FINAL FINISHING ALTERNATIVES
Although known for short product life cycles and sweeping technology changes, the electronics industry has yet to adopt an industry wide hot air solder leveling (HASL) alternative. Numerous papers have been published over the last decade predicting the replacement of HASL with organic solderability preservatives (OSPs), electroless nickel/ immersion gold (ENIG) or new metallic immersion technologies such as silver and tin. So far, none of these predictions has yet become reality.
HASL is the predominant final finish applied worldwide. A predictable, known coating, HASL is used in billions of solder joints daily. However, three main drivers are pushing the electronics industry to consider HASL alternatives: cost, technology and the need for lead-free materials.
HASL alternatives allow a lead-free printed wiring board (PWB) and also provide flat coplanar surfaces to meet technology demands. Finer pitch and area array devices have allowed increased functionality of electronics. Typically, higher technology opposes lowering cost. However, most alternatives improve high technology assembly and long term reliability while still reducing cost.
Cost savings are a function of the entire process cost that includes process chemistry, labor and overhead. Alternatives like OSPs, immersion silver and immersion tin can provide a 20 to 30 percent reduction in final finishing costs. Although the percent saving per board may be low in large, high-layer count multilayer products, the cost savings with disposable electronics, along with greater functionality and elimination of lead, will drive a dramatic increase in the use of alternatives.
The use of alternatives will not only increase but will replace HASL as the final finish of choice. The problem with alternatives today is the number of choices and the sheer volume of data that has been presented. Alternatives ENIG, OSPs, immersion tin and immersion silver all provide lead-free, highly solderable, coplanar surfaces that, under production conditions, provide significant improvement in first pass assembly yields. To unravel the mystery of final finishes, these HASL alternatives can be differentiated by matching each coating's benefits to assembly requirements and PWB design.
Assembly Requirements
The effect of HASL alternatives on the assembly process reflects the solderability of the surface and how it interacts with the soldering materials being used. Each class of alternative finishes - OSP, organometallic (immersion tin and silver) or metallic (ENIG) - has a distinct soldering mechanism. This variation in soldering mechanisms impacts the setup of the assembly process and the reliability of the solder joint.
OSPs are protective coatings that must be removed during the soldering process. The flux must come in direct contact with the OSP surface to penetrate and solder to the copper on the PWB surface.
Immersion processes like silver or tin have an organic codeposited that eliminates oxidation of the final finish. Unlike OSPs, tin and silver dissolve into the solder and will become part of the solder joint and will help the wetting speed. Both tin and silver form the solder joint directly on the PWB's copper surface.
When properly deposited, the gold on an ENIG surface is pure and, due to its solubility in solder, will provide the fastest wetting speed for soldering. However, when ENIG is used, the solder joint is formed on a nickel barrier layer, not directly on the PWB's copper surface.
All three classes of alternatives provide an optimum surface for printing with all types of solder paste. The paste is placed directly on the surface finish, providing direct contact of the flux to penetrate the OSP and wet the PWB surface. The stencil forms an effective gasket for depositing a perfect solder paste print, eliminating the smear and bridging issues of HASL. The result is high first pass assembly yields with all three alternatives and little difference in solder wetting. The difference lies in solder joint strength and reliability. Several studies have demonstrated that soldering directly to the copper surface provides the strongest solder joints. Solder joint strength becomes critical when smaller pad sizes are utilized with area array chip packages.
Although declining in use, wave soldering is still part of today's assembly process. The soldering mechanism of each final finish will affect the choice of flux chemistry and setup of wave soldering process. Metallic and organometallic coatings facilitate the wetting of solder in the through hole and typically require less flux, lower activity fluxes and less turbulence in the wave. No-clean materials work well with OSPs under production conditions, but some optimization may be required to increase flux and/or solder penetration into the through hole. Typically, this optimization increases the amount of flux applied, substitutes specific types of flux chemistry, or increases solder penetration through higher turbulence or temperature.
Methods to replace the traditional wave soldering process are being implemented globally. Intrusive reflow, selective solder fountains and compliant pins are being used on virtually all final finishes. The work done to date indicates that the turbulence in the selective solder fountains improves through hole wettability. Paste-in-hole or intrusive reflow puts the flux an flux vehicle in direct contact with the pcb surface, making through hole wettability similar in all final finish options. Finally, due to a predictable hole tolerance, HASL alternatives have an edge over HASL with compliant pins. Among the alternatives, the thicker immersion tin deposit offers the best lubricity for insertion, providing the widest operation window for compliant pins.
The assembly industry is currently evaluating lead-free soldering alternatives. While certain alloys seem to be the choice of specific OEMs, an industry alloy has yet to be adopted. However, all the alloys being tested require higher reflow temperatures and yield slower wetting speeds. Paste suppliers have engineered specific flux chemistries to improve the wetting of these new alloys. Initial studies indicate that the higher reflow temperatures do not affect the solderability or bond strength of OSP, immersion silver or immersion tin. The higher melting temperatures apparently help penetration of the OSP and wetting of the tin and silver surface even with double-sided reflow. Additional testing is underway to evaluate the effect of wetting speed and optimize specific reflow parameters for the final finishes.
