Solder Materials
The Interaction of 2 Solder Paste Alloys with 5 Surface Finishes, Part 2
SAC 305 shows faster shear strength degradation than Innolot,
while the surface finish has no effect. by Pritha Choudhury, Ph.D., Morgana Ribas, Ph.D., John Fudala and Mitch Holtzer
When a solder joint is exposed to cyclic stresses, thermally activated diffusion in the bulk solder, metallization and initial intermetallic (IMC) may take place. The growth of the interfacial IMC helps relieve the residual stress induced in the solder joint, and the growth rate corresponds to the magnitude of stress induced.1 Solder joint strength also decreases during exposure to temperature variations. Therefore, shear testing is a useful method to assess solder joint strength degradation caused by thermal cycling.2

In part one3 of this series we showed the voiding, solder spread and thickness of the high-reliability Innolot alloy compared with SAC 305 alloy solder pastes using five different surface finishes. Part two discusses thermal cycling effects on the growth in IMC thickness and solder joint strength. This study included two commonly used solder alloys in paste form:

  1. SAC 305 (96.5%Sn, 3%Ag, 0.5%Cu) powder size distribution (PSD) type 4 with novel “CVP-390” paste flux
  2. Innolot (91.95%Sn, 3.8%Ag, 0.7%Cu, 3.0%Bi, 1.4%Sb, 0.15%Ni) PSD type 4 with the novel paste flux and five variations of surface finishes, including
    1. Organic solderability preservative (OSP) (MacDermid Enthone Entek Plus HT) using two thickness levels
    2. Immersion tin (Ormecon CSN)
    3. Immersion silver (MacDermid Enthone Sterling)
    4. Electroless nickel/immersion gold (ENIG) (MacDermid Enthone Affinity).

Background

The ability of an electronic package to resist functional degradation in the intended environment of use determines its reliability4,5 and is largely dependent on the reliability of the solder joint. During operation, electronic packages are subjected to a wide range of temperature variation. Differences in thermal expansion mismatch (CTE) of electronic materials within the assembly lead to cyclic thermal loading, resulting in thermal mismatch deformation,1 crack formation in the solder joints and subsequent failure. Reliable solder joint shear strength is important because the solder joint itself must support a shear force due to mechanical shock and thermal stress.2 The measurement of the solder joint strength can therefore be a function of this microstructural damage.6
Experimental Design

This article discusses the final IMC thickness and degradation of shear strength after thermal cycling from -40o to 160oC with 10 min. dwell time at each extreme temperature, up to 2,000 cycles, of two paste alloys on five different surface finishes, as mentioned above. The growth and morphological changes of the interfacial IMC on different surface finishes in R1206 and BGA84 (with SAC 305 balls) are studied and presented. Shear testing of chip resistors is also used for evaluation of the residual joint strength after thermal cycling.

Functions of surface finishes on PCBs. Formation of a strong bond via the interfacial IMC is essential for package functionality and reliability. As solder joint size decreases, the influence of IMC layer thickness on its reliability has become significant.7 Solder joint reliability depends not only on the solder alloy but also the component, PCB finishes, and the IMC formed within and at the solder/substrate interface.5,8,9 The most important function of the PCB finish is to increase substrate solderability so reliable solder joints are achieved at the board-level assembly.4 Other common uses of surface finishes include prevention of oxidation of the copper metallization of the PCB, protection from contaminants, and damage from mishandling prior to assembly.

Significance of interfacial IMC for solder joint reliability. Interfacial IMC formation is strongly influenced by the processing parameters during reflow because of its effect on wetting and microstructure.5,10 Some surface finishes may act as a barrier layer to reduce interdiffusion between the solder and copper base, thus reducing intermetallic compound (IMC) formation.2,11 During service, joints may be subjected to elevated temperatures, resulting in IMC growth. These IMCs are generally brittle, and excessive growth can adversely affect solder joint reliability.9 The stress type is the most important factor affecting brittle fracture due to IMCs.12 High strain rate caused by rapid temperature changes, vibration, mechanical shock or bending of the assembly led to brittle fracture in most cases.

IMC formation kinetics on different surface finishes for SAC 305 – literature.

