TECHNOLOGIES FOR APPLYING FLUIDS IN SEMICONDUCTOR PACKAGING

TECHNOLOGIES FOR APPLYING FLUIDS IN SEMICONDUCTOR PACKAGING Alec J. Babiarz Asymtek Carlsbad, CA, USA ajbabiarz@asymtek.com ABSTRACT Jetting fluids in semiconductor packaging and assembly has become an enabling process. Jetting underfill for flip chip semiconductor packaging allows tighter packaging of multiple chips on a substrate because the fillet wet out area is smaller. Since the rate of applying underfill fluid for the same fillet size is faster with jetting, the throughput is higher than needle dispensing. In some cases needle dispensing and even stencil printing are not able to provide an adequate or cost effective means to apply adhesives, fluxes or other fluid electronics materials. Jetting fluxes may also allow thinner, more consistent and accurate film builds than stencil printing. Jetting has significantly reduced material costs in the underfill process for flip chips and CSPs. Keywords Capillary, Jetting, LED, Splashing, Underfill, CSP INTRODUCTION The traditional means of applying fluids in semiconductor packaging and electronic circuit board assembly has been with needle dispensers. Fluids are typically packaged in a container such as a syringe or cartridge and attached directly or through a tube to a valve or pump. A needle is attached at the outlet end of the valve or pump to direct the flow of fluid to the During application, the needle must be held in close proximity to the surface of the substrate to allow the fluid to wet the surface. When needle dispensing is coupled to an XYZ robot, the speed of moving the needle across the surface and the gap between the needle and surface must be maintained accurately to obtain high quality dispensing. Jetting fluids and adhesives instead of needle dispensing avoids quality issues associated with needle dispensing. The primary enabling feature of jetting is that the distance from the surface to the substrate to the jet nozzle does not need to be maintained as accurately as a needle. Secondly, the jetting heads can deliver higher volumes of fluid in finer streams than what can be extruded from a needle. Therefore the inherent throughput of jetting is faster than needle dispensing. JET STREAMS AND BREAKOFF Another unique feature of jetting adhesives is the fact that a viscous fluid will not immediately from a sphere once ejected from a nozzle such as observed in ink jet printing. The typical jet nozzle s inside diameter may be between 250 microns to 50 microns. Since a viscous fluid maintains the stream shape the diameter to the ejected fluid may closely follow the nozzle diameter. This fact allows jetting underfills with small fillets and into tight slots. Most fluids used in semiconductor packaging or electronics assembly are greater that 100 cps. At these viscosity ranges the surface tension forces are much smaller than the viscous forces. Therefore, a jet of fluid ejected from a nozzle remains in cylindrical from for a longer period of time before forming a sphere. If the nozzle is close to a surface, the stream never develops into a sphere prior to hitting the The physics of droplet formation on inviscous fluids is well known. The breakup of a stream into droplets is called Rayleigh jet instability. A cylinder of inviscous fluid will want to form a sphere because a sphere contains the highest volume of fluid with the lowest surface area, therefore the lowest energy state for a surface and volume. See figure 1. Figure 1. Capillary Instability: The force γ/r forces fluid from the throat, leading to collapse. γ = surface tension and r = radius of sphere. Viscous fluids are more of a mathematical challenge and beyond the scope of this paper, however given enough time any cylindrical fluid will want to form a sphere. In figure 1 u is the relative axial fluid velocity. This axial velocity depends upon the viscous and inertial forces on the fluid. The driving force is the capillary force derived from the fluid surface tension and the radius change due to a disturbance. A disturbance is necessary to initiate the

