The Observation of Solder Ball Jetting Phenomenon in Laser Solder Ball Jetting Process

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NECTEC Technical Journal, NECTEC-ACE2009 Special Edition, September 2009.1

The Observation of Solder Ball Jetting Phenomenon in Laser SolderBall Jetting Process

Sunchai Mata, Sirivit Teachajedcadarungsri*, Julaporn Benjapiyaporn

Mechanical Engineering Department, Faculty of Engineering,

Khon Kaen University 40002 Thailand*Email: [email protected]

ABSTRACT: The solder ball-jetting phenomenon in laser solder ball jetting process was observed byexperimental and numerical calculation basis. In the experiment, a laser generator applied energy to melt leadfree solder ball inside a sub-millimeter diameter of capillary tube. The solder ball was jetted by pressure of inert

gas from the capillary tip. The solder ejection was photographed by high-speed camera. In the calculation, grid

scale method was used to calculate solder ball velocity and numerically derive the velocity equation. The results

showed that the solder ball was not melted completely when it started moving out from capillary tip; its

temperature kept increasing after it was ejected from capillary tube and reaching the completely melted point,

the glowing point, when the solder ball was totally ejected from capillary tube. The molten solder ball shape

still is in spherical form while it was flying. From the experimental results, the motion of molten solder ball

increased with acceleration of 5.25 km/sec2

KEY WORDS:Molten solder ball, Solder ball jetting, Capillary tube, Laser soldering

:

5.25

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1. INTRODUCTIONThe developments of advanced semi-conductor

manufacturing technologies and microelectronics assembly

are be driven by needs of reductions in cost, higher output,

more accurate with miniaturization, automation and also

protect environment impact. To comply all of needs, lead

free solder jet technology is recently increasing in attractive

and alternative techniques for the distributions and

patterning of materials in wide varieties of applications.Solder jetting technology [1] was first developed by

MicroFab Technologies, Inc. base on inkjet technique. And

recently Packaging Technologies, (PACTECH) developedand demonstrated Laser Solder Ball Jetting process [2-3].

Laser technology was applied to melt a preformed solder

ball which was then jetted out of a capillary tube with




pressurized inert gas to form an interconnect on the targetcircuit. The process combined solder ball bumping and re-

flow process into the single system. The process gave high

output with acceptable accuracy, flexibility, and applicableon micro-sensor assembly.

Laser energy application on lead-free solder ball was

studied by Fuquan Li [4] which used both experimental andsimulation basis to study the effects of initial temperature

of solder ball to inter-metallic compound quality. The

results showed that initial temperature on solder ball was

the main factor for solder reflow quality. In the same way,Yanhong Tian [5] compared quality of solder joint

produced from conventional hot air re-flow method and

laser technique method, it was found that the surface ofsolder bumps obtained by the laser reflow method withproper parameter was smoother than that obtained by the

hot air reflow method. Zhenquing Zhao [6] reported the

same results as Yanhong Tian. The report showed that byapplying YAG laser energy could be used to improve the

wettability of Sn3.5Ag solder on copper pad. Dewan Tian

[7] concerned about assembling process that was also the

major products efficiency. By using three-dimensionalfinite element model to analyze the temperature

distribution and predict pitch motion of micro-sensor

component during laser soldering interconnection process,

the results was shown that the pitch motion was affectedby laser soldering technique which pitch angle in pre-

bumping process was smaller than that in reflow process.

Wei Liu [8] studied the sagging phenomenon of micro-solder joints fabricated by laser reflow process from the

experiment. The study results indicated that the sagging

phenomenon, happened after the laser reflow process,

came from many factors such as melting and coolingcharacteristics of the solder, pad and inter-metallic

compound, shrinkages of solidified solder and wetting

performance of solder. Yanghong Tian [9] expanded the

study by focusing on characteristics of Sn3.5Ag0.5Cu andthen compared with Sn3.5Ag solder bump by using both

simulation and experimental methods. It was reported that

shear strength of both alloys was comparable and laser

power obviously was not affected the shear strengths ofboth alloys. The formation of inter-metallic compound

growth followed temperature gradient in solder bump. And

with increasing the laser power and heating time, thecompound dissolved into the bulk solder bump.

