NOVEL APPROACHES TO BENCHMARKING SOLAR CELL TABBING ...

NOVEL APPROACHES TO BENCHMARKING SOLAR CELL TABBING SOLDERABILITY

Rick Lathrop Franklin Advanced Materials 320 Circle of Progress Drive, Suite 102 Pottstown, PA USA 19464

Karl Pfluke Indium Corporation 34 Robinson Rd. Clinton, NY USA 13323

ABSTRACT: For crystalline silicon solar cell front contact metallizations, silver thick film formulations are ubiquitous. For backside contact pads, either silver or silver/aluminum formulations are common. The trend for back contact metallizations is towards low lay-down formulations, resulting in thin-fired films. Although there are many different reflow methods used to “string” cells together, the need for fast wetting, leach-resistant and well-adhered front and rear contact metallizations are common to all methods. Quantitative tests needed to be developed due to an absence of industry standard tests in order to accurately recommend a material’s compatibility with the module assembly process. Classic thick film solder pot leaching and wire peel adhesion tests do not do a good job emulating the solar module assembly process. Although more similar in process, SMT solderability tests also lack close correlation. To fill this gap, several solderability tests specifically designed for module assembly and cell metallizations have been developed and will be discussed in detail. These tests are, in fact, a hybrid of SMT and thick film tests, but are tailored for the solar module assembly process. Wetting assessment is accomplished by measuring the reflowed area and the height of a precise volume of solder using a confocal measuring system. For ribbon adhesion, manual and automated methods are compared, as well as various peel angles. From these studies, a ribbonattach method and adhesion test emerge as suitable for benchmarking contact metallization formulations. Recommendations on how to recognize and prevent silver leaching are also discussed. Keywords: Wetting, adhesion, silver leaching, metallization, tabbing ribbon

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INTRODUCTION

The growth in the photovoltaics industry has primarily been fueled by government incentives upon which the industry is largely dependent. As government incentives continue to drive industry growth, the market has become attractive to contract manufacturers (CMs) who, until recently, have been non-existent in the industry. Viewed as a commodity, there is an opportunity for CMs to reduce the cost of module manufacturing through improved equipment utilization, lean techniques, materials improvements, economies-of-scale, and improved soldering techniques. These are the basic building blocks and core competencies of the industry. This will allow the ODMs to maintain their focus on R&D, technology assessment and enhancement, cell manufacturing, and efficiency improvements. Almost all Tier 1 CMs are now manufacturing c-Si solar modules, with an expected 4.1 GW by 2014 according to IHS iSuppli. As CMs continue to play a major role in the industry, soldering methods and ribbon and metallization standards will be required. There are four types of PV modules being produced: copper-indium-gallium-selenium (CIGS), cadmiumtelluride (CdTe), amorphous silicon (a-Si), and silicon (Si). Si and crystalline silicon (c-Si) remain the most traditional and widely used technologies, holding nearly 80% of solar module assembly market. This paper will focus on the Si and c-Si type of module. The soldering process (cell interconnect) is considered the most critical process in module manufacturing. Cell interconnect is accomplished using an automated combined tabber/stringer (CTS) utilizing one of several soldering methods or simple hand soldering. Both processes can provide sufficient wetting of a given alloy and both process are proven reliable. The reliability of either process’s output can be measured by the peel strength of the ribbon that has been soldered to the cell metallization. Alloy selection, soldering profile, cell metallization and the mechanical properties of the

ribbon all play a role in the application and adhesion process. Another variable in this process is the flux selection (and application method) used to improve wetting and remove oxides at the cell metallization and the ribbon interface. One of the issues with this semi-new industry is the lack of industry-accepted testing/reliability standards for measuring the solder quality of the PV cells that have been strung together. The same also holds true for the bussing interconnects and the materials used for attaching and joining the cells together. Since RoHS and WEEE initiatives do not apply to the solar industry, many manufacturers use tin-lead (Sn/Pb) solder alloys for interconnects, with Sn60 and Sn62 as the popular choices [1]. Tin-silver (Sn/Ag) alloys are occasionally used, and some manufacturers are exploring the use of tin-silvercopper (Sn/Ag/Cu or SAC) alloys, specifically SAC305. Solder thicknesses on the base copper ribbon can be up to 1 mil (0.001") per side. By far, the most challenging attribute for module assembly is building a module that is guaranteed for 25 years. These modules do come with a 25 year warranty so measuring and testing the reliability of interconnects is a crucial part of the module manufacturing process. This paper will discuss some ways to measure reliable interconnects for different bonding methods. 2

