Figure #1 shows the effect on shot size of lower fluid levels in a syringe

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1 New Piezo Actuated Dispensing Method for Consistent Solder Paste Dots Steven J Adamson, Alan Lewis, Dan Ashley, Brian Schmaltz, Jaynie Park Asymtek 2762 Loker Ave West, Carlsbad, California 92010, USA sadamson@asymtek.com Abstract Dispensing solder paste has always presented several problems for the SMT industry, problems such as wet-out, dot size, throughput, and clogging. When choosing a paste, particle grade size, the ratio of flux to metal content must be considered. The valve selection is also very important especially if small dots, with high accuracy, or high throughput are required. How a solder paste dispensing valve controls the flow of fluid is critical to the process, if consistent and repeatable results are required. In this paper the formulation of solder paste will be discussed and its impact on the choice of dispensing technologies. Also, two types of solder paste dispensing valves will be reviewed, leading to the rational for a design of a new, highly controllable dispensing valve based on piezo-electric drive technology. Solder Paste and its Impact on the Dispensing Process Solder paste comprises several metal alloys such as tin, silver, lead, zinc, bismuth, indium, and copper. Often there are other trace metals in pastes but these are the ones most often encountered. The density of the metals used in the paste and the metal loading of the paste can impact its flow through small diameter needles. For example Tin is lighter than Lead and therefore has a greater surface area for the same weight of metal in the paste. The greater surface area takes more flux to wet the paste to a similar viscosity as a lead based solder. No-lead formulations therefore need more flux to keep the paste viscosity low enough to flow through fine needles. Screen printable pastes can have a higher metal loading than dispensable pastes because the geometry of an aperture in a screen has very little surface contact area, compared to a needle. Ref #1 The grade of solder paste which is usually based on the particle size needs to be carefully considered. Finer particles have less resistance to flow through smaller needles. Therefore it is logical that if very small dots of solder paste are required, then a finer particle / grade solder paste is needed, at least type 4-6. However this has cost implications, because finer particle solder pastes are more expensive than pastes with larger particles. The length of a dispensing needle is proportional to resistance to fluid flow. Longer needles have more resistance to flow than shorter needles. For a small solder dot in a deep access cavity or adjacent to tall components, then a longer needles will be required. This complicates dispensing because not only is there greater resistance to fluid flow but there is increased delay between the start of fluid flow and shut off, making it more difficult to precisely dispense a small dot consistently. Dispensing Technologies / Valves Time Pressure Simple time, pressure, needle dispensing systems is used in many applications and they are perfectly adequate for non-critical applications. However they require constant adjustment by operators as the solid solder particles tend to compact at the base of syringe. At the beginning of a work shift the solder paste can be flux rich and later in the day can see flux squeezed from the paste. The paste getting more viscous and the assembly people have to make regular adjustments to either the air pressure applied to a syringe or the ontime of the air. When air pressure is applied to the fluid in a syringe, it forces the solder paste through the needle to dispense onto a board or component. However as the level of solder paste falls in the syringe, the air pressure now acts on syringe side wall. The syringe wall expands slightly, and absorbs some of the energy that was originally used to expel the paste from the needle. When the air pressure is removed the energy from the syringe side wall returns to the paste. See Figure #1. This is a major cause of drooling of paste after the fluid / air pressure is removed. This is most prevalent in manual dispensing tools where the air pressure is frequently cycled on and off. Figure #1 shows the effect on shot size of lower fluid levels in a syringe

