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Lead-free Processing: Thermal Effect on Components
December 31, 1969 |Estimated reading time: 5 minutes
By Yuichi Tenya and Tom Adams
Exposure to the higher temperatures needed for lead-free processing may shock surface mount devices (SMD) because most designs and encapsulating materials are geared for the lower temperatures of Sn/Pb.
A major concern in the switch to lead-free solders is the impact higher reflow temperatures will have on surface mount components. Although numerous lead-free alloys have been proposed as replacements for conventional Sn/Pb, the feasible replacements mostly are Sn/Ag/Cu alloys. The National Electronic Manufacturing Initiative (NEMI) proposed Sn/3.9Ag/0.6Cu for reflow applications while the IPC Association Connecting Electronics Industries endorsed several variations of this alloy, some with small amounts of a fourth metal. The Sn/Ag/Cu alloys have a melting point in the 217°C range vs. 183°C for the conventional 63Sn/37Pb. The 34°C thermal jump raises reflow temperatures from 225° to 260°C.
Thermal DamageA component's internal thermal damage may not be detectable by electrical tests. This compounds the problem. The internal damage eventually may trigger a field failure. Acoustic micro-imaging research demonstrated that exposure to high temperatures can damage plastic encapsulated components in various ways. The most common damage was popcorn cracks and molding compound delamination from the lead frame or die paddle. Although popcorn cracks are common at conventional solder processing's lower temperatures, they may be more likely at lead-free processing's higher temperatures because of lowered adhesion.
Figure 1. (left) Using a higher temperature molding compound, this PQFP was subjected to lead-free reflow conditions at 260°C. Acoustic imaging at 30 MHz reveals no internal damage.
Figure 2. (right) An acoustic image of a control component from the same test lot shows no significant difference.
A plastic quad flat pack (PQFP)-style package test lot was produced using a molding compound designed to withstand higher temperatures. The critical temperature was 260°C, which was the processing temperature manufacturers anticipated would be reached during lead-free processing. The molding compound used for the test lot was a biphenyl epoxy*. The biphenyl matrix makes it possible to increase the load of filler particles in the molding compound without sacrificing flow properties. As a result, the compound has a lower water absorption rate. To ensure adhesion to the lead frame, the new material contains adhesion promoters. The high filler content meets UL94/V-O requirements for flame-retarding properties without using halogen compounds. These compounds, along with leaded solders, are being phased out for environmental reasons.
Test lot preconditioning was conformed to Joint Electronic Device Engineering Council (JEDEC) Level 1 standards (85°C, 85 percent RH, 168 hours). Two samples from the test lot were selected. One was subjected to IR reflow conditions at 260°C, the second sample was held as a control. Both samples were submitted for acoustic imaging and analysis.
Sample AnalysisThe two samples were imaged extensively to search for differences and internal damage resulting from exposure to 260°C processing temperatures. The most likely internal defects resulting from high temperatures follow:
- Popcorn cracks are caused by the sudden expansion of moisture within the molding compound. Popcorn cracks are almost always large. In an acoustic image, a popcorn crack may cover half the package area. Accordingly, popcorn cracks are more likely than other internal damage forms to be revealed by electrical tests. Large cracks usually break an interconnect.
- Delaminations, including very small separations along the lead frame or die paddle, are unlikely to cause an immediate electrical failure or to be found by electrical testing. Rather, these defects become collecting points for atmospheric humidity and contaminants. Chemical reactions can lead to lead finger corrosion, bond wire breakage, metal migration between adjacent lead fingers and other electrical problems resulting in device failure. The accidental creation of such small defects during higher temperature lead-free processing can result in long-term reliability problems.
Acoustic-imagingThe two samples were imaged with a reflection-mode C-SAM** at a frequency of 30 MHz (Figures 1 and 2). They were imaged from both sides using return echo gating to cover all depths within each sample (e.g., gating at the surface of the molding compound, inside the compound, at the interface between the compound and the die face, etc.). Electronic gating of return echoes limits the resulting acoustic image to the gated depth within the sample. Both samples also were imaged with a second transducer below the sample to collect ultrasound that traveled through the entire device***. This image is an acoustic shadowgraph, which shows gap-type defects at any depth in the device.
Figure 3. Not part of the higher temperature study, this field failure shows the pattern of internal delaminations likely to result from thermal shock.
This comprehensive acoustic-imaging program found no significant differences between the two samples. Specifically, the sample exposed to the 260°C reflow temperature showed no signs of internal damage. There were no delaminations or cracks of any size and no changes in the molding compound's acoustic properties, which would suggest more subtle changes. Nor was there any delamination in the die attach another site vulnerable to thermal shock. The high-temperature molding compound accommodated 260°C reflow conditions without damage to the component.
Damage that can result from exposure to higher temperatures is shown in Figure 3. This device, not part of the study, uses a conventional molding compound in a lead-solder system and was a field failure. The red and yellow areas denote molding compound delaminations from the lead fingers and die face. These delaminations could collect enough moisture and contaminants to cause a failure, but in this case, it is more likely that the die face delamination broke one or more wires.
ConclusionAs production lines are redesigned and requalified to accommodate lead-free solders, engineers will be required to establish and verify numerous parameters to ensure that product reliability is not compromised. Although this early investigation was limited in scope to two components, it provides the following insights for engineers involved in the transition to lead-free solders:
- Properly designed molding compound materials successfully can withstand the higher processing temperatures without internal damage to the component.
- Acoustic micro-imaging is a fast and reliable method for viewing and characterizing internal changes in components.
*KE-3400.** D-9000.*** Thru-Scan mode.
YUICHI TENYA can be contacted at Toshiba Chemical Corp., 3-9, 3-chome, Shimbashi, Minato-ku, Tokyo 105-0004 Japan; 81 (0)3 3502 3218; Fax: 81 (0)3 3501 9989; E-mail: yuichi.tenya@chemi.toshiba.co.jp. TOM ADAMS, a consultant for Sonoscan Inc., may be reached at 2149 E. Pratt Blvd., Elk Grove Village, IL 60007; (847) 437-6400; Fax: (847) 437-1550; E-mail: info@sonoscan.com.