STATE OF THE MARKET

2.1 Soldering Methods The interconnection of Si solar cells can be made utilizing the solder coating supplied on tabbing ribbon. The ribbon is used to carry current between cells, but it also forms a mechanical connection. The solder coating can be reflowed to bond onto the metallization of each cell by a few different methods. Conduction (such as hot bar, hot pin, hot plate, soldering iron, etc.) is used in many manual and automated tabbing and stringing operations. IR heating can also be used, mostly in automated equipment. Convection is unpopular due to

Published in the proceedings of the 26th European Union Photovoltaic Solar Energy Conference September 5-9, 2011 Hamburg

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slow throughput and is unrealistic for CTS. Examples of this would be belt ovens or batch furnaces. Although these technologies seem to be competing, many CTS machines use two sources of heat. Bottom side conduction may be used to supplement topside conduction or IR heating. Other combinations are common as well. 2.2 Solder Alloys Although lead-containing alloys (such as 62Sn/36Pb/2Ag and 60Sn/40Pb) remain most common for tabbing ribbon coating, there is still interest in Pb-free alloys. 96.5Sn/3.5Ag is a favorite for certain electronics manufacturing applications, but the >230°C processing temperature creates considerable cell stresses during module assembly. A series of Bi-based alloys (of which 58Bi/42Sn and 57Bi/42Sn/1Ag are most popular) offer low temperature alternatives to traditional leadcontaining solder coatings. This paper focuses on work with the 62Sn/36Pb/2Ag alloy. 2.3 Test Methods Most cell foundries have some form of an adhesion test, but there appears to be no industry standard as can be found in the more mature electronic sectors such as SMT and thick film hybrid. This is likely due to the explosive growth of c-Si in recent years. Since silver contact metallizations are thick film materials, some reflection on adhesion tests common to the hybrid industry may have value. One standard adhesion test for conductors is a wire peel test that dates back to the early 1970s and remains the same today. A pre-formed 20 gage tin-coated copper wire is fastened to a ceramic coupon with square pads of conductor paste under the wire. This assembly is dipped in flux then dipped in a solder pot. After the test coupon cools, the wire is then bent 90° to the surface and pulled to failure. Wires pulling out of the solder or ceramic failing are ideal failure mechanisms. Thick film to ceramic failure is generally defined as an inferior failure mechanism. A tabbing ribbon to wafer 90 peel test [2] either by design or tradition is very similar to the wire peel test described. Variations include a 135 (45) peel [3] and a 180 peel [4]. Soldering methods also vary from hand soldering [2,4] to a tabber-stringer using an infrared heating element [3]. The rate that the ribbon is peeled, ribbon width, solder alloy, solder thickness, and ribbon thickness vary widely. 3

temperature solder pot, and examining the surface [5]. The drawback to this method is that it is subjective. This same reference reported success by using a wetting balance. This method is quantitative, but requires careful sample preparation to only expose metallization to the solder globule, as any wafer surface touching the globule during the test will add buoyancy to the sample and counteract wetting forces. In the absence of a PV industry standard wetting test, one has been developed using the IPC TM-650 number 2.4.45 from the SMT sector as a baseline. The basic concept is to measure the degree of solder flow or spread on a metallization surface. This has been a standard in the electronics assembly industry for over 20 years. There are two geometric attributes that indicate good solder wetting. For a defined volume of solder, the post reflow height of the melted solder should minimize and the wetted area should maximize. This will result in a low

