Wave Soldering Guidelines & Guidelines for Lead Free Wave Soldering Audit
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Wave Soldering Guidelines
Tooling
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. Companies are now beginning to coat solder pot fixtures with ceramic and Teflon to reduce this damage.
The wave solder pot shall not run continuously or idly for extended periods of time. The solder pot shall 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 4. Example of a solder pot damaged with Lead Free solders.
Wave Solder Material Control
Tin Lead (SnPb) and Lead Free solder bar stock are close in appearance. Wave solder personnel shall identify Tin Lead (SnPb) and Lead Free solder bars. When the wave solder machine is charged with Lead Free solders, the area shall be swept for Tin Lead (SnPb) and unidentified solder bars (Figure 5). All Tin Lead (SnPb) and unidentified solder bars shall be removed from the workcenter.
Figure 5. 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 Directives.
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.
Solder Pot Analysis
A sample from the wave solder unit’s solder pot shall be analyzed for constituents. This analysis shall be performed on a monthly basis. This analysis may be performed on a more frequent basis, depending upon production requirements.
Lead (Pb) and Copper (Cu) from component leads can sufficiently contaminate the Lead Free wave solder unit’s solder pot above the 0.1% upper control limit for Lead (Pb) as defined by the RoHS Directive, and above the 1% limits specified by IPC J-STD-001D.
If the materials analysis indicates that the contamination levels are above the levels specified by IPC J-STD-001D and the RoHS Directive, the manufacturer shall take corrective action to reduce solder pot’s contamination levels.
Corrective action includes:
- Replacing the solder pot’s contents with fresh solder.
- Adding a sufficient amount of pure Tin (Sn) to the solder pot to reduce the contamination level.
All solder pot analysis shall be made available for review.
Wave Solder Unit Maintenance
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, with 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.
Flexible Wave Soldering Manufacturing
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, their 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:
- Turn off the power to the solder pot and allow the solder to solidify before removing.
- Disconnect the power via a quick connect.
- Using the motorized pot drive, via a push button, allow the pot to load onto the rollaway solder pot cart.
- Disconnect the drive shaft.
- Move the lead free pot and cart into load position.
- Reconnect the Drive shaft and via push button allow the pot to load into the machine.
- Reconnect the power via a quick connect electrical connecto.
- Turn on the power.
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 workcenter 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 6).
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Figure 6. Wave Soldering Conversion Process
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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.
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Wave Solder Thermal Profile
For every assembly, the wave soldering process parameters shall be documented. These process parameters include:
- Belt / Conveyor Speed
- Top Pre-heating Temperatures
- Bottom Pre-heating Temperatures
- Solder Pot Temperature
- Solder Flux
- Solder Flux Application Parameters
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 temperatures the board experiences during the process is critical (Figure 7).
Avoid Top Side from becoming too hot. If the Top Side becomes too hot, the top side solder joints can reflow.
Overheating Bottom Side can warp board. If the bottom side becomes too hot, circuit card can warp. Boards have been know 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 10oC to 20oC higher than for Tin Lead (SnPb).
Figure 7. Example of a Wave Solder Thermal Profile
Lead Free Wave Soldering Audit
Note the tooling finish in the wave solder pot. If the tooling is not finished, using Lead Free solders represent a major risk due to Tin (Sn) erosion.
Is the bar stock in the wave solder workcenter clearly identified?
When the Lead Free solder pot is in operation, is there any Tin Lead (SnPb) solder in the workcenter?
What fluxes are used in the wave soldering process? Is the flux identified in IPC J-STD-006?
How often is the solder pot contents analyzed for contamination?
How often is solder dross removed from the solder pot?
Is the solder pot clean, with no solder splashed internal to the unit?
Are the conveyor’s fingers clean, with no solder attached to them?
Is the wave soldering process documented?
- Belt / Conveyor Speed
- Top Pre-heating Temperatures
- Bottom Pre-heating Temperatures
- Solder Pot Temperature
- Solder Flux
- Solder Flux Application Parameters
For every assembly, is there a thermal profile for the wave soldering process?
Are there top side and bottom side pre-heaters to support Lead Free soldering?
Is there a board guide through the middle of the wave soldering system?
Run a fully populated board through the wave solder unit, using Lead Free solders. Inspect per IPC-A-610, Class 3 and document findings.
Document how the operators clean their hardware? With cleaning operations, can operators remove flux residues from Lead Free wave soldering operation?
