Mixed metal manufacturing is, in fact, quite commonplace in today's manufacturing environments. Most often, due to component and board availability issues, we see lead-free finished devices being used in an otherwise Pb-free process. But, the reverse – as you've asked here – is also a fairly frequent occurrence.
In either case, the most common problem with leaded devices and mixed metal manufacturing is improper wetting and through analysis and reflow profile adjustment, sometimes acceptable wetting can be achieved. But more severe problems exist when processing BGAs and CSPs in this environment.
In this case, incomplete bump melting or collapsing occurs. The goal is to promote even distribution of lead throughout the solder joint, but, if the bump doesn't melt completely, achieving this condition is impossible. Without the lead evenly dispersed within each solder joint, premature joint cracking and early thermal fatigue are the most likely outcomes, yielding uncertain device reliability.
Mixed metals may also put you out of tolerance with RoHS. Shipping product to Europe with mixed metal assembly methods may be setting your company up for penalties or fees due to lead content in the solder joints.
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Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.