Low Temperature Lead-Free Production Versus SAC

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1 Low Temperature Lead-Free Production Versus SAC Designing a product that has minimal environmental impact for its entire life cycle is very demanding and a real challenge for designers and manufacturers. The ultimate goal is to develop a sustainable world. In addition to complying with EU RoHS and China RoHS, the unsung mission of the conversion to Lead-free electronics is to improve on 63Sn37Pb performance in an effort to meet future electronics performance needs. The main constraint in implementing Lead-free electronics is avoiding raising the process temperature above that which has been established in the SMT manufacture infrastructure over the past 26 years, thus eschewing any likely disruption and uncertainty. In such a way reliability uncertainty can be avoided, production yield is assured and energy is conserved. Due to voluminous data, it is the intent of this article to present low temperature Lead-free production in the areas deemed most relevant to production. There is also a comparison of the low temperature solder alloy s performance with SAC as observed over the last few years of production. Why a low temperature process is essential Due to the complexity of PCB design and the versatility of materials in the makeup of a PCB assembly, a higher process temperature may result in various potential mishaps or problems. These problems may be exhibited on the production floor or revealed during the service life of the PCB (Jennie S. Hwang, Lead-free Implementation: A Guide to Manufacturing ). Possible disadvantages or hazards can be seen in the box above. Materials and process options Combining manufacturability and long-term reliability, the top three performance parameters of a solder material are the melting temperature of the alloy; the intrinsic wetting ability of the alloy on typical metal substrates such as Cu and Ni; the mechanical properties of the alloy and its behaviour under cyclic strain conditions. by Dr. Jennie Hwang, H-Technologies Group Figure 1 Relative intrinsic wetting ability of viable Lead-free alloys Potential problems caused by higher process temperatures PCB de-lamination; blistering; warpage; thru-hole barrel crack; discolouration Connector plastic housing changes (dimensional, functional) Wave soldering disadvantages (dross, energy consumption) Effects on flux residue cleanabil ity, if water-soluble or solventcleaning flux is used Effects on large PCB assembly during reflow Effects on IC components; aluminium electrolytic capacitors; MLCC; internal solder joints of modules or packages; BGA (solder balls drop at package-solder interface) Component co-planarity (BGA) and moisture sensitivity Effects on flux chemistry Higher energy cost Residual stress Others As established, proven SMT manufacturing processes comprise a reflow peak temperature of below 235 C with general range of 215 C to 235 C, and wave solder pot at C. This means that the solder joint alloys that can be reliably reflowed below 235ºC and wave soldered around ºC can serve as a replacement for 63Sn37Pb without requiring any changes to the process, boards or components. Nonetheless, there are options with regard to solder alloy materials (Jennie S. Hwang, Environment Friendly Electronics: Lead Free Technology ) and therefore process options. In addition to maintaining the low temperature soldering (drop-in) for Leadfree, a higher process temperature is being adopted, which may require modifications to components and/or equipment. A sound manufacturing approach relies on the compatibility OnBoard Technology April page 30-8-

2 between process and material. They must work together. In other words, the categories of materials that have a melting temperature above 210 C are expected to work together in the modification process, mainly a higher process temperature. Materials with a melting temperature lower than 210 C are expected to work well with the drop-in process. Table 1 outlines the viable alloys and their corresponding melting temperatures, the recommended reflow peak temperatures and the recommended wave pot temperatures. Table 2 outlines mechanical properties of viable Lead-free alloys. Equally importantly, Lead-free solder alloys must possess characteristics that are compatible with practical manufacturing techniques, other constituents of the system and the enduse environment. The understanding of practical and process parameters in relation to the fundamental alloy properties is paramount to the success of Lead-free implementation. Specific attention should be drawn to the ability to accommodate a wide array of assemblies and applications. Attention should also be paid to the capacity to absorb the inherent fluctuations in SMT manufacturing so that the necessary process window required in the mainstream SMT manufacturing is provided. (specifically designed compositions) has been carried out successfully under existing reflow profile parameters without increasing the process temperature; thus, no changes in components and boards materials are needed. Intrinsic wetting ability comparison Figure 2 - Solder joints made of SAC305 exhibited surface cracks under a set of designed temperature cycling parameters (-25 C to 85 C, 2-4 hours dwell, 200 cycles) Figure 1 illustrates the relative wetting ability of the viable Lead-free alloys using the established Wetting Balance method per J-STD-002 (time to cross the zero force). The results show that the designed quaternary alloys provide an enhancement not only by lowering the alloy s melting temperature but also by increasing the intrinsic wetting ability over SAC alloys. In a practical temperature range tested, the results indicate that the low-temperature alloys are expected to impart better wetting ability under the existing SMT process conditions than SnAgCu, SnAg or SnCu eutectic. SnAgCu, SnAg or SnCu eutectic alloys are also expected to require a higher process temperature, as clearly indicated by their melting temperatures in Table 1. Both lower Figure 3 - thermogram of SnAgCuIn solder doped with both Bi and Pb, indicating the absence of detectable 96 C phase LEAD-FREE FOR LATE ASSEMBLY BIRDS For several years, high-volume production using the quaternary SnAg- CuIn, SnAgCuBi, and SnAgBiIn alloys PROBLEM: Overheating: Increased Power Decreased Package Size SOLVED: Solder TIM for Heat Dissipation Flux-Coated SAC Preforms Compliant Alloys Thermal Reliability Problem? Solved: reliability@indium.com Q U A L I T Y A S S U R A N C E OnBoard Technology April page 31