Figure 1: Wetting Angle wetting angle  as in Figure 1. By simply dividing the measured wetted area by the peak height of the reflowed solder, a “wettability index” can be determined. The larger this ratio is, the better the surface wettability. To accomplish this, a consistent volume of solder and flux must be deposited on the surface of the metallization being tested. More specifically the buss bar on the wafer as this is the surface to be soldered in stringing. The best way to accomplish this is to stencil print solder paste. The solder paste employed in this test is SMQ92J from Indium Corporation. The paste is 90% by weight of prealloyed spherical solder powder and 10% by weight of no-clean paste flux. Each solder powder particle is prealloyed. For all testing in this paper, 62/36/2 (Sn/Pb/Ag) alloy was used. The stencil used for this test is a laser formed 4 mil thick stainless steel material. Solder paste is printed with a metal squeegee blade through ten 1.4mm diameter apertures onto the center of a buss bar, leaving consistent disc shaped deposits as shown in the 3D scan in Figure 2. The next step is to reflow or melt the solder paste deposits. This is accomplished in an infrared reflow oven. Just as there is a need to have repeatable solder volume

SOLDERABILITY

In the broadest sense the term, solderability suggests the ease at which a solder joint can be made. This includes the wetting of the surface with solder and the adhesion of the solder joint. For contact silver metallizations, we associate good surface wetting and high peel adhesion with good solderability. This adhesion is highly dependent on the control of the soldering conditions, the sintered film fired microstructure, and the formulation of the contact paste. 3.1 Surface Wetting Surface wetting or simply “wetting” is the capability of the fired contact paste to accept solder under controlled conditions. This can be measured by fluxing a piece of wafer with a buss bar, dipping it into a fixed

Figure 2: Solder Paste Print on Buss Bar deposits, the time-temperature profile that the solder is processed in must be repeatable. This is best accomplished with a belt driven tool that brings the wafer and solder paste up to reflow temperature together and controls how long the solder is liquidus. The time above 179C is kept below 10 seconds and the peak temperature between 200C and 210C to avoid leaching or dewetting. An ideal profile for this test is in Figure 3.


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After reflow the deposits are de-fluxed with acetone. This is because we only want to measure the solder geometry as the confocal sensor utilized will include the clear flux residue when it measures solder area and height.

Figure 5: Multi-Angle Peel Fixture thermal goals to the one described for wetting. The main difference would be that the profiling thermocouples would be buried under the tabbing ribbon on the wafer. It was noticed that there was a significant difference in profiles depending on the wafer design. This was attributed more to the differences in the IR absorption properties of silver, aluminum, and silicon, than due to differences in thermal mass. A typical adhesion test ribbon reflow test profile is in Figure 6.

Figure 3: Wetting Test Reflow Profile To measure the wetted area and solder height we use a Cyber Technologies Vantage 3D with a blue laser confocal sensor. The wafer is secured on a vacuum stage during the measurement process. Each of the 10 reflowed solder paste deposits is raster scanned to produce a 3D image. The image (Figure 4) is normalized (de-tilted) using reference cursors (green) placed on the silver buss surface. A 3D edge detection algorithm places a 25-sided measurement polygon (red) around the deposit and computes the area as well as the peak height of the deposit. The wetting index is calculated by dividing the area by the peak height. The higher the wetting index, the better the surface wetting. From this test we can quantify surface wetting, and compare different materials and the wetting effects of different firing temperatures and fired thicknesses. Strong wetting should correlate to ease of soldering and a wide stringing process window, but does not ensure that there is adequate adhesion of the metallization to the wafer. Figure 4: Solder Area 3.2 Adhesion Testing With the absence of an industry standard method for adhesion testing, and the goal to have a repeatable and relevant method of test, reported methods were tested and, when possible, improved. Goals for the adhesion test were to use industry standard ribbon, have maximum contrast in results between materials, have a total solder time of less than one minute, test contact metallization’s in-situ on the wafer, and demonstrate the least variability among alternatives. To begin work in defining an adhesion test method, a fixture was designed that will hold any size wafer at either 90, 135, or 180 with respect to the ribbon pull direction, as in Figure 5. This fixture mounts to slide bearings to permit free movement in both X and Y directions for initial positioning and to maintain the peel angle during the test. A reflow profile was then developed with similar

