Lead Free Manufacturing: Reflow Soldering
From B2P Portal
| Fred W. Verdi | |
| ACI Technologies, Inc. | |
| One International Plaza, Suite 600 | |
| Philadelphia, PA 19113 | |
| 3/6/09 |
Contents |
Introduction
| This Lead-Free Electronics Manufacturing Guidelines is meant to establish practices and procedures that may be used to allow the utilization of Lead-Free electronics in military systems.
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. |
Lead Free Stencil Printing
| There are little differences found between the Tin-lead (SnPb) and Lead-Free solder pastes when stencil 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 and for the stencil printing process, Lead-Free solders were equivalent to Tin-lead (SnPb) solder pastes. |
Solder Paste Maintenance
| Tin-lead and Lead-Free solder pastes are maintained the same way. The only caution is to avoid mixing the two. |
Stencil Printing and Design
| Stencil design guidelines support Lead-Free printing should 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 1). |
Figure 1. Examples of Stencil Aperture Designs.
Stencils must be accurately aligned with board. Off-center prints will not result in complete pad coverage.
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 are equivalent to Tin-lead (SnPb) solder pastes. |
Reflow Soldering Equipment
| For lead-free 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 in 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:
To reach the higher reflow soldering temperatures, it is possible to reduce the oven’s belt speed. The slower belt speed 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 2). |
Figure 2. Examples of Reflow Soldering Equipment from ACI Technology’s Demonstration Factory.
Number of zones can be critical to the reflow soldering process. Older 5 zone ovens may not be 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 the experience of ACI Technologies, it is recommended that 7 to 10+ zone reflow ovens be used. The more zones, the better the higher lead-free profile can be controlled. 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 1. 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 for Pb-free electronics manufacturing as this portion of the profile may cause intermetallic compound issues in the solder joints.
|
Table 1. Reflow Soldering Thermal Profile Issues
| 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 |
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 3). Note that the peak temperature is at 221oC.
Figure 3. Typical Tin-lead (SnPb) Thermal Profile
Figure 4 shows a reflow soldering profile for SnAgCu Lead-Free solder alloy. Note that the peak temperature is at 250oC. ACI Technologies did successfully build lead-free hardware with SnAgCu lead-free solder alloy with the peak reflow soldering temperature set at 240oC. This difference was the within the solder paste vendor recommendations.
Figure 4. SnAgCu Thermal Profile
Hybrid (Mixed Alloy) Reflow Soldering Guidelines
| When RoHS compliance is exempted, such as for electronic assemblies in the military or aerospace industries, there are situations in which lead-free components will be used while continuing to produce tin-lead soldered surface mount assemblies. In most components, except for BGAs, with lead-free finishes such as tin or nickel gold, using lead-free components with tin-lead solder paste does not cause soldering or reliability problems.
Reliable solder joints can be formed when a lead-free (SAC alloy) BGA is reflowed with eutectic tin-lead paste under a lead-free reflow profile (peak temperature typically 230 °C to 250 °C). This is called Hybrid Lead-Free Reflow. Under this situation the lead-free solder spheres will melt and Pb will freely diffuse into the solder sphere forming a homogeneous joint. These lead-free peak temperatures sometimes may not be desirable for the boards or components that are not able to withstand high temperatures. Conversely, running the hybrid assembly under the standard tin-lead reflow profile (peak temperature typically 210°C) can be risky to the lead-free BGA solder joint because in some cases, the lead-free balls melt partially, or not at all depending on the range of the peak temperature. In order to maintain the reflow temperature under manufacturer's recommendation, a specific profile called hybrid reflow profile is recommended. An example of the targets is shown below in Table 2. |
Table 2. Suggested hybrid profile for reflow of lead free finished components attached with tin-lead solder.
| Hybrid | |
| Ramp Rate | 0.5 to 1.2 ºC / sec |
| Time above Sn-Pb Liquidus | 80 to 90 sec |
| Peak temperature | 220 to 230 ºC |
| Cooling stage | < 2 ºC / sec |
A common recommendation is for peak temperature to be above 220 °C and the dwell time above liquidus to be 60 to 90 seconds. The goal is for this peak temperature to collapse the solder spheres to allow for homogeneous solder joints and yet the entire temperature profile is still within the manufacturer’s recommended specification limits of the tin-lead profile. This provides the benefits of soldering the lead-free components without elevating the board assembly to the high lead-free temperature. The key to run this profile lies in the capabilities of the reflow oven to achieve the profile with narrower windows.
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:
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? |
References
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
