Investigation of Lead-free Package Reliability
A new, reliable material is essential
By Jeffrey C.B. Lee and Simon S.M. Li
Solder containing lead has been widely used in the electronics industry as an interconnect and surface finish for PCBs for more than 50 years. It offers the best in classes from the perspective of cost and reliability. Due to environmental and health concerns, the EU's WEEE and RoHS directives require lead be reduced or completely eliminated before July 2006. Developing a drop-in alternative technology is of great interest to the industry.
The lead-free conversion involves the entire supply chain in the electronics industry, requiring consideration of its impact on the current process, equipment compatibility, cost, operator training, etc.
The major concerns arising from promising replacements of lead-free solder, such as a ternary SnAgCu alloy in laminate package and binary SnCu, and SnBi alloy and matte Sn in a lead frame package, are high melting points in comparison to the eutectic SnPb alloy. Components will be subjected to a stringent 30° to 40°C higher temperature surface mount soldering process to induce significant thermal stress and vapor pressure inside the packages. This gives rise to typical failure inside the package, such as delamination, popcorn, package cracking, etc. To reduce the propensity of package failure, a new and reliable material is essential.
Solder Ball Selection. Sn4 Ag0.5Cu was selected, and its appearance is far different from that of the eutectic SnPb ball because of the dominate Sn grain on the surface (Figure 1).
Figure 1. Appearance comparison of solder balls: a) Sn4 Ag0.5Cu, b) 63Sn37Sb.
Die Attach Selection. Two types of die attach were selected for comparison. One was a single epoxy-based resin; the other was a hybrid type containing two kinds of resin-base besides epoxy to optimize balance of the adhesion and moisture uptake, as shown in the physical properties comparison in Table 1.
Table 1. Physical properties comparison of die attach.
Molding Compound Selection. Two molding compounds were selected for comparison. One was a conventional multifunctional type A, while the other was a unique multifunctional green compound B without Br and containing Sb2O3. Their properties comparison is shown in Table 2. Type B possesses higher filler content, but better spiral flow; lower Tg, but lower CTE and higher flexural strength and modulus at high temperature; and lower water absorption than Type A.
Table 2. Physical properties comparison of molding compound.
Substrate Selection. Two types of BT substrate with PSR4000 AUS5 soldermask were selected. One was a conventional pumice type, using a jet scrub process to Cu before printing soldermask. The other was a CZ type using a chemical etching process on Cu (Figure 2). Generally, CZ treatment on Cu possesses better adhesion strength with solder mask than pumice treatment.
Figure 2. Surface treatment to Cu: a) Pumice, b) CZ treatment to Cu.
Reflow Profile Comparison in Precondition. The package assembled with the above two category material sets is subject to JEDEC MSL3. Sequentially, SAT and SEM analysis were applied to identify the failure mode.
The moisture absorption curve under 30°C/60% relative humidity circumstances and desorption curve at baking 125°C over the time (Figure 3). The weight percentage of moisture absorption is higher in the 'A' material set than in the lead-free 'B' material set, as well as in the moisture desorption under 125°C baking. The phenomenon explains how the combination of higher moisture material will generate higher vapor pressure under high-temperature reflow soldering such as 240° or 260°C, so that the thermal stress induced in the package leads to risk in interfacial delamination between individual materials, package cracking and popcorning.
Figure 3. Moisture absorption and desorption curve comparison between two category material sets.
Failure with the two category material sets under various reflow soldering temperatures, after MSL3 moisture absorption, is shown in Table 3. Higher moisture material set 'A' survived at 220° to 225°C, but began to fail at 240° to 245°C and 250° to 255°C. Lower moisture set 'B' can survive at 250° to 260°C, but fails at 260° to 265°C. This finding shows the limits in each material set and identifies the vapor pressure impact on package reliability. The application in lead-free packaging with the material set limit will be to decide how wide the process window is in the SMT process, which indicates that higher MSL also should be pursued.
Table 3. Failure rate under MSL3 with various reflow profiles.
Five typical failure modes for the two material sets are shown in Table 4. Material set 'A,' with a higher moisture absorption, encounters delamination between the soldermask and compound if using CZ substrate with high adhesion strength between the soldermask and Cu. The absorbed moisture will penetrate into a weaker interface between the soldermask and compound.
Table 4. Failure mode description with the two material sets.
If the substrate is a pumice type in material set 'A,' the failure mode can be obtained. This results from poor adhesion between soldermask and Cu in BT core material. High temperature, weaker interfaces generate the popcorn effect.
The failure mode in the interface between die attach and soldermask occurred at >240°C reflow soldering with material set 'A.' The top and bottom are different. It looks perfect in its adhesion with the Si die. Although the epoxy-based resin is compatible with the similar resin based in soldermask AUS5, but the moisture accumulated always exists in the organic interface. The vapor pressure induced when exposed to high temperatures will damage limited adhesion in the organic interface if chemical bonding is insufficient. The epoxy-based die attach contains higher moisture, around 0.5%, and its modulus presents lower at high temperature. However, the tremendous vapor pressure effect will be a predominant factor even if the lower modulus performance is capable of releasing thermal stress from high-temperature soldering.
Enhancing adhesion strength in the organic interface by adding adhesion promoter with the soldermask improves chemical bonding. Curing density adjustment in the soldermask is a solution to overcome delamination. However, moisture reduction with a new, resin-based die attach development seems to be the essential direction.
Material set 'B,' with lower moisture absorption after MSL3, was subjected to reflow soldering 250° to 255°C, 255° to 260°C, and 260° to 265°C. No failures were found, except at 260° to 265°C, with a popcorn failure in the die attach. The adhesion with Si die and soldermask present perfectly after high-temperature reflow. However, the thermal stress induced from high-temperature (260° to 265°C) is severe enough to compress and shear the die attach and initialize the crack, resulting from a lack of cohesive energy of die attach. A solution to pursue is enhanced fracture toughness and thermal resistance for hybrid-based die attach to mitigate crack propagation.
Based on this discussion of package integrity under various reflow conditions, each material set has a capability limit to withstand high temperatures. Higher soldering temperatures will be beneficial to solder joint reliability from well soldering, especially in higher melting point SnAgCu, but also will negatively affect package integrity.
The test vehicle in this study was PBGA 35 mm × 35 mm. As package sizes increase, the thermal stress induced and warpage concern under high reflow temperature increases as well. MSL capability to withstand package failure will be downgraded with dimension increases. With a package size decrease, the MSL capability based on PBGA 35 mm × 35 mm will be upgraded to a higher level, because of reduced warpage concerns and lower thermal stress from CTE mismatch.
A material set to survive at higher reflow temperatures is possible, as long as a new material is developed. Lower moisture uptake, higher adhesion strength, optimized modulus and lowest CTE mismatch with related materials will be major factors to address the capability in higher reflow temperature. Compatibility between new materials and current equipment remains a concern.
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JEFFREY C.B. LEE, lead-free and green program manager, and SIMON S.M. LI, VP at central processing engineering, may be contacted at ASE, 26 Chin 3rd Rd., 811, NEPZ, Kaohsiung, Taiwan, ROC; 886 7 3617131; e-mail: firstname.lastname@example.org.