Reflow Soldering Guidelines & Guidelines for Lead Free Reflow Soldering Audit
Go to Reflow Soldering Soldering Guidelines
Go to Lead Free Reflow Soldering Soldering Audit
Wave Soldering Guidelines
Screen Printing
There are little differences found between the Tin Lead (SnPb) and Lead Free solder pastes when screen printed. With respect to print quality and snap off, there were found no differences between SnPb and Lead Free Solders.
Therefore, until further evidence warrants, it can be concluded that for the component placement process that for the screen printing process, Lead Free solders were equivalent to Tin Lead (SnPb) solder pastes.
Screen Printing Stencil Design
Stencil design guidelines support lead free printing shouldl be based on IPC 7525A
Cookson found that under stencil design may require aperture changes. A modified Inverted Home Plate aperture design for chip components may be used to reduce solder ball potential (Figure 8).
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Figure 8. Examples of Stencil Aperture Designs
Stencils must be accurately aligned with board. Off-center prints will not result in complete pad coverage.
Component Placement
There have been little differences found between the solder pastes with respect to component placement. Tin Lead (SnPb) and Lead Free solder paste tackiness were considered equivalent. Therefore, until further evidence warrants, it can be concluded that for the component placement process, Lead Free solder pastes were equivalent to Tin Lead (SnPb) solder pastes.
Reflow Soldering Equipment
For reflow soldering, the differences between Lead Free solders and SnPb solders are wider. Typically, the peak reflow soldering temperature for Lead Free solders can range between 240oC and 260oC. The thermal profile will be dependent upon the Lead Free solder alloy used and the vendor. For example, ACI discovered that for the same SnAgCu alloy, that 2 vendors had different recommended reflow soldering profiles and peak reflow soldering temperatures.
It is recommended that soldering be performed in an inert atmosphere, such as nitrogen. This will improve solder wetting and reduce solder residues. It is possible to perform Lead Free soldering without nitrogen. The JG-PP / JCAA Lead Free Soldering Program proved that nitrogen is not required to meet IPC-A-610 Class 3 inspection requirements. However, using nitrogen opens the process window wide enough to successfully solder hardware that is less than optimum condition.
With respect to the equipment, depending upon the vintage of the reflow soldering ovens used, current equipment can reach the reflow soldering temperatures required to process Lead Free soldered hardware. The equipment settings will be dictated by:
- The solder alloy used
- The hardware’s thermal mass
To reach the higher reflow soldering temperatures, it is possible to reduce the oven’s belt speed. The slower belt team will reduce productivity. Due to the higher temperatures, more preventative maintenance – specifically belt lubrication and panel maintenance – will be required.
New ovens on the market are capable of supporting Lead Free soldering (Figure 9).
Figure 9. Examples of Reflow Soldering Equipment from ACI’s Demonstration Factory.
Number of zones can be critical to the reflow soldering process. Older 5 zone ovens may not practical. These ovens may not provide the flexibility to build Lead Free soldered hardware in a production environment.
The newer 3 to 5 zone ovens appear feasible to solder Lead free soldered hardware, but their value will depend on hardware requirements and process variables used to manufacture the hardware:
- Belt speed
- Hardware thermal mass - Differences in thermal mass from one design to another.
- Heater Panel output – Describes the oven’s thermal capacity.
- Can you reach the peak temperatures?
- Where do you reach peak temperatures in the oven?
Based on ACI’s experience, it is recommended that 7 to 10 zone reflow ovens be used. This will provide better thermal profile control to the process. The additional temperature zones will improve the oven’s capability, allowing more design flexibility into the process.
Reflow Soldering Profiling
Because Lead Free soldering requires operating at a higher temperature – from 240oC to 260oC peak reflow soldering temperatures for most Lead Free soldering alloys – the problems associated with poor reflow soldering profiles become paramount.
These are a few of the process control indicators when a reflow soldering process goes out of control, as per Table X. When using Lead Free solders, extra attention must be given to the thermal profile.
Most consideration is given to reaching the peak temperatures. However, the cool down portion of the thermal profile should be controlled.
- Cool Too Fast could thermal shock solder joint and component. However a fast cool down will result in the solder joint having a smaller grain size. This should improve the solder joint’s fatigue resistance.
- Cool Too Slow will result in a solder joint with grainy appearance due to the larger the grain size within the solder. There is the potential for excessive intermetallic layer growth which creates brittle solder joints.
Problem |
Possible Thermal Profile Root Cause |
Cracked Chip Capacitors |
Excessive rise rate in the preheat zone |
Solder Balls |
Incomplete drying before reflow
Dry-out section too cool and too short a duration
Excessive drying temperature
Improper gas atmosphere: Air versus Nitrogen |
Cold Solder Joints |
Insufficient time over reflow temperature |
Solder Not Wetting To Leads |
Excessive drying time causing flux to deteriorate
Excessive reflow temperature/time causing oxidation |
Solder Not Wet On Pad |
Lead is heating faster than board (too much airflow) |
Component / Board Burning |
Excessive reflow temperature |
Table 1. Reflow Soldering Thermal Profile Issues
Courtesy of the Research International Solder Reflow Technology Handbook
Typically, the cool down rate is 3 to 4oC / sec until approximately 130oC. Therefore, from 240oC to 130oC it will take approximately 28 to 36 seconds to cool down.
This is a typical SnPb reflow soldering profile (Figure 10). Note that the peak temperature is at 221oC.
Figure 10. Typical Tin Lead (SnPb) Thermal Profile
This is the reflow soldering profile for SnAgCu Lead Free solder alloy. Note that the peak temperature is at 250oC (Figure 11). ACI did run successfully hardware with SnAgCu Lead Free solder alloy with the peak reflow soldering temperature set at 240oC. This difference was the vendor recommendations.
Figure 11. SnAgCu Thermal Profile
Lead Free Reflow Soldering Audit
Acquire a pair of test vehicles for assembly. Both test vehicles should represent the product line. Both vehicles should have different thermal masses.
Print Lead Free solder paste. Capture printing settings. Examine print quality and document.
Place components onto screen printed board. Document the accuracy of the component placement process.
Document how many zones the reflow oven has.
Generate a thermal profile with the reflow oven using 2 assemblies. Record the following parameters for each assembly:
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Belt Speed
- Zone Temperatures
- Cooling Settings
Reflow solder hardware. Inspect hardware to IPC-A-610. Class 3 inspection requirements. Document findings.
Compare the Zone Temperatures of the thermal profiles to the maximum oven temperature for each zone. This will determine if the oven has the thermal capacity to perform Lead Free Soldering.
Can reflow soldering be performed in Nitrogen or an inert atmosphere?
