bumps with gold on top were etched in air, oxygen and helium. As a noble metal, Au reacts neither with C nor O. Actually, oxygen absorption increases the resin cross-link density in UV laser depolymerization and may strengthen the adhesion to substrate. In Fig. 9, FTIR spectra were shown for the materials left on Au/Cu bump etched in various gas environments and laser fluence. For curve a (epoxy resin of WLCSP), the broad peak in 2850–3000 cm
1 corresponds to the stretching vibration of C–H groups, while peaks from 1050 to 1200 cm
1 correspond to the stretching vibration of C–O groups. The weak and broad peak centered in 3430 cm
1 is attributed to the H–O groups. The range around 3000 cm
1 which corresponds to C=O cannot be clearly identified in FTIR spectra due to influences from moisture. After laser cleaning, it can be seen that most epoxy has been removed in evidence with absorption peaks of function groups decreasing greatly. The C–H groups nearly disappeared after etching with oxygen assistance which is consistent with the UV light degradation in air (d, e, and f curves in Fig. 9) Nevertheless, especially in high fluence or with oxygen assistance, the C–O groups was always detected on surface. Only by low laser fluence and with helium assistance can the C–O and C–H groups be eliminated thoroughly. In Fig. 10, the splitting peaks of Au 4f spin–orbit remained constant at 84 and 88 eV, respectively for all samples. This is in agreement with previous reports [25]. The peak intensity or area below was taken as an indication of the composition amount. It is shown that oxygen tends to attack and oxidize the carbon composites in polymer. Carbonate, ester and aromatic groups are rapidly destroyed by

Table 1 Success rate of solder reflowing on bumps cleaned in different gases, laser energy and steps (Totally 50 bumps in test). Gas and energy bump

O2 with high-energy ablation at 1.3 J cm
2 (%)

O2 with two steps ablation at 1.3 and 0.1 J cm
2, respectively (%)

He with high-energy ablation at 1.3 J cm
2 (%)

He with two steps ablation at 1.3 and 0.1 J cm
2, respectively (%)

Au/Cu Pb–Sn/Cu

20 26

20 26

24 28

82 86

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photon/thermal decomposition and chemically modified by laser (/induced plasma) with oxygen assistance. Volatile products dispersed, such as carbon oxides. Therefore, the highest C/O atoms ratio is for etching in helium and lower ratio for etching in oxygen. The O 1s peak positions at 532 and 534, separated by 2 eV, are attributed to C=O and –O–C–O functional groups,

(a) epoxy resin on WLCSP;

C-O Transmittance (arb. units)

2

2

etching in (b) He, 0.1 J/cm ; (c) He, 1.3 J/cm ;

(f)

(d) air,1.3 J/cm2; (e) O2, 1.3 J/cm2; (f) O2, 1.3 J/cm2.

(e)

C-H

(d)

H-O

(c) (b) (a) 800

1200 1600 2000 2400 2800 3200 3600 4000 Wave number (cm-1)

Fig. 9. FTIR spectra of (a) epoxy resin on WLCSP; etching in (b) helium, 0.1 J cm
1; (c) helium, 1.3 J cm
1; (d) air, 1.3 J cm
1; (e) oxygen, 1.3 J cm
1; (f) oxygen, 1.3 J cm
1.

respectively. It is seen, in Fig. 9(a)–(c), that C=O groups accounted for much in O 1s at the surface etched by laser fluence 1.3 J cm
1. This deviation could be due to the oxygen loss considerably in laser etching since no C=O composites exist in the original epoxy. In addition, Au possibly interacts with and donates 5d electrons to the C=O groups of the polymer [26]. As a result, the Au–C bonds become stronger while C=O bonds weaker. In our experiment, Cu bump was also tested with laser etching and solder reflowing. The even low success rate (2%) can be accounted for by a similar mechanism of the formation of even stronger Cu–C bonds. In UV laser (193 nm) irradiation of polypropylene, it was found by FTIR spectrometry that C=O groups increased more in oxygen environment than in helium, particularly in low fluence at 0.1 J cm
1. They increased with laser fluence [27–29]. Some of the newly produced function groups, like carbonyl- (–C=O) or hydroxyl(–OH), are strongly polar groups that can strengthen the surface adhesion to other materials. In high laser fluence as in our case, photo thermal excitation dominated and might result in decomposition too by overheating. Laser-induced plasma, together with the heat produced in laser irradiation, added to modify the surface properties in gas environment. Friedrich et al. have

laser fluence 1.3 J/cm2, in oxygen O 1s, 532.7 1200 C 1s, 285.19 600 Si 2p Si 2s Au 4f 0 50 100

(a)

Intensity (arb.unit.)

