High Reliability Lead free Solder SN100C Sn 0 7Cu 0 05Ni Ge

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In Figure 3 the tensile strength elongation and creep. strength are plotted, More stress is imposed on components when soldering Set up a test piece. with lead free solders than with Sn 37Pb because their. higher melting point requires that the joints are formed at. higher temperatures And any stress in Sn 37Pb joints 5mm Figure 4 Set up a test piece. tends to be relieved by flow of the solder itself Because Strain. of their higher yield point the residual stress is not so easily. relieved in lead free solder joints Therefore if the stress. imposed on boards and components is to be minimized Chuck Testpiece. ductility is a more important property in a solder than. Since SAC305 which has a high silver content has a. high yield point any stress that builds up on components as. the assembly cools remains unreleased We can see from. Figure 3 that the elongation of SAC305 at 32 is lower 350. than the 48 value for SN100C To confirm the effect 300. of this lower ductility we subjected a test piece to cyclic Development of SN100C. strain until fracture occurred,o Sn 3 0Ag 0 5Cu,Test conditions 150. 1 Solder alloy Failure, Sn 3 0Ag 0 5Cu 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25. of strain cycles,Figure 5 Peak stress at point of strain reversal. 2 Manufacture of the test piece Figure 5 shows the peak stress at each cycle with the. number of cycles to failure noted The load required to. Cut solder bar into 7x20x50mm pieces to ensure equal achieve the specified strain decreases with each cycle. volumes for each alloy melt at 400 C and pour into a because of the reduction in the effective test piece cross. mould with cavity dimensions 160mm long 12mm wide section due to necking and crack propagation. and 10mm deep,Load required, 3 Tensile tester set up and test method see Figure 4 for first tensile Cycles to failure.
Alloy strain Numbers, Clamp the test piece in the tester with a distance of 60mm. between chucks,Sn 3 0Ag 0 5Cu,Strain rate 20mm minute. Strain 5mm Figure6 test result cycles to failure, Measure the peak load in the tensile part of each cycle. until fracture occurs As shown in Figure 6 the ranking of the alloys on the. basis of the peak load required to achieve the initial 5mm. deformation was in ascending order 4 Excellent Resistance to Impact. SN100C 212kg Sn 3 0Ag 0 5Cu 272kg Sn 37Pb, 347kg By contrast the ranking of the alloys in terms of. the number of cycles to failure was in ascending order. Sn 37Pb 5 cycles Sn 3 0Ag 0 5Cu 17 cycles, SN100C 25 cycles Although requiring a lower load to.
achieve the initial 5mm strain the SN100C has a greater. capacity for accommodating cyclic strain than Sn 37Pb or. Sn 3 0Ag 0 5Cu SN100C survives five times as many,strain cycles as Sn 37Pb before failure. Figure 8 Reference Ratchev et al AStudy of Brittle to. 3 Excellent Resistance to Thermal Fatigue, Ductile Transition Temperatures in Bulk Pb Free Solders EMPC. 2005 IMAPs Europe June 12 15 Brugge Belgium, Figure 7 Cross sections showing incidence of cracking in surface. mount components Figure 9 The hummer used for Sharpy impact test. Figure 7 shows the changes that occur in SN100C Sn The results of Charpy impact tests conducted by a. 3 0Ag 0 5Cu joints as a result of thermal cycling carried Belgian research laboratory are shown in Figure 8 In the. out under the following conditions Charpy impact test the energy per unit of cross sectional. area required for a swinging hammer Figure 9 to fracture. Thermal Cycling Test Conditions a test piece is used as a measure of the property of fracture. 45 C 15 minute dwell 125 C 25 minute dwell toughness In general the smaller the impact energy the. more brittle is the alloy The results indicate that SN100C. Board FR 4 Immersion tin finish has excellent resistance to impact at temperatures as low as. Cracks appeared in the Sn 3 8Ag 0 7Cu joints after 2000. cycles with complete failure after 4000 cycles No major 5 Excellent Resistance to Vibration. cracks appeared in Sn 0 7Cu until 3000 cycles and until We have reported the excellent resistance of SN100C to. 4000 cycles for SN100C The conclusion is that SN100C thermal cycling and high impact strength We will now. has excellent resistance to thermal fatigue report the results of vibration. Further tests were conducted on 1 6mm thick double. sided FR 4 boards with an OSP finish, 1 Immerse the board in a rosin based flux for 5 seconds. 2 Solder alloys SN100C Sn 3 0Ag 0 5Cu,3 Solder temperature 250 C 260 C.