2 All of these effects can lead to problems with inconsistent amounts of solder on a pad or lead. This in turn can be a cause of yield loss, reworks and a less reliable product. If too much solder is used, this a waste of material, which lowers the profitability of the company assembling a product. So while there is a very low investment in equipment for time pressure dispensing systems the overall costs can be hidden, and can be considerable. Augur Valves / Pumps When more control over solder paste dispensing applications is desired, auger valves are used to replace time pressure dispensing. Auger pumps use an auger that turns inside a cylinder that is feed from a syringe. These pumps are similar to the way that plastic extrusion screws on injection molding equipment work. There is a common misconception that these pumps operate on the principle of the Archimedes screw. In fact, the equation that governs the material flow is a complex relationship between the auger screw geometry, feed pressure, fluid viscosity, and nozzle/needle geometry. (See Ref#2) dispensing see figure #3. The auger needs to have enough clearance between the screw and the sleeve to allow the screw to rotate freely. However if the gap is to large, fine solder paste particles can get between the screw and sleeve, and be sheared. The sheared particles tend to be flat, break-off and/or bond with other particles and often result in a blocked needle. If the gap between the screw and sleeve is too narrow this can constrict the motion of the screw, making movement difficult if the run-out of the screw is not perfect then it can interfere with the sleeve side wall. Clearly an auger screw used for a type 3 paste may not work for a type 6 paste as the tolerances on the screw and sleeve may work in one case but not another. Figure #3 shows the problem of determining how large a gap should be engineered between the auger screw and the wall of the pump outer body Ref #3 Figure #2 Auger Pump One draw back to moving solder particles is that they are made from soft metals, which tend to be flattened or coined, if struck by some type of pumping action. The Auger screw applies pressure to the paste and is able to move particles with a very gentle motion, so as not to flatten the particles. If particles of solder become flattened they can lead to needle clogging and stoppage of production. Auger screws come in many different designs. The pitch, angle and depth of the screw can change the pumping action of the auger. Some augers use dual screw cuts and some change the pitch from the beginning of the screw to the end. Course pitch screws can move large amounts of material fast, while fine pitch screws are used for precise dispensing. The gap between the auger and the outer body or sleeve of the auger pump can also impact solder paste In addition solder paste is a thixotropic material and the viscosity of the material changes when a shear force is applied, meaning that the viscosity is dependent on the shearing rate, or how fast and how often the valve is running Let s assume that an auger screw is correct for the type of paste, and the thread count on the screw is ideal for dispensing a small dot. The next problem a dispensing company faces is the ramp up speed and down speed of the pump and how long it is powered for. In modern auger dispensing pumps, the pump has an encoder attached to the auger shaft. In this case the number of revolutions of the screw can be counted and in the case of very small dots where less than one rotation is required, the number of encoder counts. Modern pumps today can have upwards of several thousand encoder counts per revolution. Even with this kind of resolution dispensing companies still have to be able to ramp up the pump to speed and stop it on an exact encoder count. Clearly the ability to control the ontime of dispense pump is critical. As solder paste dots get smaller in diameter the surface tension of the needle tip starts to become a large factor in the ability to form a droplet and how it

3 breaks from the needle tip. Another consideration with needle dispensing is the distance from the needle tip from the substrate. Too large a gap and you can get stringing between the needle and the board. Too small a gap and dots get larger than desired. As most substrates are not flat, this requires constant measurement of the dispense gap. As can be seen from the above there are many parameters to control to achieve consistent solder paste dot. The ability to set up an auger valve and keep it performing consistently requires skill and experience. Turning on and off a motor to drive a screw or auger is time consuming and limits the speed that dots can be dispensed. In this paper we will describe a new piezo mechanism that can be operated at high speed to give highly consistent dot properties, at high speed, 20,000 dots per hour, and can typically do this with type four solder pasts or smaller. It can be used to make dots as small as 125 microns in diameter in eutectic or no lead solder pastes. Piezo Dispensing Tools Piezo dispensing is often associated with ink jet printers that are commonly found in offices and homes. There are several types of piezo actuation but one of the more common types uses a deflection of a piezo ceramic to squeeze fluid in a chamber through a hole or orifice plate. The droplet of fluid hits the substrate and the jet fluid is replenished from a reservoir. The reservoir refills the piezo cavity. These cavities can be built using one or two piezo elements. In this case in figure #4 it is showing one piezo ceramic element and the opposite side of the jet could be silicon, stainless steel or ceramic. The piezo ceramic only moves a few microns and typically there needs to be a large area where the squeezing forces are applied. While this technique is widely used in commercial ink jet printers and is now starting to see use in electronics manufacturing, it is severely limited in the viscosity of fluid that can be dispensed through the orifice. Fluids used with this type of Jet are often limited to a viscosity on no greater than 20 cps. Conventional solder paste is much too viscose to dispense with this type of jet. Piezo Ceramic Figure #4 Typical construction of Piezo ink jet New Piezo Dispense Method The new design for the piezo dispense valve (see Fig #5) modulates a fluid flow through the valve. Air pressure is applied to a syringe of solder paste at approximately 345kPa, this pushes the paste into the valve and provides the dispense force to move the paste from the valve to the board. The piezo actuator (see fig #5) can open or close the valve in less than 5 ms, which gives it a resolution about ten times better than an auger valve and provides a positive shutoff. Therefore it has the capability of dispensing very small dots of fluid / solder paste at high speed. Repeatability of the smaller dots is also improved since dispensing thixotropic Solder Paste through the positive shutoff Piezo valve does not have as high of a shearing rate as the auger valve. Needle Signal Solder Inlet Piezo actuator Sliding ceramic plate Figure 5 Piezo slider valve with cover removed on the right hand image Ink Reservoir Orifice The construction of the piezo valve uses two ceramic plates with a hole in each one. When the two holes are aligned solder paste flows through both holes, through a needle tip and onto a PCB pad. When the piezo stack is not energized one of the ceramic plate s moves into a park position, and the two holes are no longer concentric, and the fluid flow stops. Precision needle assembly tips and fluid pressure can be altered to optimize the dot size. See Figure# 6 Fluid Drops