Intensity (arb.unit)

C 1s, 283.99

1800

1800

O 1s, 531.9 laser fluence 1.3 J/cm2 in air

1200

600 Si 2p Si 2s 0 50 100 150 200 250 300 350 400 450 500 550

150 200 250 300 350 400 450 500 550

(b)

Binding energy (eV)

800

Binding energy (eV) 400

600

C 1s, 284.9

O 1s, 531.9

Au 4f 400

200

0 50 100

(c)

Intensity (arb.unit.)

Intensity (arb.unit.)

Au 4f laser fluence 1.3 J/cm2 in helium

laser fluence 0.1 J/cm2, in helium 200

100

0

150 200 250 300 350 400 450 500 550 Binding energy (eV)

300

50 100 150 200 250 300 350 400 450 500 550

(d)

Fig. 10. Au/Cu bump etched in (a) oxygen, (b) air, and (c) helium in 1.3 J cm (l ¼ 248 nm).

Binding energy (eV)
1

and (d) helium in 0.1 J cm
1 using excimer laser

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4. Conclusions

Fig. 11. (a) Top view of re-flown of PbSn/Cu bumps; (b) Crosssectional view of a re-flown PbSn/Cu bump.

found that crosslinking of polymer function groups is a very important process in oxygen-plasma treatment of polymer [30]. Based on the signal intensities in Fig. 9, it is found that helium-assisted etching resulted in the least amount of carbon and oxygen left on bumps while the oxygen resulted in the most. Helium gas, physically blew debris away and formed an inert environment instead of a chemical reaction agent. By using low laser fluence in helium gas environment, the success rate was increased to 86% for Pb–Sn/Cu bump and 82% for Au/Cu bump, as shown in Table 1. In Fig. 10(d), laser etching in helium with a low fluence 0.1 J cm
1 resulted in a clean Au surface without carbon or oxygen contamination. The UV laser produced less C=O composites. The ablated or unablatd epoxy resin is removed by thermal expansion essential like silica fillers. The success rate for solder reflowing is about 82%. The Pb–Sn/Cu bumps have a higher rate about 86%. Fig. 11 shows the successful reflow of Pb–Sn/Cu bump. In order to increase the rate further, some other methods were in consideration, e.g. additives added to the epoxy resin in order to increase the absorption or reduce the adhesion force to the bump, changing the composition of SnPb, etc.

A new decapsulation way by laser ablation has been developed for WLCSP which is encapsulated with transfer molding. Firstly, an oblique incident angle from 551 to 701 was found optimal for Au/Cu and 62–701 for Pb–Sn/Cu bumps. All silica fillers and most epoxy resin were removed in high laser fluence of about 1.3 J cm
1. Some of the large fillers were compressed into the Pb–Sn layer. Therefore, the larger the fillers, the higher fluence needed. The explosion from the epoxy underneath the filler, together with the transient thermal expansion of bump surface, was reasonably attributed to the filler removal. Oblique angle helps to increase the absorption depth in the epoxy layer while reduce the heat reaching the bump. In addition, p-polarized laser reach a maximum absorption in epoxy layer and epoxy/ bump interface at an incident angle, while the absorption in surrounding areas is nearly 100% constantly. The bumps etching in this way looked clean and even under optical microscope. Low laser fluence about 0.1 J cm
1 is necessary in a second step to move the remained epoxy on bumps for a ‘‘chemically’’ clean surface. This is confirmed by XPS and manifested by a high success rate in solder reflowing. Helium gas was found useful in blowing the irradiated area in prevention of epoxy oxidation, as confirmed by FTIR. The photon decomposition, together with thermal oxidation, resulted in increased C=O groups, which were attributed to the strengthened adhesion of the epoxy resin to the bump surface. A success rate of solder reflowing was reached about 86% for Pb–Sn/Cu bump and 82% for Au/Cu bump.

Acknowledgments The authors thank for the assistance in experimental work from colleagues Ms. J. L. Tan and Mr. K. M. Teh. The authors are grateful to Ms. Y. C. Liu for FTIR measurements. The authors would like to thank Dr. B. Luk’yanchuk of Singapore Data Storage Institute, for helpful discussions.

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