4 Height of solder wave above nozzle 5mm,5 Immersion depth 2mm. 6 Immersion time 10 seconds 20 seconds,Figure 10 JCAA JG PP vibration test result. Figure 10 shows the results of testing done as part of a. project on lead free solders carried out by the US. Military s Joint Group on Pollution Prevention JG PP. Figure 12 Erosion of the shoulder of a plated through hole on. According to the Joint Test Report SN100C outranks Sn. double sided board, 3 9Ag 0 6Cu and Sn 37Pb in vibration testing of wave. soldered through hole components It is expected that on. the basis of this performance SN100C will be selected for. use in conditions of severe stress,6 Reduction of Copper Erosion. Figure 13 Thickness of remaining copper pad at the shoulder of a. Copper laminate,plated through hole,Sn Cu Ni Sn 3Ag 0 5Cu.
Copper laminate, The arrow in Figure 12 indicates the direction of solder. flow Since it was found that there is a difference in the. 50 m Solder resist Solder fillet Solder resist extent of erosion between the right and left sides the. 50 m Solder fillet,thickness of the remaining copper was measured. SN100C Sn 3 0Ag 0 5Cu separately on each side The results are plotted in Figure. Figure 11 Copper erosion in single sided board Erosion by the Sn 3 0Ag 0 5Cu is substantial and. increases with immersion time After 20 seconds the. Figure 11 shows the results of a copper erosion study copper on the right side of the hole exposed to Sn 3 0Ag. carried out on single sided boards It can be seen that the 0 5Cu has eroded completely while for the hole exposed. copper of the trace connecting to the land has been almost to SN100C for the same time 81 6 of the original. completely eroded The effect of the solder wave is thickness of copper remains in that location It is clear that. apparent in that area SN100C erodes copper more SN100C erodes copper more slowly even at high. slowly than Sn 3 0Ag 0 5Cu temperature,Formation of Stable Intermetallic. Figure15 SEM mapping Upper line Sn0 7Cu0 05Ni Lower. line Sn0 7Cu0 05Ni0 05P, Figure14 Magnified cross section Aging test at the high. temperature The element mapping of solder joint cross sections in. Figure 15 is shows where the Sn Cu and Ni are, Figure 14 shows the changes that occur in the concentrated When P is added to the solder as an.
intermetallic compound layer formed at the interface antioxidant the Ni is dispersed throughout the joint rather. between a copper substrate and Sn 0 7Cu and Sn 3 0Ag than concentrating in the interfacial intermetallic so the. 0 5Cu as a function of time at 120 C benefit of its stabilizing effect on intermetallic growth is. The Ni in the SN100C stabilizes the intermetallic so not obtained. that it does not grow even during extended storage at high. temperature 768 hours at 120 C By contrast there is Summary. substantial growth in the intermetallic in the Sn 3 0Ag Since SN100C behaves almost perfectly as a eutectic it is. 0 5Cu alloy possible to achieve smooth bright fillets free of shrinkage. Although the intermetallic layer initially formed in the defects. Sn 0 7Cu and the Sn 3 0Ag 0 5Cu is thinner than that The high melting point and high creep resistance of lead. formed in the SN100C after long term aging at 120 C it free solders means that large strain is imposed on solder. has grown to a thickness greater than that of the SN100C joints as the result of the repeated expansion and. A thick layer of brittle interfacial intermetallic provides an contraction that occurs during thermal cycling The result. easy pathway for crack propagation so that the reliability can be cracking of chip components and separation of the. of the solder joint is compromised land from the laminate To avoid overstressing of the. The trace addition of Ni in the SN100C incorporates in components and the printed circuit board it is important to. the interfacial intermetallic stabilizing it against further choose an alloy with the ductility to accommodate this. growth strain Although the strength of solders that contain silver. the most common of which is Sn 3 0Ag 0 5Cu is high. their low ductility means that they are not able to. accommodate strain By contract that high ductility of. SN100C means that it can accommodate substantial strain. without embrittlement and cracking and that is apparent in. the results of the cyclic strain test thermal cycling test. impact test and vibration test A further advantage of. SN100C is that slower growth of interfacial intermetallic. during ageing The consequence of all of these advantages. is the high reliability of joints made with SN100C. Introduction While the situation varies from country to country nearly one year after the EU RoHS Directive came into force implementation of lead free solder is

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