4 Sliding plate Solder paste Needle assembly Holes in plates out of alignment no dispensing Holes in plates in alignment fluid is dispensed Figure #6, sliding disks in park and dispense positions With high air pressure pushing the paste and a high frequency valve, there is enough energy transferred into the paste to quickly form a dot. The ability to switch the valve from closed to open at a very high speed is a precise mechanism to control the volume of solder paste dispensed. Piezo Slider Valve Advantages Testing shows that piezo actuated dispense technology is capable of 256 micron diameter dots with +/- 8% variation at 3 sigma. To date dots as small as 150 microns have been dispensed with solder paste but with a larger standard deviation. The valve has been tested with numerous solder pastes using traditional leaded pastes and no lead pastes. The data shown below is representative of the capabilities of the new piezo valve Dispensing Test Results with a SAC Alloy Needle 27 gage (ID 0.21 mm) Solder SAC Alloy Sn 96.5, Ag 3.0, Cu 0.5 Metal content 86% Particle size microns Valve on 4.5 ms Fluid pressure 276 kpa Dispense gap 0.1 mm Retract distance 0.5 mm Retract speed 20 mm/sec Retract accel 7620 mm/sec^2 Substrate 96% Alumina Table # 1 Dispense results for three different dot diameters

5 Figure #7 shows solder dots 0.260mm dia As can be seen from the test data in table #1 the standard deviation on the dot size increases as the dot gets smaller. This is not unusual with any dispensing technology. Speed Comparison Tests Comparison tests have been performed to assess at what speed an auger valve can run to obtain similar results to the piezo valve. Both valves were optimized to give similar quality dots at the fastest speed possible. The best the auger valve was able to achieve was 11,250 dots per hour. The slider valve was running at a speed of 20,000 dots per hour. Solder Paste Dispensing Applications BGA & QFN Rework A common technique for adding new paste to a site that is being reworked is to use a micro-stencil for selective screen printing. Because it s a manual process, there are variations in the results. If the speed or angle varies when the micro-stencil is pulled off, the volume of paste on the pad will vary. Getting the stencil to fit onto a populated PCB with components placed in close proximity is another problem. Frequently, the outer perimeter of the microstencil conflicts with previously placed components. In addition every BGA with a different geometry, I/O count, or pitch requires a unique, individual micro-stencil. Employing a dispensing technology capable of producing paste deposit geometries that fit onto the component land pad is desirable. If the paste is dispensed rather than screen printed, the variation in volume is minimized and the result will be a successful rework operation the first time. Automating the rework process with a dispense platform removes operator-tooperator variation and eliminates the need for a large inventory of micro-stencils. Small Geometry Passives There are several scenarios where screen printing has limitations. Circuit boards with a large variety of component sizes, including small geometry passives (0402, 0201, 01005), can be challenging. The stencil thickness must be specified for the range of apertures dictated by each component s size. Larger components require larger apertures and thicker stencils while smaller components require smaller apertures and thinner stencils to ensure adequate paste transfer for each component size. When thicker stencils are used with smaller components, too much paste can be transferred onto the small pads. When thin stencils are used with large geometry components, there is the risk of depositing too little paste hence the need for stepped stencils. Many manufacturers screen print and use dispensing to put extra solder on a location for example RF Shield pad locations often require more solder paste. If only a few very small passives are used it may be more beneficial to dispense the dots after screen printing for the larger components. The piezo valve provides excellent control of the on / off time of a valve. Now that we have this degree of control of the valve our testing is indicating that other factors in the process, such as wetting of the paste to the substrate are having a large impact on the application. Solder dispensing has always been considered a slow operation. The new piezo valve is cable of increasing throughput significantly when compared to the traditional auger valve running at the same speed. With dot capability below 0.3mm, it can be used to put paste onto the smallest pads used with today s or larger components. The valve can be used with a variety of pastes No Lead or more traditional eutectic solders. The new piezo valve provides a significant improvement in control of solder paste for making small dots. It is also a faster operating valve when compared to auger valves and provides much greater consistency when compared to time pressure dispensing. References Ref #1 Is Your Solder Paste Dispensable, Eric L Austin, Alan R Lewis, SMT, July 1996 Ref #2 Engineering Principles of Plasticating Extrusion, Robert E. Krieger Publishing Company, Inc. Krieger Drive Malabar, FL Copyright 1970, Chapter 6, pp 217 Ref #3 Formulation Considerations for Automated Dispensing of Lead Free Solder Paste Alan Lewis (Asymtek Web page Articles & Papers)