Challenges in Manufacturing Reliable Lead Free Components

The recent push for lead free products is resulting in significant changes in packaging materials. Manufacturers of electronic equipment are requiring materials that consistently withstand peak reflow temperatures of 240?C to 260°C. Reflow soldering at these ext reme temperatures, especially after extended moisture exposure, introduces several challenges that must be solved to produce reliable products . Plastic packages use various organic compounds for molding and die attach that have formulations consisting of an epoxy resin, filler particles and other additives. The epoxy resin in equilibrium with the ambient atmosphere absorbs a small percentage of moisture, which turns into saturated steam during the printed circuit board reflow process. The extreme pressure from the steam and the drop in flexural strength of the mold comp ound and die attach materials can result in catastrophic failures of the packages. Typical failure modes of packages include encapsulant cracking, substrate cracking, severe package deformation, and delamination of one or more material interfaces. The problem is further exacerbated by the use of very large dies, which are typical of Altera devices. In this paper, we will discuss some of the modifications necessary to meet product reliability and usability requirements for lead free products. We will present the test results to demonstrate the reliability of PQFP, TQFP, BGA and Flip-chip BGA components and relate the results to the different approaches that were taken for different package types to successfully qualify the components to the higher reflow temperatures. For some package families, the assembly processes were improved and optimized to ensure the reliability to Pb-free reflow temperatures. For some other components, new packaging materials were used to overcome the limitations of existing packaging materials . We will also present effect of the reflow temperature on the moisture ratings of a representative member of each package family. In addition to the traditional responses such as pop-corning and interfacial delamination during reliability testing, the importance of looking at the effects of absorbed moisture on the component warpage at elevated temperatures will also be discussed. Absorbed moisture is known to increase package warpage. The higher warpage for large BGA components at Pb-free reflow temperatures requires the monitoring at the rated MSL level, since it impacts the manufacturability and reliability of final assembly. Introduction Altera’s Lead-Free (Pb-free) solutions are motivated by the requirements being imposed on the semiconductor and electronics industry to reduce or eliminate the use of Pb. A directive by European council on Waste from Electrical and Electronic equipment (WEEE) proposed restrictions on the use of Pb in electronics by 2006. In Japan, the ministry of international trade and industry has set a maximum amount of Pb for automobiles (excluding batteries). While there is no legislation mandating the reduction in Pb in the electronic devices in Japan, the electronic industry is actively marketing select electronic devices as Pb-free. Based on these proposals and directives, Altera has proactively committed to working with its suppliers to offer Pb-free packaging solutions. There are several competing options available for Pb-free finishes. Working with the assembly sub-contractors Altera has evaluated packages with matte Sn and/or Sn-2% Cu lead finish for leaded packages, and Sn-3-4%Ag-0.5%Cu solder balls for ball grid array packages. Additional testing on pre-plated Ni/Pd finish is also being pursued as a possible alternative finish. For the leaded packages, 12 micron thick Sn or Sn-2% Cu plating is used on standard lead-frames. In addition to the change in the lead finish, the thermal robustness of the package was improved by selecting appropriate materials and processes to allow for the higher reflow temperature compatibility required for assembling boards using Pb-free solder pastes. The qualification data gathered demonstrates the technical capability to assemble most devices offered by Altera with process and/or material modifications. Some of the devices tested had very large die sizes that posed some unique challenges. Not all available assembly process and materials strategies from the assembly sites were easily portable because of the very large die sizes for typical Altera devices. Various approaches were used to make the components reliable to Pb-free reflow temperatures, including the use of new materials and changes in the processes to achieve better reliability. Verification of Moisture Sensitivity Levels The standard Sn-Pb components without any additional modifications have been tested to the standard JEDEC surface mount simulation test to different Moisture Sensitivity Levels (MSL). The components were tested to different peak reflow temperatures – 220C, 235C, 245C. While almost all components meet MSL3 at 220 C, some of the larger components meet only MSL4 or worse for higher temperatures (see Figure 1). For peak reflow temperatures of 245 o C testing on the larger components (and larger die sizes) resulted in die surface delamination (Figure 2) at MSL level 3. Degradation in moisture sensitivity was observed on each package type to different extents. Smaller TQFP, PQFP and FBGA packages were able to withstand 245 C reflow. For the case of Flip-Chip BGAs (FCBGA) underfill delamination and solder smearing (Figure 3) was observed when the solder bump composition was eutectic. When high Pb bumps were used, no underfill dela mination was observed to 245 C on all components 35mm on the side and smaller. To improve manufacturability, Altera prefers to achieve MSL 3 rating for all the components. In order to achieve MSL level 3 for all the components some modifications were necessary for each package type. Different approaches were required for each package type. Requirements from various industry groups require qualification of the components to a peak reflow temperatures of 245C to 260C depending on the component size. MSL ratings for different packages