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Flexible circuits processing, performance and reliability using lead-free soldering process

Final Report Summary - FLEXNOLEAD (Flexible circuits processing, performance and reliability using lead-free soldering process)

The project FLEXNOLEAD was aimed at assisting Small and medium-sized enterprises (SMEs) in electronics products manufacture by establishing solutions to the lead-free soldering assembly of flexible circuits and through this opening up new opportunities via development of new environmentally friendly technology for current and next generations of electronics products.

Flexible circuits are most commonly manufacture using one or two base materials, either polyimide or polyester. The former is favoured where soldering is required; the latter is generally used in low cost applications. Both flexible circuit material systems are sensitive to temperature (continuous service temperature: polyimide about 177 degrees Celsius, polyester about 74 degrees Celsius), which raises considerable concerns as to their capabilities of withstanding the higher soldering temperatures, which will be imposed by lead-free solder and its impact on their operating properties. As flexible circuits are developing into such a significant technology for electronic products, with a major role in manufacturing and assembly being played by SMEs, it is essential that an understanding of the impact of this major change in technology on flexible circuits is understood and solutions found.

The scientific and technical objectives were:
1. to develop flexible circuit material processes and design techniques to ensure product manufacture, performance and reliability are not compromised by the introduction of lead-free assembly;
2. to develop predictive techniques for lifetime assessment, including modelling and virtual qualification techniques, to enable confidence in product performance and reduced design and testing time;
3. to undertake comprehensive testing to generate information on failure modes / criteria to enable predictive model validation;
4. to provide specific recommendations on the use and maintenance of lead-free flexible current assemblies.

These were realised by the achievements of the following tasks:
- the definition of sample designs, components and attachment materials;
- processing of substrates and analysis of IPC tests carried out;
- development of optimised substrate materials;
- completion of circuit assembly development with tin / lead and lead-free solder reflow processes;
- completion of circuit assembly development with conductive adhesive curing;
- completion of circuit assembly development using laser soldering techniques;
- completion of life performance testing of the assembled flexible circuits;
- completion of life predictive model for flexible circuits;
- production of design for manufacture guide for the lead-free assembly of flexible circuits.

The exploitation of the knowledge varies depending on the partner organisation; hence, the academic partners will see exploitation as being the increase in content of their courses and research offerings to business, the industrial partners will aim at using the knowledge gained to further business. The organising partner, TWI, will be able to increase its consultative offerings to industry.

Knowledge gained through the course of the project may be split into several parts:
- understanding of the way that typical materials behave in lead-free solder processes;
- alternative manufacturing methods and materials for the production of flexible circuit materials;
- manufacture of circuits using lead-free solder parameters;
- alternative manufacturing methods for the lead-free assembly of FPC and components;
- assessment of the reliability of flexible circuit boards through physical testing and numerical modelling.

The responses from each partner have been amalgamated and placed into the following four categories:
A) materials;
B) flexible circuit manufacturing;
C) assembly of components on flexible substrates;
D) understanding the behaviour of flexible circuits in lead-free processing.