In the meanwhile, there were many research projects

about solder droplet that similar to laser solder ball-jetting

process. Started by Lord Rayleight (1878), the mathematicmodels of uniform droplet formations from stream liquid

issuing from an orifice were described. The growth of

radically symmetric initial diameter disturbance on an in

viscid jet was investigated. In 1892, he extended his firsttheory to analysis viscous jets. However, due to the

complexity of the relationships made the theory lessinterest. Weber (1931) used similar approach as Rayleight

but his equation produced much more practicable results by

making simplified assumption. David B Wallace [10] usedboth theories to quantify the breakup liquid metal of

capillary. The results had shown that the oxygen had

significantly effects on molten solder metal jet breakup. Byusing both Rayleight and Weber theories, they could be

used to predict only on the magnitude of the radial

disturbance however both theories were not valid for the

liquid metal jet case. M Essien [11] studied solder jetdriving pressure pulse by using computational model for

magneto-hydrodynamic solder (MHD) jet. The results were

shown that the dispensed molten solder diameter wasdirectly related to the magnitude and duration of pressurepulse. In 2004, Hsuan-Chung Wu [12] used both

experimental and simulation method to develop model of

micro-inkjet base on piezoelectric solder drivingtechnology. The study covered the whole process from

droplet formation to droplet impact on the target. The

simulation model and experimental provided similar results

about droplet morphology, break-up time, flying distanceand droplet volume. Rajneesh [13] focused only on droplet

impact by using numerical technique to investigate the

impingement of liquid micro-droplet onto a grass substrate

at different temperatures. Rajneeshs model valid only foreutectic solder (63Sn-37Pb) but not for FC-72 and

isopropanol. Lou Jun [14] used similar technique as

Rajneesh to analyzed the mechanics of jet breakup forcedby Amplitude-Modulate (A-M) and found that frequency of

jetting was directly effects to jet breakup and it could be

predicted by the linear instability theory. D. W. Tian [15]

extended Hsuan-Chung and Rajneesh approach whichfocus on the groove impact. Tian investigated the process

of solder droplet impact on a groove with two-rectangular

pad by using 3-D modeling. The study scoped on

maximum shear stress location, gas film between droplet,and pads effect, system energy and velocity of solder

effect.

According to the literatures, the study of laser solder

ball jetting was widely focused on only the end products.There were few literatures focused on Laser Solder Ball

Jetting process. In this process, molten solder ball jetting is

the major concerned for jetting performance in term ofproduction. Thus it is necessary to understand about what

happen while the molten solder is ejected from capillary

tube. Whilst the behavior of solder jetting in this process

was still unclear, the objective of this study was to observemolten solder ball jetting phenomenon while it is ejected

from capillary in laser solder ball jetting process. The

observation would give a further understanding of the laser

solder ball jetting process.




2.

MATERIALS AND METHODSA lead free solder ball was first placed onto a capillary

tip which its diameter was smaller than the solder ball asshown in Figure 1. A laser unit with small spot diameter

was used for the heating process. The laser irradiates to the

solder ball to make it melt. Then inert gas pressure was

applied to push the molten solder out from capillary tip tothe target [2-3].

Figure 1: A solder ball was hold by capillary tip and laser

energy applied to melt the solder ball [2-3]

To observe the phenomenon of molten solder ball

jetting, a high-speed video system was used for capturingthe solder shape at any moment. The main components ofthe system are the solder jetting device with capillary and

the high speed camera with data acquisition system. A

schematic plot of the system is shown in Figure 2. Thehigh-speed camera (MotionPro X4, 5,000 frames per

second with halogen light source) equipped with a long-distance microscope was used to observe the jetting

phenomenon. The magnification of the microscope can be

adjusted so that the image could accommodate themaximum size of the jetting. The rapid motion image in the

solder jetting was captured with the MotionPro Studio

acquisition system [16] which was used to control the

beginning of the capture and save the captured images.