droplet formation. When r becomes larger than R, the capillary force developed by γ/r is significant to cause an instability and droplet formation. Rayleigh s analysis determined that instability in non viscous fluids will occur at a wavelength of 4.51 X 2R. This means that a 50 micron diameter cylinder of fluid could reach a maximum length of 225 microns before breaking up into spherical segments. Rayleigh also determined that viscous fluids could extend much longer before breaking up into droplets 1. Later work on Rayleigh instability for viscous fluids has shown that the maximum rate of instability may be correlated to the Ohnesorge number 1. Oh # = (ργd) 0.5 /μ = (Reynolds #) 0.5 Where: γ/μ = Capillary velocity μ = fluid viscosity γ = fluid surface tension ρ = fluid density D = Diameter of cylinder Figures 2 through 4 show high speed camera photographs of a jet stream of a viscous fluid exiting the jet orifice. In most applications of jetting adhesives the jet nozzle is 0.5 to 5 mm away from the And, the velocity of the fluid is between 1 to 10 meters per second. At those distances and fluid velocities, a viscous fluid does not have time to form a droplet before impacting the However, since the stream velocity is typically much faster than the XY translation speed, the stream forms a dot on the of 75 microns from the surface and at least at high as the largest filler in the fluid being dispensed. In semiconductor packaging applications, there may be 10 substrates in a boat, each with a flip chip that requires underfill. The boat may be 300 mm long. Therefore a height sense would be required at each position to obtain accurate dispensing. Since a jet can dispense from 500 microns to 2000 microns from the substrate, only one height sense is required. Since it may take 500 milliseconds to height sense, this can be a significant amount of wasted cycle time. Also, on long dispense lines, a substrate may be warped to such an extent that the needle gap varies so much that fluid is not dispensed properly along the length of the line. Needle dispensing requires that the fluid be concurrently in contact with the needle and the This leads to three quality and accuracy issues. At the beginning of a line, a needle must wait until the fluid begins to flow and then the XY mechanism may begin its move. At the end of the line, the flow must be slowed and cut off in unison with the XY mechanisms deceleration to prevent a bulbous end point. Last, as the needle lifts up to move to the next dispense point, some fluid remains on the needle. The amount of excess material left on the needle is dependent upon the dispense gap at the end of the line and the amount of plowing that occurred. Plowing occurs if the needle XY speed is slower than the fluid flow out of the needle orifice. Jetting does not have any of these problems because the fluid is jetted from the nozzle and impinges upon the Figure 2: Figure 3: Figure 4: In the case of low viscosity fluids, the stream would collapse into a sphere rapidly. See Figure 5. JET VERSUS CONTACT NEEDLE DISPENSING Some of the advantages of jetting over needle dispensing are as follows. 1. Non contact dispensing. a. High tolerance for dispense gap variation. b. No concurrent needle & surface wetting. 2. High flow rate at small steam diameters. 3. High clearance from dies or other components Non contact dispensing eliminates many of the difficult to control variables associated with needle dispensing. When dispensing dots of fluid, the fluid exiting the needle must make contact with the substrate immediately in the vicinity of the needle orifice. Typically this dispense gap is ½ the needle inside diameter. Therefore a 150 micron inside diameter needle would have to be positioned to a maximum The flow rate from a jet nozzle is less restricted than the flow rate through a dispensing tip. The flow through a tube is given as follows: 4 πdn Q= ( P2 P3) 128μn ln Where: d n = inside diameter of needle or nozzle μ n = fluid viscosity l n = length of needle or nozzle P = pressure drop Q = Volumetric flow rate

In this case for a given inside diameter d the length of the needle is 5 to 10 times longer than a nozzle orifice. Consequently at a constant pressure source the jet nozzle will flow 5 to 10 times more fluid. The reason this is an advantage is that the jet can put down a fine stream of material faster than a needle that extrudes the same diameter of material. Also, if the needle s inside diameter is the same diameter of the jet nozzle the equivalent needle s stream size is the ID of the needle, but the needle must be held close to the surface which means the OD of the needle must fit between the die or a safe distance away from the die. Since the fluid wets the outside of the need, the wetted area is actually larger then the outside diameter of the needle. See Figure 7. Figure 9 Needle path is not always square at corners & can damage the die New technologies bring new challenges. When jetting materials it is important to provide enough energy to quickly extrude an adhesive at approximately 5 meters a second and provide break off, but not provide so much energy that the drop or stream impacts the surface with enough energy to cause a splash. SPLASHING Several papers have been written around the physics of drops impinging upon a dry surface as well as a thin film of liquid 3, 4, 5. The propensity for a drop to splash upon impact is estimated by an experimental approximation called the Sommerfeld Parameter. It has been shown that the probability of splashing is high if the dimensionless Sommerfeld number is greater than 50 4, 5. Therefore the jet can put down finer fillets and lines much faster than a needle. In fact jets are used to put material between die as close as 350 microns without wetting the top side of the die. See Figure 8. Figure 8 30g needle 25μ space Jet Stream 75μ space Further, it would be impractical to do the same applications with a needle. A 30 gage needle has a 105 micron inside diameter and an outside diameter of 300 microns. Such a needle would have 25 microns of space on either side of the needle. A 100 micron jet stream would have 3 times the space margin at 75 microns. Needles can touch and destroy the die in normal underfill applications. Since the jet is moving above the die by at least 0.5mm there is no chance of a collision. See figure 9. Sommerfield Parameter = We 0.5 Re 0.25 Weber # = We = ρu 2 D/γ Reynolds # = Re = ρud/μ μ = fluid viscosity ρ = fluid density γ = fluid surface tension U = drop velocity D = drop diameter Splashing is not well understood but is initiated by instabilities and perturbations that occur at the point of impact. Splashing depends on other conditions such as surface roughness, surface and fluid temperature, surface conditions, and the dynamic surface wetting angles 6, which may be different while the fluid is advancing versus retracting. Upon retraction the surface is already wet, but there is a capillary line force restraining retraction, while upon expansion the capillary line force may be helping or hindering depending upon the wetting angle. Analyzing typical fluids used in underfilling and calculating the Sommerfeld Parameter over a range of velocities from 1 m/s to 10 m/s and viscosities from 10 cps to 10,000 cps as shown in Figure 10. This shows that jetting typical underfill materials lies in a safe zone where it is probable that splashing will not occur.

