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Measuring the impact of component solderability on lead-free solder joint reliability.

Wickham, M; Zou, L; Dusek, M; Hunt, C (2005) Measuring the impact of component solderability on lead-free solder joint reliability. NPL Report. DEPC-MPR 038

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Whilst the impact of poor component solderability on lead-containing solder systems is well understood, little information is available on the impact on the new lead-free assembly processes being introduced to conform to European legislation. Previous NPL work has demonstrated relationships for SnPb solder systems between component solderability, solder fillet rise, and process yield, and between solderability and thermal fatigue. This report summarises similar work on assembly yield and subsequent reliability of joints made using a lead-free soldering systems, employing visual inspection, micro-sectioning, electrical conductivity, and shear testing to evaluate the solder joints.
Components with lead-free finishes (R0603, R1206, SOIC and BGA) were artificially aged to produce a range of reduced solderabilities and assembled into test vehicles using lead-free solders. In the cases of the resistors and SOIC components, the reduced solderability had little effect on the process yield. The previous work with SnPb solder did show an adverse effect on the solder joint fillet, and this improvement for the leadfree assembly is attributed to the greater soldering temperatures, and/or the higher flux activity levels associated with lead-free solder pastes. In the case of the BGAs, the artificial ageing process significantly degraded the component substrate, causing warping during assembly, and reduced yield for the most severely aged components.
The reliability of the solder joints was again examined after thermal cycling. There were no electrical failures with the R0603 or SOIC components, and only a few with the R1206 components – insufficient to differentiate between the various component solderabilities. In the case of the BGAs, as thermal cycling progressed, the number of electrical failures increased for the most severely aged components, particularly in the outer rings of the BGAs. After 2000 cycles, joint cracking was present close to the component interface in samples from all ageing groups (i.e. different solderabilities). This is attributed to stress concentrations associated with solder mask definition of the pads.
The salient conclusion from this work is that, for the normal range of solderability associated with quality lead-free components likely to be experienced by end users, there should be no adverse effect on assembly yield or subsequent reliability, in a controlled lead-free assembly process.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DEPC-MPR 038
Subjects: Advanced Materials
Advanced Materials > Electronics Interconnection
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3367

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