3 melting temperature and better intrinsic wetting ability facilitate high yield low defect production. Aging tests For comparative purposes, all viable alloys were subjected to aging tests at +125 C and 20 C for 96 hours, respectively. They were subsequently measured in terms of strength, strain and modulus properties. Table 3 and Table 4 summarise the test results of SnAgCuIn, SAC305 and 63Sn37Pb in strength and strain after aging for 96 hours at 20 C and +125 C, respectively. The results indicate that SAC305 exhibited a significant degradation in Maximum Percent Strain after aging at 20 C for 96 hours, in comparison with SnPb and SnAgCuIn. Under high temperature aging at +125 C for 96 hours, SAC305 showed lower maximum tensile strength than SnPb and SnAgCuIn. Temperature cycling tests A variety of testing parameters were performed in reference to industry standards in conjunction with specially designed parameters by specific manufacturers. Details of each test will not be included in this presentation. For example, under a set of designed temperature cycling parameters (-25 C to 85 C, 2-4 hours dwell, 200 cycles), the solder joints made of SAC305 exhibited surface cracks as shown in Figure 2, while the solder joints made of V349 (SnAgCuIn) remained intact. Under another set of temperature cycling parameters (-55 C to +125 C, minutes dwell), SAC305 (SnAgCu) showed solder joint cracking at 750 cycles, while V347 (SnAgCuIn) did not show any at 1000 cycles. BGA solder ball drop with SAC solder paste It was observed that, in some cases, BGA solder balls made of SAC alloy experienced package solder ball dropping during reflow using SAC solder paste. A similar effect also occurred with BGA solder balls made of 63Sn37Pb alloy when using SAC solder paste. Although the detailed mechanism occurring in the process and the corresponding remedies may vary with the specific component, PCB and process, the primary cause is largely associated with the high reflow temperature required by SAC solder paste. Through-hole solder joint fillet Due to the metallurgy of SAC alloys, it was observed that through-hole filling can be inadequate or marginal on some occasions. This problem was eliminated by running a low temperature solder alloy such as SnAgCuIn. Whisker test In an attempt to promote whisker growth, V347 and V349 (SnAgCuIn) solders were subjected to whisker testing environments in accordance with ineme s recommendations. No whiskers were detected. Drop test PCB assemblies made of V349 (SnAg- CuIn) were inserted into their casing and a drop height was set at one metre above ground level. The assembly was then dropped onto the concrete floor. Under inspection and examination, no failure in the solder joint made of V349 was detected (some failures occurred at the Copper pad de-lamination). In contrast, SAC305 experienced solder joint cracking. Effects of Bi inclusion In order to verify that any Bi introduction from component Lead coating would not impart any detectable or adverse effects (as SnBi is one of the common component coatings), SnAgCuIn was tested with the introduction of Bi. Table 5 summarises the test results with Bi content at 0.5% and 1%, respectively. No detrimental effects were detected. Effects of both Bi and Pb inclusion The possible presence of SnPbBi metallurgical phase at 96 C when there is concurrently Pb contamination and Bi introduction into the solder joint has been a concern. Figure 3 is a thermogram of SnAgCuIn solder doped with both Bi and Pb, and indicates the absence of detectable 96 C phase. Cost The solder material cost per unit weight of SnAgCuIn is higher than SAC305 due to the untimely high price of Indium metal. Metal prices fluctuate according to various factors, such as supply and Table 1 - Viable Lead-free alloys and recommended reflow peak temperatures and wave pot temperatures Table 2 - Comparative data on material properties of SnAgCuIn vs. SnPb vs. SAC OnBoard Technology April page