Figure 11 Flux edge Containment Line FLUX JETTING The contact pads must be fluxed prior to flip chip attach. There are several methods in the industry ranging from dip fluxing, stencil printing, needle dispensing, spraying and jetting. Generally the least amount of flux applied is the best amount. In most assembly processes it is desired to apply a sticky flux. The sticky flux holds the flip chip in place while the substrate is moved to the reflow oven. Upon reflow, the melted solder bumps are self centering. Noncontact jet fluxing is used to apply thin layers of flux. Jetting the thin fluxes will apply a thinner and more uniform deposition than any other application process and is the preferred method. In the process of jet fluxing a swirl and curtain of air surrounds the jet stream. The air helps contain the flux aerosol and helps force the flux down to the surface for good wetting. If there is too much energy applied during the jet fluxing the flux may move along the surface. To prevent and capture the flux along a straight line for good edge definition, two parallel lines of flux may be jetted without swirl air within the application area. The bands of flux lines contain the spray as it impinges the surface thereby providing excellent edge definition. See Figure 10. Underfill Applications As is well known in the industry, flip chips must be underfilled with an adhesive. The purpose of the underfill adhesive is to prevent strain on the solder bumps due to thermal mismatch between the silicon die and the substrate material. Flip chip devices typically have ball diameters of 125 microns or less. The strain on a ball is directly proportional to the ball height. The strain is determined by the difference in thermal coefficients of expansion between the die and substrate material. Each time a device is thermally cycled the bump undergoes strain. The number of thermal cycles to bump failure is inversely proportional to the square of the strain. Consequently, underfill is used to prevent bump strain. The underfill adhesive completely encapsulates the solder ball, which prevents bump from moving (strain) and the epoxy shrinkage keeps the ball in hydrostatic compression which also helps prevent crack initiation. This underfill process is called First level underfill. The customer use model for handheld and mobile products requires that the product survive several drops. The typical user of a cell phone will drop the phone at some time in the product s life. Consumers expect a dropped phone to work. Most mobile electronic products contain CSPs or BGA packages. These packages have bump heights that are typically 5 to 10 times larger than flip chip bumps, therefore a CSP can last 25 to 100 times more thermal cycles. However, the larger mass and size of CSPs makes the package vulnerable to excessive bending strain that occurs when a product is dropped. Consequently, underfill adhesive is applied to CSP packages to provide mechanical stability. This process is called secondary underfill. First and secondary underfills are applied during assembly by an automatic dispensing robot. As mentioned earlier, jetting allows less material in the fillet due to the narrow stream of fluid. The material savings can be significant enough to pay for automated jetting equipment, even if the underfill is being applied by low cost hand held needle dispensing processes.

Table 1 shows data from 4 types of CSPs in used in a mobile device. CSP Size (mm) I/O Ball Size 8x11 88 I/O 0.375m ball 10x10 100 I/O 13x13 288 I/O 15x15 550 I/o Area under CSP mm 2 Needle Disp mg. Jet Disp mg Savings mg % savings over needle 88 60 26 34 57% 100 60 26 34 57% 169 90 46 44 49% 225 135 106 29 31% 3. Christophe Josserand and Stephane Saleski, Dorplet splashing on a thin liquid film, Physics of Fluids, Volume 15, Number 6, June 2003 4. M. Bussmann, S. Chandra, and J. Mostaghimi, Modeling the splash of a droplet impacting a solid surface, PHYSICS OF FLUIDS, Vol 12, Number 12, December 2000 5 C. Mundo, M. Sommerfeld, and C. Tropea, Droplet wall collisions: Experimental studies of the deformation and breakup process, Int. J. Multiphase Flow 21, 151 (1995) Figure 12 shows a comparison to actual material savings achieved at a manufacturing site after switching to jet dispensing versus needle dispensing and the calculated material savings based on an analytical volume model. Figure 12 CONCLUSION The advantages of jetting adheseives over needle dispensing has enabled more economical and practical packaging solutions that utilize adhesives. In equivalent needle geometries, jetting allows higher throughput, greater margin on dispense gaps and tighter spacing guidelines, and greater material savings. REFERENCES 1. T. Funada, D.D. Joseph/J. Non-Newtonian Fluid Mech. 111 (2003) 87-105 2. A. Babiarz, Jetting Adhesives and other materials for semiconductor and electronic component packaging. Pan Pacific Conference, SMTA, Feb 2007.