So, rather than re-inventing the wheel, we'll concentrate on basic information you, as a buyer or purchasing agent, can use to make your life easier. The following information helps you to understand some of the key particulars involved in selecting Pb-free PCBs and their associated costs.
What's Your PCB Application?
If the Pb-free PCBs you are purchasing are to be used in medical, industrial, military/aerospace, consumer, computer, or network systems, the Pb-free PCB requirements for each of these system applications are vastly different from one another with different costs and delivery times. So, it's a good idea ato get a sound understanding of the Pb-free PCB design, fabrication, and design steps associated with your particular PCB application.
Higher Temperature Requirements for Lead Free Process
The higher temperatures used for Pb-free processing and rework place greater stress on components and boards, which is further aggravated by cumulative heat exposures. The combined reflow and rework heat cycles can result in a single assembly being exposed to as many as five thermal heat excursions that includes two reflow passes, a wave solder pass and two localized heating passes for rework.
The internal package structure within components and PCBs must survive all processes and still provide long-term reliability. Component effects include increased moisture sensitivity levels (MSL), thus resulting shorter exposed floor life, while PCB laminates must minimize internal layer de-lamination, via cracking and board warpage.
SMT first pass reflow processing typically has the following characteristics:
- Four to six minute cycle times.
- Temperature ranging from 230 to 250ºC for joints and body temperatures.
- Delta T ranges from 5 to 20ºC on single assembly.
- Time above liquidus ranging from 60 sec to 90 seconds.
The minimized superheat required for Pb-free assembly and rework means that profiling becomes critical. It is not uncommon for an engineer to attempt to use one profile for a family of assemblies of similar size and complexity. This is usually workable in an SnPb process since it is characterized by a generous process window. As long as all the assembly reaches about 200º C, one can be reasonable confident that all the components will properly solder. This approach does not work with Pb-free assembly and rework, which requires more precise reflow temperature cycles.
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