Process Capabilities

We support our customers conversion efforts to a “lead free” assembly process.

Process Capabilities BESTProto has the capability to be your “lead free” prototype assembler having the processes, necessary equipment and trained and experienced personnel in place. We have been working with the engineering development community for several years on the refinement of lead free processes and assembly techniques. BESTProto has the engineering, processes and capabilities to make your transition go smoothly.

Successful assembly methods have been developed by BESTProto for “lead-free” solders for all types of components. BESTProto has developed manufacturing techniques for the most common lead-free solders including Sn/Ag/Cu and Sn/Ag alloys. The soldering manufacturing parameters are adjusted to accommodate the higher melting temperature and slower wetting characteristics of the “lead-free” solders. We also have developed practices for inspecting “lead free” PCB assemblies.

After confirming that the parts and materials to be used in the lead-free assembly are available, suitable for the process, BESTProto focuses its attention on paste handling, printing, reflow, wave soldering, inspection, rework & repair and cleaning to insure that the quality of “lead free” assembly is maintained.

Paste Handling Shelf Lives

“Lead-free” solder pastes whose storage conditions tend to be slightly more stringent than their tin/lead counterparts, may be reduced when compared with tin/lead alloys. For example, BESTproto process technicians allow paste to come to room temperature before use and they do not mix old and new paste in the same jar. You can be assured that BESTproto follows proper paste handling procedures, which helps maintain consistent quality products that we build.

Printing

There are no major equipment or process condition changes with respect to paste printing of lead free alloys. In general, no major changes to the printing process are necessary. Stencil life, aperture release, print definition, high-speed print capabilities, print repeatability, remain relatively unchanged.

However, “lead free” solder alloys have poorer wetting characteristics compared to tin/lead alloys, requiring some stencil design modifications to maximize paste spread and counteract inferior wetting.

Reflow

This is the SMT process area that is the most affected by a switch to lead-free assembly. Most “lead-free” alloys require higher reflow temperatures than the 210-220ºC peak temperature of tin/lead. An increase to the 235-260ºC temperature range is common. In terms of the reflow profile, making sure that the critical areas of the actual board are profiled based on the solder paste manufacturers’ recommendations and BESTProto process engineering experiences is the key to achieving repeatable results.

Lead-Free SMT Assembly

One aspect of “lead free” solder alloys we have noticed impacting SMT assembly is the poorer wettability characteristics and ensuing smaller spread compared to tin/lead pastes. In addition, surface mount devices tend not to self align as well during reflow. As a consequence the placement accuracy of the BESTProto hand assembly or our pick and place machine has proven invaluable.

Wave Soldering

This is one area of the PCB assembly process where “lead-free” solders have major differences when compared to their tin/lead counterparts. Depending upon the alloy selected, “lead-free” wave soldering will require a pot temperature of 260-275°C. This increase in temperature and the change in solder alloy requires additional process changes. Correctly formulated fluxes for the lead-free solder alloy are used to overcome the poor wetting of the “lead free” alloys and higher thermal stresses of the wave process.

BESTProto has adopted the use of separate selective wave soldering systems for use in lead-free processing as the increased caustic nature of the high tin content solders as well as the higher processing temperatures is maintained.

Inspection

There are a number of visual differences between tin/lead and “lead free” solder joints. The latest version of the IPC-A-610D, has been up updated to account for the changes in the visible appearance of the solder joints. BESTProto inspects the assemblies to these new IPC-A-610D inspection criteria.

Rework and Repair

Operators have been trained for “lead-free” rework, as the lead-free solders do not flow as well as their tin/lead counterparts. One change is in the higher flux content in wire-cored solders. As with any change of flux chemistry. BESTProto technicians pay close attention to the operating conditions in the new operating window.

All rework performed uses the same lead-free solder alloy as originally used on the solder joint; different lead-free solder formulations are not mixed on the same solder joint. If more than one alloy is in use in production (i.e., Sn/Ag/Cu for SMT and Sn/Cu for wave soldering), operators are trained to use the correct solder wire for each part. For this reason alone, it is advisable to use a single solder alloy for all assembly operations. BESTProto uses a single standard SAC305 alloy for both reflow and selective wave soldering processes for this reason.

As part our assembly process controls, work orders have a visual color coding as an indication (red=lead, green=”lead-free”) of the alloy that is to be used on a given assembly. Materials, including both flux and solder, are released per job order based on the alloy specified by the customer.

Lead-Free Through Hole Assembly

It isn’t practical to run both tin/lead and “lead free” processes with a single selective wave soldering machine. We have a selective solder system dedicated to “lead-free” solders as well as one dedicated to the legacy tin/lead processes.