|Ask the Experts|
February 9, 2009
Set up for lead-free wave soldering
What is the best set up for a lead-free wave soldering machine? Concerns include temperature for top and bottom pre-heaters, settings for conveyor speed, pressure for the flux, and most importantly temperature for the solder pot.
|Expert Panel Responses|
Wave soldering setup is dependent upon the materials used, and the configuration of the PCBA.
Considerations for Leadfree Soldering:
* Flux Type : RMA/Low VOC/ NoClean or WaterSoluable
* Flux Delivery System: Foam/Ultrasonic/Pnuematic Spray
* Solder Alloy: SAC 304, 305, Sn100C etc
* PCB Laminate : Td Temperature
* PCBA Type :Single Side PTH, PTH/SMT Mixed Technology
* PCBA Component Limits : Validate Processing Temperatures of Top and bottom side components
* PCBA Handling ; PCB Edge, Palletized, Selective Palletized
* PCBA Mass or Loading and Thermal Properties : Artwork Design and Heat Dissapation
* PCBA Lead/Hole Ratio and Contact Patch : Duration in Solder, and Alloy Selection
* Conveyor Angle and Machine Options: Angle of approach coupled with Wave Type and Atmosphere
All the above are factors to be considered in Wave Soldering Process development.
For a standard PTH technology, 305SAC RoHS, Water Soluable Flux Configuration, we approach the wave soldering below;
Coverage of the Fluxshould be controlled to minimize overspray to enhance this drying requirement, we generally reduce pressure to just allow the PCB coverage, and minimize any pooling on topside.
General guidelines are to get a topside or preheat temperature of the assembly to approximately 140-160 C for the topside of the assembly, ramping linearly the preheat settings are commonly set at 200-210C. This is done in conjunction with the amount of Flux applied to ensure the flux used is fully evaporated, especially on RMA or Low VOC types of No Clean.
At a 3 degree slope our PCBA is checked for entry depth into the lambda wave. A calibration plate is used to determine the solder contact patch or size. From this patch, we can determine the solder time used. No Lead wetting is best at 7-10 seconds, allowing the flux to work.
Solder Temperature is set on our machine to approximately 275C, for SAC 305 Alloy with conveyor speed of 30-35M/Min pending PCBA size and Contact patch above.
The combination of the component leads, annular rings, thermal relief and Lead:Hole Ratio should be considered when designing the PCB for Wave soldering. If SMT devices are also added to the mix, Wave settings and the use of Nitrogen might be considerations to carefully monitor their effect on solder finish and yield.
Capital Equipment Operations Manager
Specialty Coating Systems
While optimal process settings will be determined by the alloy, flux, equipment type and individual assembly being processed, some good rules of thumb are: Lead-free processes respond and perform better when you approach them with patience. Lower conveyor speeds and slightly longer contact times will benefit hole fill, reduce bridging and provide overall greater wetting and spread to components and pads. Slower conveyor speeds therefore are preferred over faster. Surface tension of lead-free alloys is higher than tin/lead, so their "peel" characteristics; which affect shorts/bridging, react better to lower conveyor speeds. Lead-free alloys also tend to wet slower, so again, lower conveyor speeds allow a compensation for this inherent characteristic. It is recommended to begin process setup by adjusting contact time in the wave. This can be accomplished in a number of ways such as adjusting the laminar flow duct, or through software settings such as pump speed and lead-clearance. The simplest and most common approach would be to lower the speed of the conveyor. Most leaded applications will run between 3 feet ~ 5 feet/minute with emphasis on the latter, but it is rare that lead-free applications ever see 5 feet/minute. For lead-free applications, normal speeds are in the 2.5 feet to 3.5 feet/minute range, but let your contact time dictate this - a target contact time in the wave for lead-free is 4 seconds ~ 6 seconds. Follow up with optimizing your pre-heat settings, which are normally dictated by the product application and flux requirements; however, a typical starting temperature will usually be in the 100C /212F (top-side) prior to contacting the wave. The actual process window again will be flux and product dependent, but may be in the 85C to 130C range. Flux volume, pressure settings, etc. will vary widely between chemistry types and activator packages. Do not get hung up on what the actual equipment settings are. Optimize your flux application last by reducing the amount applied until you induce a failure - noticed through spider webbing, bridging, poor hole fill, etc. Increase the amount slowly until you have achieved the desirable results. Remember that the flux application is also dependent on your pre-heat temperatures. Hotter pre-heat temperatures generally require a larger volume of flux to be applied, inversely cooler pre-heat temperatures generally require a lower amount of flux, which also results in lower post-process residues. Alloy temperatures for lead-free tend to hover around the same as tin/lead, but will perform better if kept slightly higher. Most commonly we recommend a temperature in the 510F to 515F range. A common concern in lead-free alloys is copper dissolution, and not all lead-free alloys are equal in this regard. The amount of copper dissolution you experience will be a function of the lead-free alloy you select, the process temperature of your solder pot, and the exposure, or contact time an assembly experiences during manufacture. Do not be afraid to think outside the box!. Datasheets are good starting points, but they cannot take into account every variable which will affect the overall process. Experiment will flux volume and pre-heat temperatures on both ends of the spectrum to achieve the best possible/most optimal results, and work closely with your solder and flux supplier. By approaching each new process in this manner, you will learn much more about the products you are using, and the true process window of a each application.
This is somewhat a difficult question to answer. With wave soldering the process parameters are more related to the assembly and application then they are to machine settings.
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