Some have said that I am a fan of lead-free assembly. It's not true. In a world drowning in electronic waste, I am a fan of re-cycling. Removing lead and the other 5 RoHS restricted materials makes re-cycling easier and safer. Before WEEE and RoHS were inacted the world only recycled 10% of "e-waste." We need our recycle target to asymptotically approach 100% in time.
On to lead-free assembly. Some products (network and telecommunications infrastructure, etc) are exempted from the lead restriction in RoHS, these products can contain lead (only in the solder) but must be RoHS compliant in the other 5 substances. Many are now referring to these types of electronics as RoHS 5 products.
RoHS 5 products can face a particular challenge however. Currently it can be difficult to find BGA components with tin-lead solder balls. So, people have to assemble RoHS 5 products with BGAs that have lead-free (SAC) solder balls. This type of assembly is commonly called RoHS 5.5 assembly. The SAC solder in the BGA solder balls doesn't melt at temperatures below 217C. So if the assembler uses a typical tin-lead reflow profile with a peak temperature below 215C, in RoHS 5.5 assembly, the solder joints formed with the BGA will look like the one in the figure above (courtesy C. Key Chung etal). These types of solder joints have been shown to have poor mechanical properties.
Therefore, when assembling RoHS 5.5 products, the reflow oven peak temperature should be about 225C to assure complete reflow of the solder balls which will result in a more robust solder joint.