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September 2015 • SMT Magazine 41 conditions to which the unit is exposed. This would require that the solder alloy and all solderable surfaces be charac- terised to ensure performance is met. It is of particular im- portance with small area array bottom terminated lead-less components. 2. Electrochemical reli- ability: Material selection and process parameters are of para- mount importance for such sensitive applications. There are two major factors which bring this aspect of reliability into sharp focus for vision and detection systems: Firstly, the small gaps between conductors require that any residues left on the assembly from fluxes or PCB fabrication chemicals to be extremely benign and sec - ondly, the harsh and variable climatic conditions under which the modules need to operate. While there is no one test which can characterise materials for these applications, a broad range of SIR, electromigration and corro- sion tests exist, which in combination can give a strong indication of material suitability. 3. Process Performance Requirements: A lot of expertise in assembling miniaturised electronics exists in the hand-held devices sec- tor, but for automotive this is a relatively new experience. Process performance will direct- ly affect product reliability in several ways. It starts with a robust and repeatable solder paste printing process to ensure that solder joint vol- ume is maintained on critical small footprint devices such as cameras, active and passive de- vices. Solder paste formulation and stencil de- sign know-how is the key to a repeatable print process. Beyond printing, a major focus needs to be on thermal processes to ensure that both solder joint formation and correct activation of the flux has been achieved. This is most critical in the use of any liquid fluxes in a wave/selec- tive process typically used for connector attach. mAterIAL CONsIDerAtIONs FOr ADvANCeD DrIver AssIstANCe systems AssembLy continues FeAture In conclusion, it is imperative that design and manufacturing engineers consider the cost of product failure versus the cost of processes and materials, or in other words, "the total cost of ownership." There is continual pressure on automotive tier 1 suppliers to reduce costs, but at the same time it is widely understood that the choice of assembly materials can make the dif- ference between a product surviving its warran- ty period or not. Assembly materials typically cost approximately 1% of the BOM for a vision detection unit, and as a result it is somewhat surprising that compromises are sometimes made on alloy choice and material selection. The cost of product failure in the field is the full cost of recall and unit replacement, which is of course >100% of the original BOM costs. smt steve brown is the director of automotive oeM marketing for alpha.