Solderability Testing Lead-free soldering

If the reactive blending is stopped at an intermediate stage, the micelles and the shrunken particles coexist and a bimodal particle distribution is realized, as shown in Fig. 8.39. In the case of Fig. 8.39, PA-6 was mixed with polyethylene (PE) modified with a small amount of MAH (0.1 wt%) and glycidyl methaciylate (3-12 wt%), at a 70/30 (PA/PE) blend ratio. The bimodal system can be easily crosslinked by electron beam irradiation at a low dose level, the same as that used for neat PE (Pan et al. 2002). The crosslinked PA/PE alloy shows good heat resistance in a lead-free solder test thus, it may be applied in making construction parts with melt-down resistance in fires, e.g., a window frame. 

Since the process temperature for most lead-frees are higher than for eutectic- or near-eutectic Sn-Pb solder, any flux residues associated with No-Clean soldering are more thoroughly baked onto PWB snrface metals. This inhibits electrical test probe contact. Even with today s No-Clean solder pastes and with Sn-Pb solders, electrical probing can be a challenge. Often the residues that cover test points necessitate multiple seating cycles of the test probes to penetrate the flux residue. 

The solder shock test is one of several methods to assess the thermal resistance of copper-clad laminates. It is easy to perform and represents another key test during the early assessment of a material. There are a number of different methods to choose from, which will be described in detail in Section 12.5.2. During the initial assessment of the material, it is important to choose at least one of the described test methods to make certain that the material meets the minimum requirements, especially if the material is nsed in higher-temperatnre lead-free assembly processes. Aside from solder shock testing of bare laminate material, it is also recommended that the PCB engineer consider PCB-level temperatnre shock as well as repeated reflow testing with a particnlar focns on resin-reinforcement delaminations. This will ensnre that not only the raw material bnt also the completed PCB will be able to withstand the required temperatnre regime. 

Solder Float Resistance. This test addresses the thermal resistance of the laminate material floating on the solder bath. Becanse this method subjects the sample to a thermal gradient across the z-axis of the material similar to an actual wave solder operation, the resnlts of this test are particnlarly important and—as mentioned previously—either solder pot temperatures or exposure times should be increased if the laminates are intended for use in lead-free assembly processes. 

Lead-Free Solderability and Fluxes. For lead-free solderabihty testing, a further extensive study was undertaken by the J-STD-002/003 committees again.The ontcome of the testing was to recommend and specify the nse of a more active flux. The activation level has been increased to 0.5 percent as measnred by chloride content. This flux and the increase in test temperature are specified in the J-STD-002C and 003B. 

The acceleration transforms that can be used to estimate the field life of tin-lead and lead-free solder joints based on the results of temperature cycling tests... 

A tremendous number of researchers have explored the reliability of Pb/Sn solder alloys. A wealth of data, material properties, and proposed constitutive relationships have been captured in the literature. However, there is relatively little information in the literature on the material properties of lead-free solders. Moreover, the material property information available is on slightly different compositions of lead-free solders, with tests performed in different ways. 

Lead test kits are inexpensive. The pink color obtained on reaction of lead with the kit reagents is very distinct and easy to interpret. Tests appear to be specific for lead when a pink color is obtained. They do not give a positive reaction with several metals used in lead-free solders. The chemicals used are stated... 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Failure analysis is useful for providing the failure locations, modes and mechanisms, verifying the reliability test data and reliability prediction, and providing insights into the physical, chemical, electrical, mechaiucal, and thermal behaviors of solder joints (Ref 104-105). The failure modes of lead-free solder interconnects are similar to those for the tin-lead solder (Ref 104), under various loading conditions. Techniques for failure analysis for lead-free solder joints will be presented in Chapter 10. 

IPC Solder Products Value Council White Paper, Round Robin Testing and Analysis of Lead Free Alloys Tin, Silver, Copper, http.V/leadfree. ipc. org/LeadFreeWP006. asp, 2005... 

J. Lau, N. Hoo, R. Horsley, J. Smetana, D. Shangguan, W. Dauksher, D. Love, I. Memis, and B. Sullivan, Reliability Testing and Data Analysis of High-Density Packages Lead-Free Solder Joints, Solder. Surf. Mt. TechnoL, Vol 16(2) 2004, p 46-68. Also, Proceedings of APEX 2003, March 2003, p S42-3-1/24... 

It was observed that the creep data were readily fit to the two-term or two cell version of Eq 28. Qualitatively, two-terms for Eq 28 is suitable because the creep behavior of lead-free solders is usually a combination of bulk diffusion and fast diffusion processes as has been demonstrated by the apparent activation energy values cited previously. Values of the parameters used in Eq 28 are listed in Table 3 for a number of Pb-free solders and the Sn-Pb baseline alloy. Clech validated the resulting equations, using creep and stress-strain test results that are independent of those data used to establish the parameters in Table 3. 

A cursory look at steady-state creep data (Ref 28, 34-39) for Sn-Pb and lead-free solders suggests that over a wide temperature range (— 55 to 125 °C, or -67 to 257 °F) and under high enough stress, many of the common lead-free solders (except perhaps for eutectic Sn-Bi) creep at similar rates or faster than standard Sn-Pb. Figures 12 and 13 are plots of isothermal steady-state creep data in shear and tension, respectively, for near-eutectic Sn-Pb and lead-free solders of various compositions. The shear data in Fig. 12(a), 12(b), and 12(c) are for test temperatures of approximately 25, 75 and 125 " C (77, 167, and 257 °F). The tensile data in Fig. 13(a) through 13(d) are for test temperatures of approximately — 55, 25, 75, and 125 " C (— 67, 77, 167, and 257 °F). 

First-order test life correlations (cycles to failure versus applied shear strains) were developed for lead-free and Sn-Pb assemblies. The empirical correlations show different slopes for SAC, Sn-0.7Cu and standard Sn-Pb assemblies. The crossover points of the various trendlines suggest that the rankordering of solder joint lives for the different alloys change with the applied stress or strain level. The correlations that were shown are purely empirical and should not be used to calculate acceleration factors or for life prediction purposes. 

A. Schubert, R. Dudek, R. Ddring, H. Walter, E. Auerswald, A. Gollhardt, B. Schuch, H. Sitzmann, and B. Michel, Lead-Free Solder Interconnects Characterization, Testing and Reliability, 3rd International Conference on Benefiting from Thermal and Mechanical Simulation in Microelectronics Proc., 2002, p 62-72... 

T. Woodrow, Reliability and Leachate Testing of Lead-Free Solder Joints, IPC Lead-Free Conference Proc., May 2002 (CD-ROM)... 

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