  • As-reflowed condition: Soldering of copper substrate involves (i) eutectic melting (reflow) of solder bump and (ii) reaction of molten solder with substrate, resulting in the formation and growth of one or two intermetallics, i.e., Cu3Sn (ε-phase) and Cu6Sn5 (η-phase). Mechanical bonding is provided mainly by the η-phase that has a peculiar scallop-like morphology.12 The -phase changed from a scallop to a flat structure in the presence of as little as 0.05 wt% Ni.12 Thermodynamically, a Cu3Sn layer must exist between the Cu and Cu6Sn5. However, the Cu3Sn layer, being very thin, is not visible after reflow and can be observed only after prolonged aging/reflow.13 The presence of Ni changes the composition of the intermetallic to (Cu,Ni)6Sn5. The Cu6Sn5 layer thickness increases linearly with the square root of reflow time.6 Cu6Sn5 and Cu3Sn have been identified between SAC 305 solder and copper pads on IAg and ISn finishes. Ternary (Cu,Ni)6Sn5 forms on ENiG finish.9 SAC 305 on IAg shows higher lead-pull strength than ENiG and ISn finishes.15
  • Effect of thermal cycling: SAC 305 reliability on Cu OSP, IAg and ENiG was studied using chip resistors.16 Solder joint lifetime was highest for OSP, followed by ENiG and IAg surface finishes. Increasing the amount of copper in SAC 305 on OSP increases the amount of copper precipitates, thereby strengthening the microstructure. On the other hand, excess void formation during reflow on IAg results in poor performance.16 SAC 305 in BGAs on OSP and ENiG finishes have been tested during thermal cycling.17 Multiple crack path fatigue, vertical cracks and typical component side fatigue have been observed in this study.

Effect of thermal cycling on IMC and joint strength of Innolot and SAC 305.

  1. IMC thickness.
    • BGA84: FIGURE 1 shows the IMC thickness on different surface finishes for Innolot and SAC 305. BGA84 is a hybrid joint with greater volume of available alloy at the joint compared to the resistor, resulting in thicker IMC after thermal cycling for both alloys. ENIG is an effective diffusion barrier for both the alloys, resulting in minimum growth of IMC. On ENIG finish with Innolot, Au, Ni, Cu and Sn have been observed on both the board and component surfaces of BGA84. The Ni-Au finish on the component adds to the presence of these failures on the component side regardless of the surface finish on the copper pad.
    • R1206: FIGURE 2 shows the final IMC thickness on different surface finishes in R1206. As an efficient diffusion barrier, ENIG results in minimum growth of IMC for both alloys. The average growth in IMC is similar in both alloys for the remaining surface finishes.
graph illustrating final IMC thickness (µm) on different finishes in CTBGA84
Figure 1. Final IMC thickness (µm) on different finishes in CTBGA84.
graph illustrating final IMC thickness (µm) on different finishes in R1206
Figure 2. Final IMC thickness (µm) on different finishes in R1206.
  1. IMC morphology. Extensive cracks are observed (FIGURE 3) in both alloys for the BGA and the resistor. The interfacial IMC being brittle relative to the matrix, cracks are formed at or near this IMC for both alloys and components. The presence of Bi and Sb in Innolot strengthens the Sn matrix by solid solution strengthening. Ni in Innolot substitutes for Cu in Cu6Sn5, resulting in the formation of (Cu,Ni)6Sn5. A detailed study of the interfacial IMC on different surface finishes is presented in FIGURE 4. The extension of cracks is less in Innolot than SAC 305. The distribution of Ag3Sn needles in the Innolot matrix also enhances the matrix strength. All the above factors help retain the strength of the Innolot, even when exposed to extreme environmental conditions, thus resulting in a lesser extension of cracks. For SAC 305, all the surface finishes play an equal role in crack extension in either component. Innolot on “Entek Plus HT high” finish has a very small crack extension in the BGA (Figure 3). The IMC thickness on this finish is higher than the rest (Figure 1), and its brittleness is reduced by the presence of Ni, thus reducing crack formation. Crack formation and extension is similar across all surface finishes for Innolot and is mostly located at the corner in the resistor. The higher stress in this region results in greater crack formation and subsequent damage to the joint.
morphology of the joints in CTBGA84 and R1206 after 2000 thermal cycles (TC)

Figure 3. Morphology of the joints in CTBGA84 and R1206 after 2000 thermal cycles (TC).

final IMC thickness (µm) on different finishes in R1206
Figure 4. Final IMC thickness (µm) on different finishes in R1206.
  1. Shear strength degradation.Both alloys have similar shear strength after reflow as shown in FIGURE 5. Surface finishes do not have any effect on the initial shear strength of either alloy. The loss in shear strength due to 2,000 thermal cycles is 66% in Innolot, while that in SAC 305 is nearly 80%. The reduction of shear strength in SAC 305 is nearly twice that of Innolot. The greater retention of joint strength in Innolot is also supported by the lesser extension of cracks, as shown in Figure 3. After thermal cycling, surface finishes do not seem to have any effect on the shear strength for either alloy.
shear strength (kgf) degradation with thermal cycling on different finishes on R1206
Figure 5. Shear strength (kgf) degradation with thermal cycling on different finishes on R1206.
Summary
Ever-growing demand for high-reliability solder joints is pushing requirements of solder alloys to higher levels. With a deeper understanding of the interaction between solder paste chemistries and surface finishes, the summary of the findings of this study are:

  1. ENIG had the lowest IMC thickness after 2,000 thermal cycles.
  2. Both tested solder pastes produced similar IMCs on a given surface finish.
  3. Innolot solder paste with the novel flux outperforms SAC 305 solder paste with the same flux after thermal cycling for a given surface finish.
  4. The complex microstructure of Innolot provides improved strength and thereby improved thermal cycling reliability.
  5. Thermal cycling has a significant effect on shear strength degradation of both solder alloy joints.
  6. Shear strength degradation is faster in SAC 305 than Innolot after thermal cycling.
  7. The loss in shear strength is 66% in Innolot and 80% in SAC 305 after 2,000 thermal cycles.
  8. PCB surface finishes do not have any effect on shear strength of either alloy before or after thermal cycling.
References
  1. J.W. Ronnie Teo and Y.F. Sun, “Spalling Behavior of Interfacial Intermetallic Compounds in Pb-free Solder Joints Subjected to Temperature Cycling Loading,” Acta Materialia, vol. 56, 2008.
  2. James Webster, Jianbiao Pan and Brian J. Toleno, “Investigation of the Lead-free Solder Joint Shear Performance,” Journal of Microelectronics and Electronic Packaging, vol. 4., no. 2, 2007.
  3. Pritha Choudhury, Ph.D., Morgana Ribas, Ph.D., John Fudala and Mitch Holtzer, “The Interaction of 2 Solder Paste Alloys with 5 Surface Finishes,” PCD&F/CIRCUITS ASSEMBLY, July 2021.
  4. Nan Jiang, Liang Zhang, Zhi-Quan Liu, Lei Sun, Wei-Min Long, Peng He, Ming-Yue Xiong and Meng Zhao, “Reliability Issues of Lead-Free Solder Joints in Electronic Devices,” Science and Technology of Advanced Materials, vol. 20, 2019.
  5. Jianbiao Pan, Jyhwen Wang and David M. Shaddock, “Lead-Free Solder Joint Reliability – State of the Art and Perspectives,” Journal of Microelectronics and Electronics Packaging, vol. 2, no.1, January 2005.
  6. Milos Dusek, Jaspal Nottay and Christopher Hunt, “The Use of Shear Testing and Thermal Cycling for Assessment of Solder Joint Reliability,” NPL Report CMMT (A)268, June 2000.
  7. J.W.C. de Vries, M.Y. Jansen and W.D. van Driel, “On the Difference between Thermal Cycling and Thermal Shock Testing for Board Level Reliability of Soldered Interconnections,” Microelectronics Reliability, vol. 47, 2007.
  8. A. Siewiorek, A. Kudyba, N. Sobczak, M. Homa, Z. Huber, Z. Adamek and J. Wojewoda-Budka, “Effects of PCB Substrate Surface Finish and Flux on Solderability of Lead-Free SAC305 Alloy,” Journal of Materials Engineering and Performance, vol. 22, no. 8, August 2013.
  9. Anupam Choubey, Hao Yu, Michael Osterman, Michael Pecht, Fu Yun, Li Yonghong and Xu Ming, “Intermetallics Characterization of Lead-free Solder Joints under Isothermal Aging,” Journal of Electronic Materials, vol. 37, August 2008.
  10. A. C. Spowage, L. M. Sim, C. M. Thong, P. A. Collier and G. Y. Li, “Investigation of the Effects of Surface Finish and Simulated Service Aging on the Characteristics of the Intermetallic Layer Formed in Various Lead-Free Solder Joints,” STR/04/033/PM.
  11. Wonil Seo, Yong‑Ho Ko, Young‑Ho Kim and Sehoon Yoo, “Void Fraction of a Sn-Ag-Cu Solder Joint Underneath a Chip Resistor and its Effect on Joint Strength and Thermomechanical Reliability,” Journal of Materials Science: Materials in Electronics, August 2019.
  12. Per-Erik Tegehall, “Review of the Impact of Intermetallic Layers on the Brittleness of Tin-Lead and Lead-Free Solder Joints,” IVF Project Report, June 2007.
  13. Oleksii Liashenko, Andriy M. Gusak, Fiqiri Hodaj, “Phase Growth Competition in Solid/Liquid Reactions between Copper or Cu3Sn Compound and Liquid Tin-based Solder,” Journal of Materials Science: Materials in Electronics, vol. 25, 2014.
  14. A. Syed, “Accumulated Creep Strain and Energy Density Based Thermal Fatigue Life Prediction Models for SnAgCu Solder Joints,” ECTC Proceedings, 2004.
  15. Weiqiang Wang, Anupam Choubey, Michael H. Azarian and Michael Pecht, “An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics,” Journal of Electronic Materials, vol. 38, 2009.
  16. Maurice N. Collins and Jeff Punch, “Surface Finish Effect on Reliability of SAC 305 Soldered Chip Resistors,” Soldering and Surface Mount Technology, vol. 24, 2012.
  17. Michael Meilunas, Anthony Primavera and Steven O. Dunford, “Reliability and Failure Analysis of Lead-Free Solder Joints,” 2002.
Pritha Choudhury, Ph.D., is senior scientist R&D; Morgana Ribas, Ph.D., is manager of Metals Technology Group – R&D; John Fudala is senior process specialist, and Mitch Holtzer is technical knowledge leader, Global R&D at MacDermid Alpha Electronics Solutions (macdermidalpha.com).