Figure 2: Schematic drawing of experiment setup

3. RESULTS AND DISCUSSION3.1 Solder Ball Melting

To observe solder ball melting behavior while it start

to move out of capillary tip using high-speed video, it wasstill unclear that the solder ball was melt completely while

it starts to move out from the capillary tip. From the

captured, there was color changing in the solder ball. Thecolor of solder ball changed from dark to light while it was

ejected from capillary as shown in Figure 3a) and 3b).

Changing of color means that the temperature of solder ball

was still increased [17] while it moves through a capillary

tip. The temperature in solder ball was rising up to glowingpoint after the solder ball was fully ejected from the

capillary as shown in Figure 3c). However, with the

limitation of measuring equipment on micro scale, theexact temperature was not able to be captured in this study.

3.2 Molten Solder ball velocitySince high-speed camera could not measure the molten

solder ball velocity, so, its velocity could be estimated by

using graphics method. Grid scale was generated cover the

images captured by the high-speed camera to estimate the

distance of the molten solder ball flying. With the highestcaptured rate of the high-speed camera that used in this

study, time period between capture shot to shot of 197second as shown in Figure 4.Then from using basic

physics theory, the velocity of particle moving can be

calculated by the equation:

12

12

tt

ssv

= (1)




a) b) c)

Figure 3: Solder ball jetting process 3a) The capillary with

solder ball before laser apply; solder ball is dark 3b) The

solder ball start to move out from capillary after laser

energy applied; color changing in solder ball 3c)

Completely melt solder ball jetting from capillary

Figure 4: Grid Scale for velocity calculation

The velocity calculation is applied for 3 ranges. D1 isthe distance from solder ball start ejected from capillary to

position P1, D2is the distance from solder ball at position

P1to position P2, and D3is the distance from solder ball atposition P2 to position P3. The velocity of molten solder

ball is shown in Figure 5. The molten solder ball velocity

equation provided by linear curve fitting as in Figure 5.The acceleration of the solder ball is the slop of the

equation, the value of 5.25 km/s2.

T3T2T1

3.0

2.5

2.0

1.5

Dimensionaless traveling time

Velocity(m/s)

95% CI for the Mean

Solder ball Velocity

y = 5,247.5x + 0.1773

Figure 5: Molten Solder Ball Velocity

3.3 Molten solder ball shape

To capture molten solder ball shape, the high-speed

camera needed to reduce light intensity. It was for reducing

light reflection from the molten solder ball. The resultsshowed that after the molten solder ejected from capillary,

the solder ball immediately formed to sphere shape as

shown in Figure 6. The high surface tension and high

viscosity may cause the formation of molten solder ballformed to sphere shape immediately after it move out from

the capillary. However, further investigation has to be

done in order to understand the fluidize phenomenon.

Figure 6: Molten solder ball shape after ejected from

capillary

4.

CONCLUSIONSIn this paper was to observe molten solder ball jetting

phenomenon by using high-speed camera to photograph the

solder ball jetting and using graphical calculations method

to estimate molten solder ball velocity after it was ejected




from capillary tip. The results showed that the solder ballwas not melted completely when it started moving out from

capillary tip; its temperature kept increasing after it was

ejected from capillary tube and reaching the completelymelted point, the glowing point, when the solder ball was

totally ejected completely from capillary tube. The molten

solder ball shape still is in spherical form while it wasflying. From the experimental results, the motion of molten

solder ball while it flying increased with acceleration of

5.25 km/sec2

5. ACKNOWLEDGEMENTThe authors would like to express deepest gratitude to

the scholarship of The Nation Electronics and ComputerTechnology Center (NECTEC), Industry/ University

Cooperative Research Center (I/U CRC) in HDD

Component, Khon Kaen University and Seagate

Technology (Thailand) Co., Ltd for providing financialsupport.

6. REFERRENCES[1] D.B. Wallace & D.J. Hayes, Solder Jet

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[5] Y.H. Tian, C. Wang and D. Liu,

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[10] D.B. Wallace, Capillary Instability of a JetLiquid Metal, Journal of Fluids Engineering,SEPTEMBER, Vol. 115, 1993 pp 592-532

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[16] Glenn Elert, Temperature of a Hot Object Red,www.physics.uwo.ca,3/07/2009

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The Observation of Solder Ball Jetting Phenomenon in Laser Solder Ball Jetting Process

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