MAGAZINE

SAC 305 shows faster shear strength degradation than Innolot, while the surface finish has no effect.

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
    • Organic solderability preservative (OSP) (MacDermid Enthone Entek Plus HT) using two thickness levels
    • Immersion tin (Ormecon CSN)
    • Immersion silver (MacDermid Enthone Sterling)
    • Electroless nickel/immersion gold (ENIG) (MacDermid Enthone Affinity).

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For spread and wetting performance, certain finishes stand out. 

Electronic assemblers have myriad material and process choices to make, not limited to board materials, solder masks, laminate Tg’s, components, surface finishes, assembly materials and design for manufacturing (DfM) process conditions. High-reliability alloys such as Innolot are designed to meet harsh automotive conditions and extend service life of the solder joint. Applications requiring higher operating temperatures and increased number of cycles to failure have benefited by implementing that alloy. While solder alloy selection is an important factor in determining reliability of the solder joint, considerations should be made for surface finish selection to further enhance performance. This study explores surface finish factors such as IMC formation, voiding and solder spread that contribute to reliability.

Each choice can have a significant impact on the in-service reliability and commercial success of the assembly. This multi-part article will focus on data developed from an extensive study of surface finishes and solder pastes used by many global, high-reliability assembly manufacturers. The 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

a. Organic solderability preservative (OSP) (MacDermid Enthone Entek Plus HT) using two thickness levels
b. Immersion tin (Ormecon CSN)
c. Immersion silver (MacDermid Enthone Sterling)
d. Electroless nickel/immersion gold (ENIG) (MacDermid Enthone Affinity).

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PCD&F’s annual salary survey reflects the consistency and stability of the PCB design industry. 

To say much has transpired in the past year and a half is the understatement of all understatements. When we published the findings of our last designers’ salary survey in May 2020, we were still in the early months of the Covid-19 pandemic, many of us in the initial stages of a lengthy quarantine we thought was temporary. We were unsure how the virus would affect the world in the short-term, let alone the long-term – with regard to the health of loved ones and the economy as a whole, to name two of countless concerns. It will be many years before we fully comprehend the enduring global impact of this unmitigated health crisis, but if this year’s survey is any indication, one thing that has remained consistent is the PCB design engineering profession.

The US unemployment rate in July 2020 was 10.2%, and as of July 2021, it was 5.4%, according to the US Department of Labor.1 More specifically, for engineering occupations, the unemployment rate as of Jun. 30, 2020, was 6.1%, and at Jun. 30 this year, it was only 3.4%, BLS says, and the computer systems design and related services sector added 100,000 jobs in June alone.1  

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From secure data exchange to managing EoL parts, the applications are numerous. 

In last month’s discussion of how electronics companies first began to use Blockchain technology to automate and simplify “high-friction” multiparty processes, we noted many of the earliest projects tended to focus on the relationship between a single “sponsor” company and its partners. In other cases, companies worked together as a consortium to solve a common problem. Quickly, however, electronics companies began to leverage applications originally developed for other industries, especially to leverage the “track and trace” capability originally developed for the food industry.

Basing a new blockchain network on functionality that has been developed and implemented for another network1, even in a completely different industry, lowers the cost of entry and simplifies the process of setting up that new network. That has turned out to be very important, since it also makes it easier to create a valid business case for the application.

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