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Part 9: Lead-free System Reliability Avoid Likely Failures
December 31, 1969 |Estimated reading time: 4 minutes
By Jennie S. Hwang, Ph.D.
In this ninth discourse on the subject of lead-free reliability, let's look at what it takes to maximize system reliability, achieving a product's ultimate performance and reliability. Ultimate reliability can only be achieved by planning for reliability at the outset, followed by sound design, material selection, parts selection, and executing manufacturing for reliability.
A product's reliability is gauged by fulfilling its required and intended functions for a designated time frame under a set of anticipated service conditions. Analogous to a chain, the reliability of a product is only as good as its weakest link. From the engineering perspective, the ultimate reliability is to eliminate a product's probability of failure.
Likely FailuresTo weed out the weakest links and minimize the probability of failure, what are likely failure sources in an electronic assembly? Likely failures can be ordered into several buckets: solder joint failure; component/part failure; PCB internal structure-related failure; and other failures induced by protracted time, voltage, and extreme external conditions. The following, by no means exhaustive, examples highlight some common failure sources under each of the main categories, which a producer/manufacturer should avoid. For a lead-free electronic assembly (i.e. main board level), solder joint-related failures, which occupy a significant space of the subject, will be discussed separately in my next column.
Component-related FailuresComponents such as aluminium electrolytic capacitors, multilayer ceramic capacitors, and other heat-sensitive components (crystals, oscillators, fuses, tantalum capacitors) are considered more susceptible to damages from undue manufacturing processes. For BGA/CSP packages, delamination between the layers of interposer beneath the die is a vulnerable area, which may cause electrical failure in the presence of moisture or temperature cycling conditions in end-use applications. Likely production defects in ceramic chip capacitors include overheated termination coatings or internal cracks across the layers as the result of moisture and temperature. In these situations, the damaged components may still pass production-level electrical tests. However, the thermally damaged capacitors may lead to performance errors and eventual failure during the assembly's service life. Additionally, a plastic package crack could be a source of a reliability issue.
PCB-related FailuresThe process temperature requirements using lead-free SAC alloys inevitably put additional demands on the thermal integrity of PCB materials and structure. The potential PCB internal structural issues include PCB delamination, PCB blistering, board warpage, thru-hole barrel crack, copper dissolution, Z-axis thermal expansion that weakens vias, PCB deflection-/bending-related failure, pad cratering, and copper-pad lifting. Any of these defects and problems could lead to a subsequent reliability concern.
For thicker PCBs (board thickness > 0.092" or 2.3 mm) used for servers and other high-performance applications, inadequate hole filling during wave soldering and plated thru-hole thermal stress damage due to Z-axis thermal expansion are additional potential issues.
For fragile, thin PCBs used in most portable electronics, PCB deflection/bending, mechanical shock, and board delamination are often areas meriting attention. Likely production defects also could occur in the main board, such as thermal damage in the areas between mounted components, which can hardly be detected, yet could lead to functional issue during its service life, particularly under harsh environments.
Time, Voltage, and Extreme Conditions FailuresAmong other major issues that might cause failures in the long run, the top two insidious phenomena that should be watched for closely are tin whiskers and eletromigration. The specifics, strategies, and methodologies to alleviate these issues are addressed in my workshops in the 2009 series of International Lead-free Conferences organized by IPC and JEDEC, and at APEX 2009.
Reliability Engineering vs. CostIn today's business and global market environment, cost always is an important factor in designing and producing a product. The cost of producing a product generally increases with the increased reliability level desired. However, a lower manufacturing cost (design and production) does not necessarily translate into a lower overall cost of the product. The total product cost is the sum of manufacturing costs and post-shipment costs including warranty, replacements, and loss of customers or sales due to inferior quality and product failure. The objective is to minimize the manufacturer's cost without compromising the product's reliability and quality. In reliability engineering, the balance between the manufacturing cost and desired reliability leads to the optimal reliability, which offers the minimum total cost over the entire lifetime of a product. It is wise to produce a product for the reliability level required.
APPEARANCES Dr. Hwang will deliver lectures on "Lead-free System Performance & Reliability Present & Future" and "BGA/CSP/WLP Pb-free Reliability in Packaging & Assembly" at IPC International Conference on March 5 in Santa Clara, CA; and "Lead-free Reliability How to Alleviate Failures" and "Interactive Discussion on Lead-free Electronics" at APEX on March 30.
Jennie S. Hwang, Ph.D., an SMT Advisory Board member, is inducted to the WIT International Hall of Fame, elected to the National Academy of Engineering, and named an R&D-Stars-to-Watch. Since the inception of SMT manufacturing, she has helped improve SMT manufacturing yield and solved challenging reliability issues. Having held executive positions with Lockheed Martin Corp., Sherwin Williams Co., SCM Corp, IEM Corp., she is currently a principal of H-Technologies Group providing business and manufacturing solutions to the electronics industry. She is a member of the U.S. Commerce Department's Export Council, and serves on the board of Fortune 500 NYSE companies and civic and university boards. In addition to technical publications, she is an international speaker and author on trade, business, education, and social issues. Contact her at (216) 839-1000; JennieHwang@aol.com.