By Vern Solberg, Tessera, San Jose, Calif.
Factors that an engineer should review before developing a product using CSPs include physical features and device construction, environmental limitations and suitable substrate materials. A general understanding of the attachment methodology is also required. Environmentally Safe Assembly Many of the electronic products under development that employ miniature CSPs are moving toward lead-free, environmentally safe assembly processes. IPC-9701, a new industry specification, is near release. It establishes specific test methods to evaluate the performance and reliability for SMT and CSP attachment. The tests are designed to replicate actual use environments of the electronic assemblies. In addition, it establishes different levels of performance and reliability of the solder attachments of all surface mount devices to rigid, flexible and rigid-flex circuit structures. The benefits of reliability testing:
The reliability of the solder joint attachment of surface mounted electronic components to the circuit board substrate may require extensive testing. The testing is to qualify a product for specific-use categories and/or environments.
During the product's use, surface mount solder joints can be subjected to a variety of loading conditions that may lead to premature failure. Reliability, as defined in IPC-SM-785, is the ability of a product (surface mount solder attachments) to function under given conditions and for specified periods of time without exceeding acceptable failure levels. The following load conditions should exist either singly, sequentially or simultaneously:
The Coffin-Manson Model, for example, is a "predictive model," which relates the number of cycles-to-failure to the applied plastic strain Most solder joint failures are a direct result of the strain developed during high and low temperature extremes. These extremes represent the difference in expansion between materials incurred during cyclic temperature changes, typical of operational use or testing, due to different coefficients of thermal expansion or cyclic differential expansion. The failures that eventually result from temperature cycling are recorded and show the statistical distribution of failures due to wear. Reliability Database To ascertain that the solder joint attachment of surface mount circuit assemblies meets reliability expectations in the intended use environments, it is necessary to produce a generic reliability database. Accelerated fatigue tests may confirm the reliability for some specific applications, but slower cycle testing typically provides more useful data. Because of the time dependent creep and stress relaxation properties of solder, the cyclic damage and the fatigue life in accelerated testing are not generally equivalent to those in operational use. A generic understanding of the reliability and failure mechanisms for a given surface mount attachment technology is the first step toward designing and assuring product reliability. For this, a generic database is required. Although failure mechanisms based on cyclic or monatomic over-stressing can occur, the most common reliability threat comes from stress-relaxation base fatigue damage. Acceleration Tests A fatigue failure database must be referenced to a combination of low acceleration and high acceleration tests. In this context, low acceleration tests produce a MTTF of the test vehicles that are about 10-20X less than actual life in field use. High acceleration tests are about 100-500X less. The higher the test acceleration (shorter duration between extremes), the less representative of performance in field conditions. Thus, low acceleration tests should closely mimic expected field conditions, while high acceleration tests are often needed by the reality of the time and resources required for the low acceleration testing. The rapid movement toward lead-free soldering is of grave concern by many experts in the field of reliability testing for electronic assemblies. Non-Lead Alternative Alloys Many companies began looking at non-lead alternative alloy years ago, knowing that any alloy selected must meet the criteria for reliability and economy, as well as assembly process compatibility. Some of the alloy compositions considered appeared to achieve all the physical goals for component attachment, but due to excessive oxide growth, acceptable flux materials often proved inadequate. Another issue is the temperature required for soldering. The liquidus temperature range is generally a great deal higher than the traditional eutectic tin/lead alloy, requiring an even greater thermal rise before reaching the wetting state. Two factors one must consider when reviewing alternative attachment alloys are availability and cost. A number of the alloys being considered for soldering are not available in the volumes necessary to maintain projected production demand. In some cases, additionally, the sources for unique materials may not be reliable. The cost of the non-lead alloy will be a factor as well. Although the material supplied in a paste form for surface mount attachment is only slightly higher than the lead-bearing paste, the basic solid alloy will be more than 100% higher in cost.
The silver alloy percentage (by weight) is a significant cost factor, and bismuth, indium and antimony alloys possess a premium cost penalty as well. When comparing the relative cost of these alloys to the cost of lead, the engineer must consider the benefits to be gained in product reliability, rather than the economy of the material itself. As far as any health risk is concerned, most experts believe the lead-free issue is more political than real. The fact that less than one-half of one percent of the lead used by industry worldwide is for electronics assembly should minimize practical concern. However, the industry may find it an advantage in the end to move ahead with lead replacement before government actions require alternatives. The other issue is consumer pressure. If "Company A" claims to have a lead-free, environmentally friendly product, and is perceived to be gaining market share, "Company B" will rapidly move to comply. This market-driven condition may have more to do with accelerating the shift to lead-free electronics than any government-imposed activity. Further Reading 1. W. Engelmaier, Design for Surface Mount Solder Joint Reliability, Society of Manufacturing Engineers (SME) publication. 2. IPC, Guidelines for Accelerated Reliability Testing of Surface Mount Attachments (IPC-SM-785), Northbrook, Ill. 3. IPC, Implementation of Ball Grid Array and Other High Density Technology (J-Std-013), Northbrook, Ill. 4. SMT Attachment Reliability Test Methods (IPC-9701), an IPC standard currently in development.
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