PWB Design
As discussed, the assembly process can be optimized to work with all final finishes. The PWB design, but more specifically the types of packages and interconnection, will ultimately determine the best HASL alternative for each application:
- Applications like key contacts, component shielding and edge connectors require low contact resistance throughout the life of the device.
- Flexible substrates typically require aluminum or stainless steel stiffeners or heat sinks.
- Component packages and some PWBs require wire bonding or compatibility with conductive adhesives for direct chip attach.
- High-density interconnect (HDI) geometry on PWBs dramatically affects yields with traditional electroless plating.
- Field failures due to poor bond strength with area array packages assembled on ENIG have been seen.
To meet all of these requirements, the electronics industry is focusing on three main alternatives: OSP, immersion silver and immersion tin. Each of these three coatings provides benefits that match specific PWB design requirements.
OSPs are the lowest cost alternative, are compatible with multi-metal surfaces, and provide the highest bond strength. New formulas are available that provide a thinner deposit as robust as the original that eliminate staining on multimetal surfaces. Multi-metal finishes like electrolytic nickel/gold for edge connectors or gold wire bonding are required due to wear resistance or bondability of electrolytic gold deposits. High cost and gold embrittlement of the solder joint require OSP as a secondary finish for soldered connections.
While production proven for key contacts, ENIG can also be used with the new OSP process if solder joint strength is critical to a PWB design. This high bond strength has made OSPs the choice for mobile electronics and area array packages. OSPs have also demonstrated a greater compatibility with conductive adhesives used in flip chip applications. Finally, heat sinks or stiffeners are most easily applied in panel form prior to final finishing. Unlike OSP, immersion or electroless processes will plate on the stainless steel or aluminum, causing discoloration.
Immersion silver is still a relatively new technology when compared to OSP and ENIG. However, over the last six years, extensive testing and high volume production have proven the reliability of this process. The solder wetting characteristics make this coating more adaptable to an existing no-clean wave soldering process. This surface finish is a potential alternative for most applications, including shielding, aluminum wire bonding, key contacts and soldering.
The contact resistance of this coating remains low after aging or reflow processes. Initial studies show that the contact resistance remains low after 300,000 contacts with conductive polymer, although more work needs to be completed. Also, as a metallic coating, immersion silver is readily inspectable under low or no magnification, making it easy for both the applicator and assembler to determine its presence.
Immersion tin has been used in the PWB and metal finishing industries for decades. However, new chemistry has been developed that co-deposits organic with the tin onto the copper surface. This co-deposited organic eliminates whisker growth, which had been a reliability concern, and retards copper-tin intermetallic growth, which affects solderability.
The result with these new immersion tin processes is a thicker final finish (30 to 50 millionths) that provides lubricity for compliant pin insertion and in-circuit test (ICT) penetration. Studies are underway to evaluate the relative ICT probe wear and performance with several final finishes. The new immersion tin processes are easily adapted to no-clean assembly and, like immersion silver, their presence is easily determined.
Conclusion
OSP, immersion silver and immersion tin will all provide high first-pass assembly yields with mixed technology and both water soluble and no-clean assembly technologies. The proper application for each product is designated by PWB design requirements:
OSP is the lowest cost coating and provides the highest solder joint strength for chip-scale and flip chip packages. New formulations have the ability to process multi-metal finishes such as nickel/gold tape automated bondings (tabs) or aluminum heat sinks and stiffeners.
Immersion silver provides a single surface finish that is wire bondable and has a low contact resistance for key contacts and metal-to-metal sheilding.
Immersion tin provides a thicker, uniform metallic coating for improved ICT probe life and lubricity for press fit pins.
Experience is being gained globally to solve process issues so an easy transition from HASL to other alternatives will be possible.
Comparison of Coatings
| Properties | White Tin | HASL | Ni/Au | OSP | Silver |
| Fine Pitch | Yes | Problem | Yes | Yes | Yes |
| Flatness of Pad | Yes | No | Yes | Yes | Yes |
| Mult Solder Cycles | Yes | Yes | Yes | Problem | Yes |
| Bare Board Testing | Yes | Yes | Yes | Problem | ? |
| Dimensional Stress | None | High | None | None | None |
| Controllability | High | Low | Med | High | Med |
| Cost Factor | Med | Med | High | Med | High |
| Compatible with All Flux | Yes | Yes | No | No | Yes |
| Solder Pot Contamination | No | No | Yes | No | No |
| Solder Mask Compatibility | Yes | Yes | Problem | Yes | Yes |
| Reworkability | Yes | Yes | Problem | Problem | Problem |
| Shelf Life | Long | Long | Long | Med | Med |
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