iNEMI's High-Reliability RoHS Task Force has published recommendations for Pb-free manufacturing of complex, thermally challenging electronic assemblies. These recommendations, which focus primarily on thermal requirements for components, laminate and PWB materials, and equipment, are intended to help ensure product reliability.
Most high-reliability products have a Pb-solder exemption or are out of scope under the RoHS Directive, and the companies that manufacture these products are evaluating the impact of Pb-free SAC solder assembly on the reliability of these more demanding applications. The complex assemblies found in high-reliability products often feature broad component mixes and pose many thermal challenges as a result of board thermal properties and the resulting thermal gradients across the assembly.
Pb-free assembly is already challenging for manufacturers due to the high temperatures required and the resulting tighter processing windows. When assembling complex assemblies, these challenges are exacerbated by the variability of component mass, large size and high layer count of PWBs, the need to rework, and the high likelihood of mixed through-hole, surface mount, and back side assembly.
"New soldering materials, maximum qualified component temperatures and primary attach and/or rework equipment all need to be addressed relative to reliability in the face of Pb-free assembly requirements," said Joe Smetana, principal engineer, Advanced Technology, for Alcatel and co-chair of the task force. "These recommendations were developed to communicate the needs of the high-reliability segment to the supply chain and the relevant standards groups that must address these needs."
Key recommendations include the following.
ComponentsIndustry standards for components have not yet fully encompassed Pb-free processing requirements, particularly for thermally complex assemblies. Recommendations address temperature requirements for non-hermetic devices, glued and wave-soldered SMD components, wave soldering, and BGA coplanarity requirements over temperature.
Laminate & PWB MaterialsThe task force recommends specific test methods, including CAF (conductive anodic filament) and thermal/temperature cycling, to validate acceptable performance of materials in the appropriate use conditions.
EquipmentManufacturers need to work with equipment providers to integrate and resolve the issues of process speed, peak temperatures, flux chemistry, solder pot contamination and soldering gas atmosphere. Key issues (from a materials perspective) that need to be addressed include the ability of flux to handle higher temperatures at longer pre-heat times (135-200C for 3-4 minutes) and total profile times of 8-9 minutes.
In the meantime, a few steps can be taken to improve processing.
- Slow reflow oven speed to guarantee a good solder joint.
Use the following techniques to improve rework processes:
- Preheat the whole board to reduce heatsinking effects of PWB power and ground planes.
- Improve adjacent component "shielding" from hot gas rework temperatures to prevent secondary reflow.
The International Electronics Manufacturing Initiative's mission is to assure leadership of the global electronics manufacturing supply chain. Based in Herndon, VA, the industry-led consortium is made up of approximately 70 manufacturers, suppliers, industry associations and consortia, government agencies, and universities. iNEMI roadmaps the needs of the electronics industry, identifies gaps in the technology infrastructure, establishes implementation projects to eliminate these gaps (both business and technical), and stimulates standards activities to speed the introduction of new technologies. The consortium also works with government, universities and other funding agencies to set priorities for future industry needs and R&D initiatives.
For additional information about iNEMI, visit http://www.inemi.org . The recommendations are available on the iNEMI website at http://www.inemi.org/cms/projects/ese/High_Rel_RoHS.html .
ABOUT THE TASK FORCEThe iNEMI High-Reliability RoHS Task Force consists of electronic product manufacturers (OEMs and EMS providers) whose products are characterized by long service life and high-reliability requirements. For these companies, maintaining product reliability is crucial to survival. The following companies support the iNEMI recommendations for Pb-free manufacturing.
Agilent Technologies Inc.
Cisco Systems Inc.
Delphi Electronics & Safety
Jabil Circuit Inc.
Sun Microsystems Inc.