Because lead-free solders do not behave or look like their lead-containing counterparts, individual PCB assemblers will need to address several issues related to hand soldering and rework. These issues include:

• Higher melting temperatures. Operators mistakenly increase equipment operating temperatures. Higher operating temperatures do not speed up the process; they actually slow it down.
• Poor wetting and spreading properties. Additional time is required when working with lead-free solders. They do not spread or wet like lead-containing solders do.
• Difficult to work with. Bridging and insufficient solder defects are common even for experienced operators.
• Dull grainy finish – makes inspection difficult.

Because lead-free solders oxidize quickly, more aggressive and longer lasting fluxes are required to keep surfaces clean and free from oxidation. Working with no-clean fluxes can be challenging, as their process window is often smaller. Once they are gone, oxidation immediately begins to form, which can result in a marginal or defective solder joint. Additionally, increasing operating temperatures creates an ideal environment for oxidation and will also lead to flux and solder ball splatter on the PCB. If the flux is splattered all over the PCB, it will not be able to do its job on the surfaces to be joined during soldering.

Lead-free solders also affect soldering and rework tools, and their effect can be detrimental. Lead-free solders contain high percentages of tin, almost always over 94%. Tin is a corrosive and active metal. When it mixes with the protective iron layer on a soldering tip, an inter-metallic compound is formed which will wear away more quickly. This causes shorter tip life and causes oxidation to form more quickly (further exacerbated by the high temperature environment the tips work in). Once the oxidation begins to form, the tip will lose its ability to wet with solder. If not cleaned off quickly, it becomes almost impossible to remove the oxidation, and the tip must be replaced.

The use of nitrogen-assisted soldering equipment helps to mitigate the lead-free soldering problems. Nitrogen helps on two fronts. First, it creates an inert environment around the soldering tip, reducing the potential for the tip to oxidize. Second, it assists in the soldering process at the PCB level by purging oxygen from the immediate area, which reduces or eliminates the formation of oxidation from the work site. This not only reduces the amount of flux required, but also helps to improve wetting and coverage, leaving a finish that is shinier and less grainy.

The EMPF utilizes various soldering stations in the Demonstration Factory and the Training Center. One of our lead-free compatible stations is the Pace TW-50 soldering system, with nitrogen provided by the Pace NF500 Nitrogen Farm.

The TW-50 uses a single-channel power supply with the Thermo-Drive soldering iron. The system can also be used with mini-tweezers. Using the proper power module (the power module selects the desired heat level for soldering operation) makes the system easy to operate. The TW-50 comes standard with three power modules – heat levels 6.5, 7.0, and 7.5.

If the power module is not installed or if it is removed during operation, the system will switch itself off. An LED indicator light on the front panel will turn red. To operate the unit, the operator must insert the desired tip cartridge and install the desired power module into the power port on the front of the unit. The LED indicator turns amber while the tip heats to the selected level. Once the tip has reached the desired heat level, the indicator turns green and the system is ready to use.

All solder tips are lead-free ready. Simply select the appropriate power module for the temperature requirements. To improve the wetting of lead-free solder, Pace’s NF500, a stand-alone nitrogen generator, separates nitrogen from standard room air and supplies it to the soldering site.

If you would like a lead-free soldering demonstration with the Pace TW-50 and NF500, please contact Jeff Stong at the EMPF at (610) 362-1200, extension 224 or jstong@aciusa.org.

For more information concering Lead Free processes and surrounding issues, please stop by ACI's new Lead Free Manufacturing Page to download articles contributed to ACI by some of the industry's most knowledgable individuals and organizations, as well as material generated by ACI, and documents on the legislation surrounding the Lead Free issue.