(This column, which originally appeared in Global SMT & Packaging magazine 6.1 (Jan 2006), is also available as a free PDF.)  

In this column a year ago, I stated that “the transition to lead-free soldering is taking place worldwide, and the train has left the station.”

The question at this time is, “Will the train you are on be arriving on time?”  Supply chain readiness and compatibility are critical to a smooth transition to environmental compliance for the worldwide electronics industry.  In this regard, there are a number of issues that need to be addressed.

1.    Tin whiskering

Many years of research point to compressive stresses as the underlying driving force for the formation and growth of tin whiskers.  The compressive stress may arise from metallurgical interactions, thermomechanical factors, and/or mechanical processes.  As the mechanism for tin whisker growth becomes clearer, predictive modeling for tin whisker nucleation and growth, which can accurately account for the thermodynamic driving force and the nucleation and growth kinetics, will become a very useful tool for life prediction and optimization.

2.    Backward compatibility

The issue of ‘backward compatibility’ arises due to the decreased availability of Sn-Pb area array packages and the continued existence of Sn-Pb soldering for products that, for various reasons, are not being transitioned to lead-free.  The ‘backward incompatibility’ of CSP and BGA with SAC balls with Sn-Pb soldering is primarily due to the fact that the SAC alloy, with a melting temperature of 217°C, will not always completely melt during reflow with the Sn-Pb solder, typically at reflow peak temperatures between 205-225°C (or even as low as 200°C in extreme cases).

The minimum reflow temperature required for ‘complete mixing’ depends on the ratio of the SAC solder ball volume to the Sn-Pb solder paste volume, and thereby varies with the CSP/BGA type.  This is because the liquidus temperature of the “mixed alloy” varies with its composition, which depends on the mass of the Sn-Pb solder paste relative to the mass of the SAC solder ball.  The mass ratio (in direct proportion to the volume ratio) in turn is related to the stencil thickness and aperture size, depending on the pitch of the package.  Generally, 225°C (and preferably >235°C) has been considered to be the minimum reflow temperature.  Such a scenario however demands that the other components on the board are able to withstand the higher soldering temperature, considering the relatively large temperature delta for large complex boards (which are typical of products not being transitioned to lead-free).

3.    Mechanical components

The availability of RoHS compliant mechanical components remains a concern.  For example, hexavalent chromium (Cr+6) is used extensively in the electrical and electronics industry in metal finishing of sheet steel, for example for enclosures, for the purpose of passivation and corrosion protection.  Currently, sheet steel for outdoor applications that do not contain hexavalent chromium are not readily available in all of the markets, and the corrosion resistance of trivalent chromium (Cr+3) limits its application primarily to indoor use.


4.    Reliability

Data are still emerging from different studies on the parameters for the constitutive equation for lead-free solder alloys.  The effort is complicated by the high strain-rate sensitivity (strain hardening) and temperature sensitivity of the Sn-Ag-Cu alloy.  Complicating the situation further is the different stress dependency of the creep rates for Sn-Ag-Cu and Sn-Pb eutectic alloys, leading to different comparisons between the two alloys at low and high stress levels.

For handheld products in particular, dynamic mechanical loading conditions (such as when the product is dropped to the ground) are often the predominant cause for the brittle failures of the solder interconnects, due to the excessive strains, at high strain rates, caused by the mechanical shock.  Due to the higher modulus of the Sn-Ag-Cu alloy, the failure location may move from the solder joint to the PCB.  Understanding of the deformation behavior and failure, and reliability prediction under dynamic mechanical loading conditions, is an area of great importance for handheld products.  Predictive modeling and ‘design for reliability’ tools and methodologies, taking into consideration the package structure, component location on the PCB, solder interconnect characteristics and the product mechanical structure, are needed for optimized design for handheld products.

For large boards, excessive board flexure during PCB manufacturing, testing, assembly and use often can cause solder interconnect failures.  Establishment of the strain rate-strain limit for lead-free solder interconnection on different PCB surface finishes can greatly help safeguard the reliability of the products.

More details can be found in “Lead-Free Solder Interconnect Reliability” (www.asminternational.org/leadfree ).

5.    Compliance assurance management

The issue of compliance assurance management may turn out to be more challenging than the technical issues related to RoHS.  ‘Due diligence’ is easy to spell in words, but not so easy to define and demonstrate.  Hopefully, a sensible approach will emerge that will help achieve the goal of eliminating hazardous substances from electrical and electronics products, but does not impose too much a burden on the industry.  This way, the industry can focus on achieving compliance without having to spend excessive energy on trying to figure out how to demonstrate it.

Together, we’ll get there.
 

Keywords : Shangguan, lead-free, environmental, compliance

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