Lead-free soldering is a technology that is gaining momentum in the electronics manufacturing world. Pending legislation in Asia and Europe have electronics manufacturers developing and implementing lead-free soldering. The American Competitiveness Institute has performed research on this subject, having completed several lead-free production runs. Here is a brief primer on issues that one has to consider when implementing lead-free soldering.

Lead-Free Solder Paste Alloys:
There is a wide range of lead-free solder paste alloys available. In general, lead-free alloys have higher soldering temperatures than SnPb (Table 1). The most popular range of alloys is in the SnAgCu family for SMT manufacturing. Eutectic SnCu has been recommended for wave soldering applications. SnBi alloys have shown promise in commercial applications due to their low soldering temperature. However, in high reliability applications with severe thermal requirements (-55oC to 125oC), SnBi does not perform as well as SnAgCu or SnCu. In addition, Bi when exposed to Pb can form a low temperature intermetallic which can reduce solder joint thermal reliability.

Moisture Sensitivity:
Due to the higher temperatures associated with many lead-free alloys, the requirement for baking out moisture sensitive components and board materials become even more important. Studies performed by Lucent and the Lead-Free Components Focus Group have found that plastic components will lose 1 to 2 levels of moisture sensitivity when exposed to lead-free reflow soldering temperatures up to 260oC as opposed to 220oC for SnPb solders. Failure to bake out moisture sensitive hardware will result in failures due to delamination ("popcorning") and measling.

Component & Board Finishes:
There are several component and board finishes available. Lead-free board finishes, such as OSP, NiAu, and Immersion Sn, are currently available from most board suppliers.

Component manufacturers are beginning to offer lead-free component finishes. NiPd, Sn, SnCu, and SnAgCu are a few component finishes being offered. Component finishes are based on market demand placed on the component manufacturer. Depending upon the interaction between the solder alloy, component finish, and board finish, there is a possibility of material incompatibilities. However, the Lead-Free Components Focus Group found minimum thermal reliability effects from solder paste alloy, component finish, and PWB finish variable combinations.

Screen Printing / Component Placement:
Some good news. From a screen printing and component placement perspective, lead-free solders perform as well as their SnPb counterparts.

Reflow Soldering:
The peak temperatures for many lead-free solder alloys range from 240oC to 260oC (Figure 1). Thermal profiles with higher peak temperatures will be required. New reflow soldering equipment can support the higher temperatures associated with lead-free solders, but what about current reflow soldering equipment. Depending upon the equipment's vintage, current reflow soldering equipment may not be able to support lead-free soldering. Equipment that can support lead-free soldering temperature requirements will require additional preventative maintenance.

Another option would be to reduce the belt speed through the reflow soldering oven. The lower the belt speed, the higher the temperatures reached within the oven for a given thermal mass. However, the lower the belt speed, the longer the exposure to the higher reflow soldering temperatures. This longer exposure may damage the components and boards. The lower belt speed will reduce the reflow soldering oven output.

Many lead-free solders do not wet as well as SnPb. Therefore, to improve wetting, reflow soldering should be performed in an inert atmosphere.

Wave Soldering:
ACI soldered lead-free and SnPb hardware on comparable production quality wave soldering equipment. The lead-free solder required more aggressive solder flux than SnPb. While the solder pot temperature for both alloys were set at 500oF, lead-free solders required higher preheat temperatures than SnPb.

Lead-free wave soldering should be performed in an inert atmosphere to improve solder wetting and reduce the amount of dross.

Cleaning:
It has been ACI's experience that no-clean lead-free solders leave minimum residues as their SnPb counterparts. However, many solder suppliers are using more active solder fluxes with their lead-free solder pastes to improve wetting. While these fluxes do open the processing window, the residues left on the hardware after soldering are harder to remove. To remove these residues, a more aggressive cleaning process may be required.

Visual Appearance:
Lead-free solder joints look different that their SnPb counterparts. Lead-free solders will have a dull grainy appearance. The IPC are beginning to change their visual inspection guidelines to take this feature into account (Figure 2).

Thermal Cycling Reliability: Based on the Lead-Free Components Focus Group, it was determined that lead-free solder joint reliability is equivalent to SnPb solder joints for 2,000 thermal cycles, with temperatures ranging from -55oC to 125oC. Failure distributions for LCCC and area array component types found to be similar to other industry studies using similar test conditions. Of significance is that wave soldered hardware built by ACI, with a SnAgCuSb alloy, went through 2,000 thermal cycles without failure.

Despite these process differences, the American Competitiveness Institute proved that it was feasible to build high reliability hardware which meets IPC J-STD-001C Class 2 and Class 3 inspection criteria. For additional information, contact the EMPF Helpline at (610) 362-1320.

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.