Figure 6: Adhesion Test Reflow Profile In addition to developing a reflow profile, a tip-controlled soldering iron was purchased with a chisel tip designed for ribbon stringing. As with every soldering process, flux is required. For the hand soldering experiments a low-solids no-clean flux (GS-5454 Tabbing Flux from Indium Corporation) was used. For the reflow process a tacky flux was used (Indium Corporation’s TACFlux 020). The reason for the tacky flux is that the ribbon is not held against the wafer in any way during the reflow method. A thin film of tacky flux ensures that the ribbon is in close contact with the wafer buss metallization until soldered. The tacky flux is also a no-clean formulation that is printed onto the wafer with a steel squeegee and a laser-formed stainless steel stencil that is 3 mils thick. With all of the tools in place, the first experiment was to compare hand soldering to reflow soldering. Twelve wafers were hand soldered and twelve more reflowed soldered. All wafers were peeled at 180 at a speed of 200mm per minute. Peel force was recorded in newton’s for the entire rip. All force data was exported into Excel. Approximately 2500 data points were collected for a 156mm wafer. The data was sorted from most to least and the average force of the top 80% of the data points was calculated. Initially the top 60% and gross average


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were also calculated, but it was found that all three figures tracked the same, preserving order of rank in experiments. The results indicated slightly higher median peel strength for the hand soldered, but with a wider

Figure 9: Reflow Solder Adhesion

Figure 7: Manual vs Reflow Tab Soldering distribution as can be seen in Figure 7. This was expected in that a manual and an automated method of soldering are compared. In the manual method, the operator is first looking for soldering to occur, then is moving up the length of ribbon, while trying to maintain constant pressure and speed of the tip. The soldering iron is controlling the tip temperature, but at a much higher peak temperature than the profile in Figure 6. Higher peak temperatures combined with potentially longer liquidus times will result in silver leaching. In hybrid applications of thick film, pure silver metallization’s are rarely used; palladium and/or platinum are added for leach resistance, which is not practical for cost-sensitive PV applications. In reflow soldering, the entire length of ribbon is soldered at once, controlled by the belt speed and zone settings very much like the firing process, but at a much lower temperature. In extreme situations the manual solder operator may leach (too long a solder liquidus time) the silver, resulting in low adhesion, as in the rip in Figure 8. Note the failure mechanism differences. Figure 9 shows a rip using the reflow solder method.

Figure 10: Various Peel Angle Comparisons

Figure 11: Ribbon View of Peel Angle Failures

The failure mechanisms of the 90 and 135 peel angles are very similar to large sections of the wafer removed. The 90 and 135 peel tests seem to test the strength of the silicon more than the paste [5]. Although these tests may have value in revealing a catastrophic adhesion failure, they serve little value in benchmarking different formulas and firing profiles. Figure 8: Hand Soldering Adhesion The next experiment was to compare all three peel angles. Twelve replicates were assembled using the reflow method for each peel angle tested. Pull speed was maintained at 200in/min. In Figure 10, the highly contrasted results can be seen. The ribbon view failure mechanisms photographed in Figure 11 seem to correlate with the contrast of the peel force data.

3.3 Balancing Wetting and Adhesion In the perfect world, the mechanisms responsible for a contact paste’s surface wetting and peel adhesion would be completely decoupled so that each could be maximized independently. The reality is that they are inversely coupled. To demonstrate this, an experimental paste was compared to a commercial product. The experimental paste “A” on the left is a pure silver, relatively high solids back contact metallization with no adhesion promoters and only a small amount of inorganic binder. Paste A has basically no adhesion but exceptional surface wetting. To the right is LunAg 699-HC2, a very


