- History & Importance of PCB - Environmental Initiatives - Reliability Consideration With High Temp Processing - Reliability Considerations for Materials to withstand Lead-Free Assembly - Plated Through Hole Failure Modes - Considerations for PCB laminate materials - Printed Circuit Materials Overview - PCB Material Properties - What is “FR4”, Laminate Composition - Overview of other laminates

This paper will explore the most common alternatives to hot air-leveled solder (HASL) as a finish for flex circuits and some of the issues one may want to be aware of when converting. Whether the reason for seeking alternative finishes is RoHS compliance or assembly process enhancement,there are factors one must consider in switching.
We will address benefits and impacts of ENIG (electro-less nickel/immersion gold),Immersion Sn (tin) and Immersion Ag (silver),electrolytic Ni/Au (nickel/gold),as well as what is commonly known as “lead-free” solder. The benefits range from ease of processing – bare flex and assembly – to a more robust interconnect concept for specific applications. Impacts are the “gothchas” - potential pitfalls – and can include electrical test
failures and significantly increased part cost.
The attendee will leave with an appreciation for the careful nature of making such a significant change in part configuration.

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In this presentation,numerous examples of counterfeited parts are provided along with the means of identification for the respective technique utilized in production.

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This presentation discusses critical material properties and test data that are often overlooked in the introduction of new lead-free solder alloys,but are critical to alloy comparison and the development of life predictive models and acceleration factors. Common gaps in property and test database are identified (e.g.,lack of creep data at low to medium stress and cold temperature,insufficient data under mildly accelerated test conditions). The importance of variations in temperature variables (cold and hot
temperatures) as well as dwell times is also discussed. Examples of thorough test conditions and test databases that have been used for the development of SAC305,SAC387/396 acceleration factors are presented. It is concluded that the “winning” alloys - i. e. alloys that end-users can work with – are those that are fully characterized in terms of metallurgy (including at interfaces) and mechanical / physical properties & their evolution; are robust enough under both thermal and mechanical loading conditions; and come with an extensive reliability test database and validated reliability models & acceleration factors.

Constitutive and failure descriptions of SnAgCu solder alloys are of great interest at the present. Commonly,constitutive models that have been successfully used in the past for Sn-Pb solders are used
to describe the behavior of SnAgCu solder alloys. Two issues in the characterization of lead-free solders demand careful attention: (i) Lead-free solders show significantly different creep strain
evolution with time,stress and temperature and (ii) The building of valid constitutive models from test data derived from tests on solder joints must de-convolute the effects of joint geometry and its influence on stress heterogeneity. In the first part of the talk I will review the common approaches to modeling solder behavior,along with their limitations and then describe our efforts in developing constitutive models of SnAgCu solders that are valid across a wide range of strain rates.
The problem of solder joint fatigue is essentially one of fatigue crack growth. However,there is little work that has been done to arrive at fatigue life estimations by means of tracking of the crack front and its growth. Common fatigue life models such as the Coffin-Manson rule are empirical and therefore,limited in their applicability and in the insight they provide. There are several challenges to employing a fracture mechanics approach to accurately track the growth of a fatigue crack in a solder joint. Key
among these,are the facts that the problem involves large-scale yielding,viscoplastic solder behavior and complex geometries. In the second part of the talk,I will describe the various approaches to
modeling solder joint fatigue and present our efforts at developing Cohesive Zone Model inspired approaches to predicting crack propagation at solder interfaces.

For many of the Pb-free solders required under the European RoHS directive,there is now sufficient information,primarily in the form of the results of accelerated thermal cycling of various levels of severity,to develop acceleration models for the creep-fatigue of these solders. In this paper the parameters for the SAC405/305,SAC205,SAC105 and SnAg to replace the parameters for eutectic SnPb in the well-established Engelmaier-Wild solder creep-fatigue model.

Due to In-Circuit Test issues probing difficult to penetrate processes such as Pb-free solder,OSP (Organic Solderability
Preservative),Immersion Au (Gold),Ag (Silver),Sn (Tin) and No-Clean,as well as the different manufacturing process
variations like wave,select wave and reflow (single and double),new innovative razor sharp probe tip styles were developed
(Figure 1). It was determined that a probe tip that has the qualities of a sharp blade edge would help to penetrate these problem processes. Edge sharpness has been determined to be a critical factor in the successful penetration of these processes within via and test pad applications. In addition,due to the sharpness of the edge,the penetration is effective even off center of the test via or in the pad area as well. Most tips used to probe vias today contain a single point design with the mentality that this sharp single point will break thru these board processes/contaminants. However,in most cases a single point probe will bottom out in the pool of flux contained within the via resulting in poor electrical contact. A razor sharp tip style will better penetrate these hard to probe fluxes and contaminants without bottoming out. Typically,Test Engineers
would be forced to increase the probe spring force to break-through these contaminants,but high-density PCB’s do not allow
the use of higher spring forces due to the increased possibility of board flex which can cause damage to expensive boards. The use of the razor sharp probes does not require a higher spring force and in some cases the spring force can be reduced. Because of these innovative tip styles,first pass yields are significantly increased,repeated fixture actuations are not needed,false failure rates and NDF’s (No Defects Found) are reduced,all of which results in less test time and faster board throughput which ultimately lowers board test costs.

All but the most simple circuit boards processed in recent times are inspected using in-circuit testing,ICT. In order to test on pads or connections printed with no-clean solder paste; the post-reflow flux residue must be easily penetrated by ICT pin-probes. Additionally,the electronics industry continues to move from tin-lead solder alloys to various lead-free alloys,particularly the tin-silver-copper alloys that usually require hotter reflow profiles. The hotter reflow conditions required for lead-free solder paste influence the consistency of the solder paste flux’s residue and its suitability for in-circuit testing,often producing a hard,brittle residue that is difficult to pin probe.
A new method of evaluating solder paste flux residue penetrability has been developed. Because most circuit assembly plants use vacuum actuated fixtures,this method uses a common,multi-probe ICT fixture,modified and wired so the probe-to-pad resistance can be evaluated using a very sensitive 4-wire resistance measurement. This method allows measurement of resistances as low as 30 mO?. Four probe forces,three tip shapes and two probe densities have been incorporated into the fixture,so the optimum probe shape and minimum force required to successfully penetrate the solder paste’s residue can be determined. Results are presented for the ICT probe penetrability of the flux residue of several different solder pastes after being processed with different reflow conditions.