Lead Free Manufacturing: Wave Soldering
From B2P Portal
| Fred W. Verdi | |
| ACI Technologies, Inc. | |
| One International Plaza, Suite 600 | |
| Philadelphia, PA 19113 | |
| 3/6/09 |
Contents |
Introduction
These Pb-free Manufacturing Guidelines are compiled from both the hands-on experience of manufacturing, reworking, and repairing electronic systems hardware using lead-free processing at the EMPF (Electronic Manufacturing Productivity Facility), which is a COE (Center of Excellence) for U.S. Navy ManTech. This is a living document, representing benchmark presently used Pb-free electronics processing. Processes will be updated as new developments and techniques become available.
| There is evidence that special tooling and tooling finishes may be required for lead-free solders. The high tin (Sn) content solder at elevated temperatures can damage tooling within a solder pot (figure 1). Companies are now beginning to coat solder pot fixtures with ceramic and Teflon to reduce this damage.
The lead-free wave solder pot must not run continuously or idly for extended periods of time. The solder pot must be on a rigid preventative maintenance operation, concentrating on the solder pot's impeller and wave solder fixtures. Following these practices will extend the life of the solder pot using lead-free solders. |
Figure 1. Example of a solder pot damaged with lead-free solders.
Figure 2. Tin-lead (SnPb) and lead-free bar stock are not easily identified.
Putting a tin-lead (SnPb) or unidentified solder bar into a lead-free solder pot can ruin the integrity of the Lead-Free solder. It has been shown that 2 Tin-lead (SnPb) bars can sufficiently contaminate a Lead-Free wave solder system's solder pot above the 0.1% upper control limit for Lead (Pb), as defined by the RoHS Directive.
All solder fluxes used in the process shall be compliant to IPC J-STD-006 is the controlling document which identifies the Requirements for Electronic Grade Solder Alloys and Fluxed and Non-Fluxed Solid Solders for Electronic Soldering Applications.
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Lead-Free solder alloys can generate more solder dross than tin-lead (SnPb) solder alloys. Dross shall be removed from the wave solder unit's solder pot.
Unlike tin-lead (SnPb), many Lead-Free alloy's solder dross may be located on the bottom of solder pot. To assure that there is no dross in the pot, an operator may have to pull samples from the solder pot's bottom. Solder splashes may contaminate the wave solder unit's solder pot. On a weekly basis, the wave solder unit shall be inspected for solder splashes, and the solder splashes removed from the unit. If a wave solder unit has the capability of being converted from tin-lead (SnPb) to Lead-Free solders, inspection for solder splashes shall be performed as part of the conversion process. The wave solder unit's conveyor and fingers may collect solder. On a weekly basis, the wave solder unit's conveyor and fingers will be inspected for solder, with the splashes removed from the unit. If a wave solder unit has the capability of being converted from tin-lead (SnPb) to Lead-Free solders, the wave solder unit's conveyor and fingers shall be inspected for solder as part of the conversion process. |
A major concern is to change the solder pot from tin-lead to lead-free alloys. While most manufacturers will convert their wave soldering alloy from tin-lead (SnPb) to lead-free solders, there will be electronic manufacturers who will require tin-lead (SnPb) to lead-free wave soldering capability who can not afford the purchase of a second wave soldering system. A flexible wave soldering configuration offers the capability to use both alloys. This configuration requires the wave soldering system to the quick disconnect electrical and mechanical connections for the tin-lead (SnPb) solder pot and the lead-free solder pot. The procedure is as follows:
This capability can be applied to selective wave soldering processes, where the solder pots are smaller than for a traditional wave soldering process. This does increase the maintenance requirements for wave soldering, due to the possibility of polluting the lead-free solder with tin-lead (SnPb) solder residues. Quick disconnect fixtures will be required on the solder pot and wave soldering system to support this concept. Solder splashes on the interior of the wave soldering system must be removed. The conveyor fingers must be cleaned frequently to avoid lead (Pb) contamination. The work center shall have all tin-lead (SnPb) solder cleared when the unit has lead-free solder. This is to avoid the possibility of a tin-lead (SnPb) solder entering the lead-free wave solder pot (figure 3). |
Figure 3. Wave Soldering Conversion Process.
| NOTE: Solder pots must be cold and the solder solid prior to changing. Removing a solder pot while the solder is in its molten stage represents a safety hazard. |
For every assembly, the wave soldering process parameters shall be documented. These process parameters include:
For every assembly, a thermal profile shall be generated and documented with the process parameters. The thermal profile is critical in developing the lead-free wave soldering thermal profile. As with reflow soldering, the temperature the board experiences during the process is critical (Figure 4). Avoid Top Side from becoming too hot. If the Top Side becomes too hot, the top side solder joints can reflow. Overheating of the bottom side can warp the board. Boards have been known to deflect 3 to 5 mil during wave soldering. Board deflection can be reduced by tooling or center board guide. Installing a center board guide is dependent upon the wave solder equipment manufacturer. These practices are consistent with tin-lead (SnPb) wave soldering processes. They become more important because lead-free wave soldering processes can operate 10 °C to 20 °C higher than for tin-lead (SnPb). |
Figure 4. Example of a lead-free wave solder thermal profile. Note the high temperatures to which the bottom of the assembly is subjected.
1. IPC J-STD-001D - Requirements for Soldered Electrical and Electronic Assemblies, IPC Standards
2. IPC A-610D -Acceptability of Electronic Assemblies, IPC Standards
3. "Issues And Solutions To Implementing Lead-Free Soldering" by L. Whiteman. American Competitiveness Institute; SMTA Boston Conference; Boston, Massachusetts
4. "Test Results From The Lead-Free Component Focus Group" by L. Whiteman, American Competitiveness Institute, Philadelphia, Pa; M. Kwoka, Intersil, Palm Bay, Fl; J. Cannis, Amkor Technology Inc., Chandler, Az; G. O’Brien, Photocircuits, Glen Cove, N. Y.; D. Hillman, Rockwell Collins, Cedar Rapids, Ia; M. Toben, Shipley Ronal, Freeport, N. Y.; R. Schetty, Technic, Inc., Freeport, NY; SMTA Boston Conference; Boston, Massachusetts, May, 2002
5. "Guidelines for Lead-Free Hand Soldering" by L. Whiteman, American Competitiveness Institute, R. Northam, American Competitiveness Institute; Circuits Assembly Magazine
6. "Converting Wave Soldering Equipment From Tin-lead To Lead-Free" by L. Whiteman, American Competitiveness Institute, J. Stong, American Competitiveness Institute, D. Alavezos, Technical Devices Company; Circuits Assembly Magazine
7. "Assembly Of JCAA/JG-PP Test Vehicles" by A. L. Campuzano-Contreras,BAE Systems, SMTA International Conference, Chicago, Illinois; September, 2005
8. "Lead-Free Process Implementation Tactics" by C. Shea, Cookson Electronics Assembly Materials Group
9. "Lead-Free Wave Soldering: Tighter Process Windows Require Tighter Controls" by C. Shea, Cookson Electronics Assembly Materials
10. "Optimizing Stencil Design For Lead-Free SMT Processing" by R. Pandher and C. Shea; Cookson Electronics Assembly Materials, SMTA International Conference, Chicago, Illinois; September, 2004