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Figure 12: Surface Wetting and Adhesion low solids, low laydown back contact metallization with exceptional adhesion and sufficient surface wetting. Basically, adhesion promoters and inorganic binders do not solder well. Their inclusion must be balanced with surface wetting. The move to low laydown/high coverage back contact pastes has presented a challenge to formulators who want to retain both adhesion and surface wetting due to the very thin fired thickness of these materials. All of the adhesion results in this paper utilized the LunAg 699-HC2 back contact metallization at a fired thickness of 4-6 microns. 3.4 Aged Adhesion In developing these tests, all of the adhesion work has been done within an hour of soldering the ribbon. To get a view the effects of time on adhesion, a preliminary

Figure 14: Loss of Adhesion from Leaching This substantiates the criticality of controlling solder peak temperature and liquidus time. In order to verify that the loss of adhesion was due to silver leaching, the surfaces of both the wafer and the ribbon were examined with SEM and EDS. Figure 15 shows a typical 180 peel test failure. There are silicon rip-outs on the ribbon and small craters in the silicon surface. There are areas of solder and thick film on both the wafer and ribbon. This is a mixed mode failure where no one material is predominant. Just the opposite is the case with a leached metallization as in Figure 16. On the wafer side a few islands of thick film can be found, but the majority of the area is the wafer surface with trace amounts of adhesion promoter and inorganic binder. The wafer surface is dark grey to the eye, noticeably different than a normal failure. The silver is almost all on the ribbon side of the fracture. There are typically no silicon rip-outs on either surface.

Figure 13: Aged Adhesion Increases study was conducted to look at aged adhesion. Two groups were assembled, one using hand soldering and one using reflow soldering. Half of each group was peel tested within an hour of soldering, and the other half was stored at room temperature for 7 days then peel tested. The 180 fixture was used for both groups. As can be seen in Figure 13, the adhesion increased in both hand and reflowed groups. Further work on aged adhesion will reveal whether this increase occurs in the first 24 hours as previously reported [4]. 4

Figure 15: Normal Peel Failure Mechanism

SILVER LEACHING

In developing these tests, we encountered silver leaching when the liquidus time for the solder was more than 12 seconds. The solder on the tab has 2% silver in its alloy to minimize, but not prevent, leaching. To demonstrate this deleterious condition we ran several experiments (A-F) in two similar reflow profiles. Belt speed in the reflow oven was kept constant, but the peak reflow temperature was raised to increase the time the solder was liquid. The melt point for 62/36/2 solder is 179C and the alloy is eutectic. The significant loss of adhesion in the experiments reflowed with a long liquidus time of 14 seconds can be seen in Figure 13.

Figure 16: Leaching Peel Failure Mechanism


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CONCLUSIONS

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The market is primarily using lead-containing solder alloys as there is no current legislation mandating the move to lead-free as in the electronics assembly industry. Surface wetting can be quantitatively assessed by dividing the wetted area by the solder height of a consistently deposited solder volume. Hand soldering is too variable for benchmarking materials. Although soldering iron tip temperatures are controlled, the contact time and pressure on the ribbon cannot be controlled. A peel angle of 180° is preferred over lower peel angles in that it better isolates the metallizations adhesion. Leaching can destroy adhesion; careful control of the time-above-liquidus (TAL) can prevent this. For 62/36/2, a TAL of less than 10 seconds is recommended. Well-adhered back contact metallization pastes with thin fired films in the 4-6 micron range are commercially available.

REFERENCES

[1] Pfluke, Karl: “Photovoltaic Module Assembly Using SMT Materials and Processes” [2] Carroll et al: “Advances in PV Metallization Technology”, Proceedings 20th EU PVSEC, 2005 Barcelona [3] Bӓhr et al: “On Lead and Cadmium Free Metallizations for Industrial Solar Cells and Modules”, Proceedings 21th EU PVSEC, 2006 Dresden [4] Moyer et al: “The Role of Silver Contact Paste on Reliable Connectivity Systems”, Proceedings 25th EU PVSEC, 2010 Valencia [5] Moyer et al: “Solder Wetting Measurement of Back Contact Paste Using a Wetting Balance”, Proceedings PVSC 34th IEEE, 2009 Philadelphia


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