4 demand, mining issues, sourcing issues, import/export situations and international trade issues. All precious and commodity metals are subject to the intricate balance of supply and demand. With increasing globalisation, this balance is even more of a challenge. At the time of writing, all metals have relatively higher prices due largely to increased demand a result of a ramp up in production as well as the emerging markets. Historically, demand has been met by supply over time. Overall cost assessment requires a consideration of the total cost of ownership. This comprises solder material cost per unit weight (wave solder ingots, solder paste); process operational cost, including energy cost; component cost; PCB cost; equipment cost; defects & yield; product performance and reliability. Reliability consideration Reliability is the ability to achieve a low or zero probability of failure. There is no single magic test that can comfortably determine reliability. However, the probability of failure under likely service conditions, or all possible service conditions, can be assessed. Although each and every solder connection is critical, both the solder joint and the package/assembly as Table 3 - Aging test results at -20 C for 96 hours Table 4 - Aging test results at +125 C for 96 hours Table 5 - The effect of Bi in SnAgCuIn alloys a whole should be considered when assessing reliability. Furthermore, there is concern about the lack of a life prediction model and/or unknown accelerating factors. Yet reliability can be logically assessed providing the key factors that affect the probability of failure and the fundamentals behind them are understood. Solder joint reliability must also be understood with a thorough consideration of mechanical behaviour, thermomechanical behaviour and damage mechanism, along with the anticipated metallurgical phenomena of each of the viable Leadfree solder alloys, respectively. There are differences between SnPb and Pb-free. But the similarities and what has been learned far exceed the differences. Reliability can be assessed with or without a life-prediction model. When Surface Mount solder joints of PCB assemblies were put in service 26 years ago in place of through-hole, no working life-prediction model or field data existed. In terms of tests, basic material property evaluation is the first step. LEAD-FREE FOR LATE ASSEMBLY BIRDS PROBLEM: Incomplete Barrel Fill: Reduced Wetting Ability Using Pb-Free Alloys Insufficient Solder Volume SOLVED: Wave Solder Products WF7742 Wave Flux Pin-In-Paste + with 0603 Preforms PTH Reliability Problem? Solved: reliability@indium.com Q U A L I T Y A S S U R A N C E OnBoard Technology April page 33

5 An inferior material cannot deliver a superior solder joint. In this case, the stress-strain relationship, for example, would be revealing. Creep, fatigue, and isothermal vs. temperature cycling fatigue characteristics are informative. With that understanding, further tests and accelerated testing such as thermal shock, HALT/HASS, accelerated aging, vibration, mechanical shock, drop test, and tin whiskers, can be selectively included. Both fundamental property testing and accelerated testing are important, and form part of the reliability equation. Environmental testing adds another dimension to the understanding of solder joint integrity and the assembly s reliability. Key is determining how many and which tests should be performed and the validity of selected test parameters. There are a number of industry standards (e.g. J-STD-001D, J-STD-004, J-STD-005, J-STD-006, IPC-9701, MIL-STD-883, ASTM standards) that can be drawn on as references. For SnPb or Pb-free solder joints, under the wide spectrum of service conditions, fatigue and creep interaction is a predominantly damaging mechanism. The intricacy is that the set conditions drive a creepdominant fatigue or fatigue-dominant creep or a damage mechanism. How this mechanism is related to the alloy is directly associated with the makeup of the alloy in question. For example, SnAgCu near eutectic alloys may be vulnerable to high stress or high strain rate conditions whilst SnPb or SnAgCuIn near eutectic alloys are not. The observations and test results should coincide with the anticipated metallurgy and mechanical behaviour of each of the alloys in question. Overall, there are many tests that one can conduct, perhaps too many. To assess reliability it is crucial to select the most relevant test methodologies and to define the testing parameters for a specific methodology. For example, is there a need to carry out a HALT/HASS test or, in a temperature cycling test, how do you choose the boundaries of the top influential parameters: upper temperature, temperature amplitude, lower temperature, dwell at Tupper, dwell at Tlower, wave form, strain amplitude, ramp rate? Having the test results is not the final step. Actually it is just the beginning of the conclusion phase. The most important step is to integrate the results of the tests and to interpret the data and phenomena. In science and engineering, we generally assume that everything is clear-cut and not subject to interpretation. However, this is not necessarily the case. In many instances, the correct interpretation of data is Lead-Free Process Optimisation CeTaQ Americas has announced the availability of a new service that verifies feeder and shuttle motion repeatability. This service was developed as one of the consequences of the move to Lead-free assembly is less tolerance for even slight variations in placement accuracy. Compo- directly related to the design of the test, the test parameters, the criteria selected and how the assembly is processed. Before drawing a conclusion, the design and the process must be considered. Solder joint integrity and assembly reliability not only depend on the materials and parts but also on the process that puts them together. Anticipated microstructure evolution in response to the external environment leads to the predicted damage mechanism. Ultimately the integrated data and congruent damaging mechanism lead to the rationalisation of reliability assessment. Globally, Lead-free production under the existing SMT settings using a temperature as low as that used for 63Sn37Pb is a proven process. The credo behind the change is not only to make environmentally friendly electronics but also to advance performance above and beyond SnPb. Dr. Jennie Hwang is a long-standing contributor to SMT manufacturing, has helped improve SMT production yield and solve various reliability issues worldwide. She is a member of the U.S. Commerce Department s Export Council, and serves on the board of several public corporations and civic and university boards. In addition to numerous technical publications, she is also a prolific speaker and author on trade, business, education and social issues. nents do not self-centre as easily during reflow with Leadfree solders. CeTaQ s feeder calibration service is critical to ensuring repeatability of pick location, feeder indexing, component location, and sprocket repeatability. This is of particular importance for handling components such as tiny 0201 and chips. Additionally, CeTaQ s Leadfree solutions testing is applied to automated dispense systems. This is especially important as SMT passive components grow ever smaller, making dispense accuracy for adhesives and solder paste increasingly critical. CeTaQ Americas P.O. Box 7332, Nashua, NH 03060, US Tel , Fax solutions@cetaq-americas.com, OnBoard Technology April page