- During stencil design, it’s important to keep in mind that lead-free pastes have higher surface tension and do not wet or spread on the surface of pads as easily as eutectic solder pastes.
- The reflow process window for conventional soldering is relatively wide. The melting point of the eutectic solder is 183ºC. The lower temperature limit for reflow is usually 200ºC and upper limit is about 235ºC. The lead-free alloy used for BGA solder balls has a melting point of 217ºC and requires a minimum reflow temperature of 235ºC to ensure good wetting. Maximum reflow temperature is in the 245ºC to 260ºC range, depending on complexity and density of the board.
- Reflow profile should be performed by attaching calibrated thermal couples embedded in the spheres of the larger BGA parts and other critical PCB locations to ensure all components are heated to temperatures above minimum reflow temperatures and that smaller components do not exceed maximum temperature limits.
The OEM seeking lead-free design, fabrication, and assembly must cast a wary eye on the system an EMS provider has for separating eutectic versus lead-free components and assuring timely and efficient lead-free board design and fabrication. The last thing an OEM wants is for his boards, supposedly populated with eutectic components, to go through the lead-free reflow oven and come out damaged. A traumatic incident like this relates back to how incoming QC for components was conducted, how well auditors were trained, and the level of sophistication of the inventory management system being used. When this happens, the EMS provider incurs the cost of its mistakes, but the OEM’s boards may be significantly delayed, as well.
But the most important pre-caution to an OEM is to avoid selecting separate design layout and fabrication contract manufacturers for a single Pb-free PCB project. For example, an OEM that goes to a design house that’s not well versed in lead-free may inadvertently spec out fabrication using a HASL surface finish, which is not conducive to lead-free assembly. Or, the design house may unknowingly use an SMT component on the PCB’s bottom side that can only go to 230ºC.
It’s vital for the EMS provider to be up-to-date and well-versed on all design and fabrication areas dealing with lead-free. This includes keeping track and implementing lead-free industry developments, understanding how solder paste compositions are changing, how component vendors are deploying part numbering schemes, which surface finishes are best for given applications, and other related details. With this in-depth lead-free knowledge base, an EMS provider can design and fabricate highly reliable lead-free boards to meet an OEM’s time to market requirements.