Summary: The first of a two-part series, Dr. Hwang takes a wide, sweeping look at the history, timeline, highlights, and future projections for lead-free manufacturing.
After more than a decade of lead-free manufacturing, millions of products have been produced and put to use across many industry sectors and for people in all walks of life. What is the track record for lead-free electronics?
In a holistic view, this two-part column lays out the landscape in 100 points by taking into consideration manufacturability, reliability, and future trajectory. Each of the summary points will not be discussed or elaborated.
However, inquiries about any of the points for scientific base, rationale, and further discussion are welcome.
1. From a supply chain perspective, lead-free electronics primarily comprises three parts: Solder joint, PCB surface finish, and components. All three components need to be lead-free for the final product to be lead-free.
2. Globally, many patents on lead-free solders have been issued, yet only a small percentage of those patents have actually worked on a commercial scale.
3. Technically, the design of a “robust” lead-free solder is an intricate task.
4. An intricate solder composition should not be translated to instability or lack of the required stability in application.
5. Metallurgical alloying, microstructure, and micro-structural evolution under an anticipated service environment are critical to the technical base.
6. For obvious scientific reasons, the greatest technical challenge comes when designing alloy compositions with a melting temperature (liquidus) less than 210°C without incorporating a high percentage of Mg, Zn Bi, In, and other low-melting elements.
7. R&D work in lead-free solder commenced long before the RoHS regulatory mandate.
8. Since inaction of the EU’s initiative, RoHS has gone global.
9. Japan is a pioneer in practice.
10. Solder alloy selection in lead-free manufacturing is critical to the quality and reliability of lead-free electronic products.
11. A working solder composition is dictated by the existing SMT establishment.
12. A working solder composition should be dictated by the performance need for a specific application.
13. Several choices of solder joint alloys are available.
14. Several choices of PCB surface finishes are available.
15. Several choices of lead-free components are available, albeit with some limitations.
16. The PCB assembly process must work with all components for a specific design.17. Constraints in the heat tolerance of components and the PCB bare board exist.
18. The choice of solder alloy drives the reflow processing temperature required.
19. The choice of solder alloy drives the wave pot temperature required.
20. The alloy drives process temperature and impacts reliability on both process and material fronts.
21. The main SMT operation modules (printing solder paste, pick-and-place, reflow, wave, inspection, testing, etc.) remain essentially the same as in SnPb production.
22. When a problem or defect occurs, the ability to separate SMT manufacturing issues from those induced or aggravated by lead-free is a prerequisite to obtaining an effective solution.
23. Production defects are often attributed to lead-free (SAC system).
24. No surprises in production results occur with the SAC system judging from the fundamentals and technical anticipation.
25. Production yield is in sync with the best practices in SMT manufacturing.
26. Practicing the assured compatibility between material and process is a prerequisite for high-yield, low-defect production.
27. Understanding the basics of compatibility in the material system is a prerequisite in implementing lead-free electronics.
28. Designing a compatible material system is a prerequisite for product reliability.
29. Contrary to perception, Bi is a relatively safer element than other metals often used in electronics.
30. If used properly, Bi and In can be beneficial to the properties and performance of solder alloys (lead-free or SnPb).
31. The manufacturing process is key to solder joint integrity by keeping two basics in mind: Solid bonding zone and well-formed microstructure.
32. The manufacturing process is key to an assembly’s reliability—minimizing the co-planarity problem, heat-induced damages, and preventing defects, etc.
33. Commonalties and differences exist between SnPb and lead-free concerning production issues.
34. A high percentage of problems/defects that separate SnPb from lead-free are induced by the higher process temperature required for SAC solders.
35. A cardinal rule for rework: “Do it right at the first time.” (Particularly for lead-free.)
36. Integrate manufacturing know-how and solder joint integrity to achieve quality and high yield.
37. Integrate manufacturing know-how and solder alloy intrinsic properties to achieve product reliability.
38. Reliability depends not only on the as-received (as-designed) materials and components, but also on the process that puts them together.
39. Reliability assessment requires the integration of material, process, testing, and
data analysis while meeting fundamental principles.
40. Pay attention to not only obtaining test data, but also to data interpretation and integration.
41. Reliability assessment takes more than accelerated temperature cycling (ATC) tests.
42. One set of ATC testing does not necessarily translate into reliability. Circumspection in drawing conclusions is warranted.
43. Reliability assessment takes more than hard data—especially when the field data is scarce or nonexistent.
44. Extrapolation, experiences/knowledge, and meeting fundamental principles should be part of the equation in reliability assessment.
45. Reliability, a relative term, is to minimize probability of failure.
46. Tests can only verify reliability.
47. Meeting fundamental principles is key to long-term product reliability performance.
48. A life prediction model alone does not guarantee reliability.
49. Materials science principles work wonders— the observed phenomena and performance coincide with the teachings of materials science
and metallurgical engineering.
50. No surprises observed thus far.
By Dr. Jennie S. Hwang, CEO H-Technologies Group SMT perspectives and prospects
