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作者：Cookson电子组装材料部区域执行副总裁 Rick Reagan
Introduction
Lead-free (Pb-free) technology has advanced rapidly since its first introduction, resulting in an electronics industry that now has more real-life manufacturing, trial and process data available than ever before. What factors have driven the move to lead-free, and what issues need to be considered in order to implement successful lead-free processes for electronics assembly?
Background
So, what are the principal drivers for the move to lead-free in the electronics industry? If one analyses the raft of everyday industrial and other contributors to the overall content of lead, it becomes apparent that electronics solder is actually a relatively small player in terms of lead consumption (Figure 1). Rather, it appears that competitive market forces are more influential, particularly in the consumer electronics area; in this case however, legislation issues begin to play a part.
Lead-free implementation: overview
First of all, it is important to realise that any lead-free formulations that are being considered are not simply a 'drop-in' replacement for traditional Sn-Pb (tin-lead) materials. Notably, the lead-free materials result in a higher processing temperature, typically 20° to 40°C higher, and this in turn introduces the issue of component reliability. In many cases, it is found that component manufacturers may lag behind somewhat in terms of developing suitable devices for lead-free processing, and in some cases it is necessary to implement mixed technology processing with some element of Sn-Pb.
Further, the reflow process window is narrower than with Sn-Pb-based processes, with the window between liquidus and peak reflow temperature reducing from 40°C to 20°C. The types and properties of assembly substrates have to be considered also, including which surface finishes are used, such as OSP (Organic Solerability Preservative), HASL (Hot Air Surface Levelling), ENiG (Electroless Nickel / Immersion Gold), Ag, or Sn.
In terms of capital equipment, the good news is that most existing assembly technology is generally already geared up to handle lead-free processing, with minor or no tweaks to standard configurations, software and hardware.
Finally, industry standards and criteria for inspection have not been fully established yet, and there is much ongoing research and qualification activity taking place in the industry, covering all aspects of adopting lead-free technology.
Set against all these challenges however, it should be noted that a significant number of applications have been successfully implemented across the electronics industry using lead-free technology.
Surface finish implications
Full wetting of all metallic surface features on a circuit board substrate rarely occurs, regardless of which surface finish is chosen. However, small spreading does not equate to poor wetting, and does still give sound interfaces. Small spreading may result in visible exposed copper after reflow, particularly when using non-OSP substrates, such as HASL, plated Ni-Au, Sn, SMT component leads, and stamped lead frames. However, non-wetted areas do not typically impact upon overall solder joint integrity, e.g. at the corner of SMT pads, and at the perimeter or top-wide annular rings; typically, a maximum of 5-10% of the solderable pad area may be exposed and non-wetted without interfering with solder joint formation issues.
In side-by-side tests using a wetting balance, it can be shown that interfacial tension of lead-free Sn-Ag-Cu alloys is greater than that of Sn-Pb at all processing temperatures, while wetting time is greater than for Sn-Pb, and wetting force smaller than Sn-Pb alloy.
Joint strength and reliability
Tests show that solder joints can have a dull or 'frosted' appearance using a Sn-Ag-Cu alloy compared with the shiny joints achieved using a Sn-Pb alloy. This should not be considered as a defect during inspection. In terms of joint integrity, assembled solder joints exhibit higher voiding levels than Sn-Pb joints, though this does not appear to degrade the reliability of strength of the joint. In fact, thermal cycling and pull tests can show that such Pb-reduced joints are stronger and more resistant to thermal fatigue than comparable Sn-Pb joints.
Fine and ultra-fine pitch challenges
As miniaturization continues to be a challenge for electronics assemblers, materials developers need to understand the unique factors introduced by the increasingly challenging dimensional constraints of today's circuit boards. For example, lead-free solder pastes must now accommodate Pb-free powders of small particle sizes, while at the same time enabling activity level sufficient for reflow of the smallest paste deposits. Indeed, all stages of the lead-free SMT process become a challenge as the pitch becomes finer: printing, placement, reflow, and inspection.
Printing Pb-free pastes
The key material factors in achieving successful Pb-free paste printing are: printability; print definition; consistent volumetric performance; repeatability at various print speeds; consistent transfer efficiency over various print speed ranges; stencil life, or ability of the Pb-free paste to maintain its properties over a period of process time; repeatable response to process pause; stability of paste viscosity over a period of time and over a temperature range.
Reflowing Pb-free pastes
As for printing, there are a number of key material factors that determine how successful the reflow process is during Pb-free assembly. These include: fine-feature reflow performance using high soak reflow profile in air; self-alignment ability to correct rotated components; results across a wide range of board finishes, including rosin-coated, Cu OSP, and Ni-Au; impact on voiding performance due to reflow profile changes (minimal = best); solder spread performance across a wide range of board finishes; mid-chip solderball performance; solder joint cosmetics (smooth, uniform); fillet wetting performance; and flux cosmetics across various profiles.
Summary
Lead-free technology has advanced rapidly since its first introduction to the electronics assembly industry. Pb-free solder materials, boards, finishes, equipment, and to a lesser extent components, have all been developed at a rapid pace, and often been shown to give better reliability than traditional Sn-Pb materials, particularly on OSP board finishes. Because of the narrow process windows that are typically demanded by such materials, careful control of design and processing is key to a successful low yield production line.
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Rick Reagan<|||>
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Regional Executive Vice President, Cookson Electronics Assembly Materials<|||>
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2005-1-18 |
