Higher SMT and wave solder process temperature: Lead-free solders reach liquidus at temperatures 20-50oC higher than the lead-based solders currently in use. It is projected that IC packages will have to be redesigned to withstand temperatures of 260oC, which brings about concerns over the effects that higher process temperatures will have on substrates and the MRT levels of plastic packaging materials. Process Window The primary challenge that lead-free solders present to electronics assemblers and IC manufacturers is higher process temperatures. The currently accepted limit on the peak temperatures that IC packages can withstand in the course of the thermal process is 235-240oC. The lower limit is generally set at 200-205oC, which is the minimum temperature required to reflow leaded solders reliably. These high and low process limits provide a delta of ˜30oC-wide enough for a carefully monitored process to produce a low defect rate and high yield with little fear of defects caused by process drift. With lead-free assemblies, the process window shrinks dramatically (Figure 1). With the bismuth alloys favored by Japanese assemblers (206-213oC liquidus), the window shrinks by one-third, to a delta of ˜20oC.
When the Sn/Ag/Cu alloys (217oC liquidus) are used, the window is reduced by two-thirds, to a delta of 10oC. Given that few assemblers want to run within 5oC of their control limits, the true process window with Sn/Ag/Cu alloys is extremely small. One obvious solution to this problem is to widen the process window by raising the temperatures that IC packages can withstand. Another solution, one that has often been overlooked, is to increase and optimize the capability of the soldering process. Real-time Thermal Management A potential solution to the problem of the decreased thermal process window, and one that many engineers and managers in semi-conductor manufacturing may not be aware of, is real-time thermal management. For more than a decade, the key to a reliable and repeatable thermal process has been real-time, continuous thermal management, which allows electronics assemblers to obtain and analyze live data on their soldering process by continually monitoring process temperatures in the reflow oven. Real-time thermal management can detect critical process temperature variations that oven control thermocouples cannot, and it can immediately reveal these temperature drifts and their location on the user's PC screen. Thirty thermocouples embedded in two slim stainless steel probes are permanently mounted just above or below the conveyor (Figure 2). The probe thermocouples monitor process temperatures continuously, taking readings as frequently as every five seconds.
These temperatures are displayed as "process profiles" on the KIC PC screen. All data is recorded permanently to the hard drive, giving users the ability to review process data from previous production runs. By creating a mathematical correlation between product profile, as measured by a pass-through profiler, and process temperature, as measured by the thermocouple probes, real-time thermal management provides a product profile for every board processed. This "virtual" product profile is calculated every 30 seconds, and virtual profile statistics, such as peak temperature, are also calculated and updated continuously. The virtual profile allows users to monitor their processes accurately, automatically, continuously and in real time (Figure 3).
Profile Prediction The first automated profile prediction program was released in 1997. At the time, the technology was capable of formulating more than one hundred potential profile recipes per second, then evaluating and ranking the recipes. This tool was capable of finding optimum oven setups that would yield a profile in the center of the process window, as well as the recipe with the highest possible conveyor speed to maximize throughput. It was the first program that enabled users to know they had found an optimal thermal profile for a given product. One issue with the original automated program was that it required an expert operator. Consider that a 10-zone oven offers literally billions of possible combinations of zone setpoints and conveyor speeds. To search all of them would take several days, so the operator was required to know enough about thermal profiling to be able to tell the program which range of combinations of zone setpoint and conveyor speed to search in order to get a solution within a reasonable time. The latest release of the tool offers several significant improvements. It is now capable of searching the entire range of possible recipes automatically, in less than a minute, so operators no longer need to set search parameters. The software package also includes a comprehensive database of solder paste specifications, including data for the new lead-free solder pastes from all major manufacturers. The operator selects the solder paste being used from a drop-down menu, enters any non-solder-paste-related process limits, runs a profile, starts the automated prediction tool, and, within seconds, has an optimal profile that is custom designed for both the oven and the product. Running a pass-through profile confirms that the oven settings are correct and that the oven is ready for production. Because the improved automated prediction tool has searched the entire range of possible oven setups, users are assured of finding the most productive profile. The automated prediction tool is designed to center the profile in a process window defined by the solder paste specification and the user-defined input process limits. This is done by ranking potential process profiles with a "Process Window Index" (PWI). The Process Window refers to how well a given profile "fits" the critical process statistics. The index goes several steps beyond telling the user whether the profile is merely in or out of spec. The PWI uses the input process limits to rank profiles numerically (Figure 4).
A PWI of 100 or more indicates that the profile will not process product in spec. A PWI of 99 indicates that the profile will process product within spec, but at the very edge of the Process Window. Less than 99 indicates that the profile is in spec and tells users what percentage of the process window they are using: for example, a PWI of 70 indicates a profile that is using 70% of the process spec. Most users seek a PWI below 80, and with the improved automated prediction tool, profiles with a PWI between 50 and 60 are commonly achieved (if the oven is sufficiently flexible and efficient). Temperature-sensitive IC packages provide an example of the way the improved automated prediction tool functions. If the package can't withstand temperatures above 240oC, but a peak temperature above 230oC is required to reflow the lead-free solder properly, the automated prediction tool will find the optimal profile and center it between the high and low process limits, giving users the most robust profile their oven is capable of achieving. Conclusions While the shift to lead-free electronics assembly presents the semiconductor industry with serious challenges, technology exists to meet the most significant of these. The higher minimum soldering temperatures required by lead-free solder alloys require that temperature-sensitive IC packages must be soldered much more carefully than in the past, but the use of existing and proven technology demonstrates that the packages can be soldered to the electronics assembly successfully. The use of real-time thermal management and an improved automated prediction tool will allow most existing packages to be attached using lead-free solders without requiring modifications to increase their thermal tolerances. References 1. For more information on no-lead efforts, visit the IPC's www.lead-free.org site. 2. J. Cannis, "IC Packaging Perspective on the Pb-Free Solder Issue," SAC Technology Session, Nov. 18, 1999.
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