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Lead-free PCBs

Number of visits: Date:2007-3-19 10:51

The conversion to lead-free soldering has created significant material challenges for PCBs with materials that must be compatible with higher soldering temperatures and longer dwell times. Lead-free soldering temperatures are approximately 25°C higher for reflow soldering and 10°C for wave soldering, and dwell times tend to be 30–60 seconds longer for reflow soldering and 2–3 seconds longer for wave soldering. These requirements put more emphasis on PCB material performance.

Critical PCB material parameters must be reviewed and materials retested to ensure compatibility with lead-free soldering processes. Material testing should be a collaborative effort between the PCB fabricator and the material supplier. Work with your PCB supplier to ensure adequate material testing is done. At this stage, it is not necessary to test individual PCBs for compatibility. However, once the materials have been tested and selected it is good practice to evaluate a few PCBs using actual reflow, wave, and rework soldering conditions to ensure thermally robust materials have been selected.

In 2006, the iNEMI High-Reliability Task Force published "Lead-free Manufacturing Requirements for High-complexity, Thermally Challenging Electronic Assemblies." This document provides a summary of key material characteristics, test methods, and suggested values for lead-free PCBs. Copies are available at www.inemi.org.

The glass transition temperature (Tg) of a resin system is the temperature at which polymer materials transition from a rigid to a soft state. Tg affects coefficient of thermal expansion (CTE) and Z-axis expansion. Tg temperature range is approximately 130–170°C. For all PCBs, a Tg >140°C is recommended; for PCBs that contain BGAs, have more than 10 layers, or have high aspect ratios (>6:1) a Tg >165°C is recommended. Remember that Tg alone is not a good indicator of thermal robustness.

The CTE is the amount of material expansion that occurs above and below the glass transition temperature, usually expressed in parts per million (ppm) per °C. Z-axis expansion is the primary concern because it impacts plated-thru-hole reliability. CTE in the Z-axis is approximately 50–85 ppm per °C; it can be three to four times higher above Tg. Low Z-axis expansion materials are desirable. Test methods include IPC-TM-650.2.4.24C in-plane (xy) CTE in ppm/°C and IPC-TM-650 2.4.41 out-of-plane (z) CTE in ppm/°C above and below Tg.

Material decomposition temperature (Td) usually is defined as the temperature at which 5% of the original resin mass is lost to decomposition, leading to laminate failure (delamination). Materials with a high Td are desirable. Td temperature range is approximately 290°–370°C. For all PCBs, a Td of value > 325°C is recommended. Test methods include IPC-TM-650 2.4.24.6 decomposition temperature, 5% weight loss by TGA.

Time to delamination (T260 and T288) is the amount of time a material can withstand at a single temperature before delaminating (T260 = 260°C and T288 = 288°C). The recommended value for T260 is >30 min.; the test method is IPC-TM-650 2.4.24.1. The recommended value for T288 is >5 min.; the test method is IPC-TM-650 2.4.24.1 modified per paragraph 6.1 to 288°C.

RoHS restricts the use of two flame retardants: polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), which have been used in thermoplastics, and, as a general rule, have not been used in PCBs. A halogenated flame retardant called Tetrabromobisphenol A (TBBPA), which currently is not banned or restricted by legislation, typically is used in FR-4 PCBs. Detailed information about PCB flame retardants is contained in IPC-WP/TR-584, a white paper and technical report on halogen-free materials used for PCBs and assemblies. Copies are available at www.ipc.org.

The ideal surface finish does not exist today. Every surface finish has limitations that affect fabrication, solderability, testability, reliability, or shelf life. Surface-finish selection becomes more challenging as component and board complexity (i.e. fine pitch, BGAs, blind vias) increases. The advantages and disadvantages of each surface finish must be compared to the application. For example, a HASL finish is adequate for coarse-pitch surface mount components; however, if fine-pitch components are involved, a HASL finish is not the ideal choice. There are five common surface finishes: hot air solder leveling (HASL — tin/lead and lead-free); organic solderability preservative (OSP); electroless nickel immersion gold (ENIG); immersion silver (ImAg); and immersion tin (ImSn).

Surface finishes can be rated using these criteria: non-BGA and BGA solder joints are reliable and predictable; solderability shelf life (storage) is >6 months; finish thickness is consistent and flat; finish is conductive (for in-circuit testing); finish maintains solderability through four soldering cycles, and acceptable hole filling occurs when no-clean flux is used. When I compare and rate the five finishes, there is not an ideal choice for all applications. Based on these criteria, my lead-free surface finish preferences are ImAg and OSP.

Conclusion
Lead-free soldering temperatures have created unprecedented challenges for PCB material manufacturers. Work closely with your PCB suppliers to ensure that PCB materials are compatible with your lead-free soldering processes. SMT

Robert Rowland is an SMT Editorial Advisory Board member, instructor, and co-author of the book Applied Surface Mount Assembly. He currently is the process engineering manager at RadiSys Corp. in Hillsboro, Ore., and technical conference director of SMTA International. He also is an active member of the SMTA and a recipient of the SMTA Founder's Award. Contact him at (503) 615-1354; e-mail: rob.rowland@radisys.com. Rob's latest CD "Lead-free Reflow and Wave Soldering" is available at www.smta.org.

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