Reliability Modeling of Electronics for Co-Designed System Applications Greg Caswell, DfR Solutions, LLC (o)

Transcription

1 DesignCon 2013 Reliability Modeling of Electronics for Co-Designed System Applications Greg Caswell, DfR Solutions, LLC (o) 1

2 Abstract Complex electronic systems are implemented in a multitude of designs covering all high reliability market segments, e.g. telecommunications, medical, aerospace, solar, etc. In many cases, these systems are expected to be exposed to environmental thermal cycling and moisture over a period of up to 25 years. Fabricators of these systems need to be comfortable knowing that their designs will meet these aggressive environments. A new reliability modeling tool has been developed that is being used to predict the failure rate of the electronics in these systems. This tool provides the designer with the ability to find and correct design flaws before release of the product, thus avoiding costly failures in the field. The failure rate is predicted for thermal cycle fatigue of solder joints and plated through hole vias as well as shorting from conductive anodic filament (CAF) formation. The tool also produces a finite element analysis of the circuit board(s) showing regions susceptible to excessive board strain during vibration or shock events. The most value comes from the ability of the engineers to perform various what if scenarios to determine the impact of any number of design choices. What if I change the mount point locations? What if I change the via diameters, the spacing, or the copper thickness on the circuit board? What if I change the laminate thickness or material selected? What component is at highest risk of failure and what if I change its packaging format? What is the reliability impact of changing from SnPb to SAC305 solder? Finally, once the design has been optimized to satisfy the competing requirements, the software can be used to predict the rate of failure over the lifetime of the product and this information used to more accurately plan for the warranty costs. This presentation will use some specific examples to demonstrate the capabilities and value that this new tool provides to the electronics industry. Author s Biography Greg is widely recognized as a pioneer in surface mount technology (SMT) and has 40 years of experience in the electronics industry. Currently he is a Sr. Member of the Technical Staff for DfR Solutions. Previously he was the VP-Engineering for Reactive NanoTechnologies and VP Business Development for Newport Enterprises. His experience encompasses all aspects of SMT manufacturing, circuit board fabrication and materials, advanced packaging, IC fabrication processes and materials, solder reflow, RoHS, and bonding utilizing specialized nanotechnology. Greg, a Past President of IMAPS NA, was the National Chairman for the IMAPS Advanced Technology Workshop program from and was the Editor in Chief for Advancing Microelectronics magazine for the past 3 years. He received his Bachelor of Science in Electrical Engineering from Rutgers University and also has a Bachelor s in Management from St. Edwards University in Austin 2

3 Introduction Any electronic system or assembly is a complex interaction of materials that depends on the interaction of their various mechanical, thermal, and electrical properties. It has been demonstrated that the overall cost and quality of a product is most influenced by decisions made early in the design stage. Finding and correcting design flaws later in the product development cycle is extremely costly with the worst case situation discovering design problems after failures occur in the field. Implementing a newly developed reliability prediction analysis tool will forever change this equation. Before a single product is built, this valuable new tool enables the engineer to import the design files and quantitatively predict the life of the product according to the assumptions made for the user environment. With margins shrinking in the electronics industry, OEMs depend more on profits from extended warrantees. Inaccurate life prediction can cut heavily into this income stream. Under prediction of the failure rate will lead to cost overruns while over estimating failure will mean lost business to competing extended warranty plans and the setting aside of funds that could instead be used for further product development. This paper demonstrates the capabilities and value that this new tool provides to the various functional units within a electronics design environment where the reliability of the chip, package, circuit board and system are paramount. Environmental stresses are also an element of this assessment. They can vary from hand held units (e.g. cell phones) projected to survive for 2-3 years to solar electronics having a 25 year life requirement. These more stringent environments can involve: a. being used at varying temperatures or temperature extremes c. having a temperature range of -55 C to 125 C d. being used in an application having a medium to high shock, pressure, vibration, or moisture environment e. being stored for later usage (over 10 years) f. having an application life span of 10 to 25 years When faced with these daunting product implementation requirements, it is vital to ascertain whether the product can survive its intended environment as early as possible. 3

4 Common Failure Modes Wire Bond Failures Wire bonding has been the most common interconnect for IC packages for over 50 years. The most common materials are gold, aluminum, and more recently copper. The most common bond pad material is aluminum. A cross section of a typical wire bond is shown in Figure 1. Figure 1 Cross Section of a Typical Wire Bond. Wire bonds tend to fail if exposed to elevated temperatures (intermetallic formation), exposure to elevated temperature and humidity (corrosion) and exposure to temperature cycling (low cycle fatigue). Printed Wiring Board Failures Printed Wiring Boards have several failure modes that are detrimental to reliable operation. Failures in PCBs can be driven by: Size (larger boards tend to experience higher temperatures) Thickness (thicker boards experience more thermal stress) Material (lower Tg tends to be more susceptible) Design (higher density, higher aspect ratios) Number of reflow exposures The failure modes that can typically occur are Conductive Anodic Filament (CAF) shown in Figure 2, and plated though hole (PTH) Failures which can be driven by voids, etch pits or fatigue. Conductive anodic filament (CAF), also referred to as metallic electromigration, is an electrochemical process which involves the transport (usually ionic) of a metal across a nonmetallic medium under the influence of an applied electric field. CAF can cause current leakage, intermittent electrical shorts and dielectric breakdown between conductors in printed wiring boards. 4

5 Figure 2 Conductive Anodic Filament Failure PTH voids can cause large stress concentrations, resulting in crack initiation. The location of the voids can provide crucial information in identifying the defective process; around the glass bundles; in the area of the resin; at the inner layer interconnects (aka, wedge voids) or at the center or edges of the PTH. Etch pits are due to either insufficient tin resist deposition or improper outer-layer etching process and rework. They can cause large stress concentrations locally, increasing likelihood of crack initiation and large etch pits can result in an electrical open. Overstress cracking can occur in the PTH due to a Coefficient of Thermal Expansion (CTE) mismatch which places the PTH in compression. Pressure applied during In- Circuit-Testing (ICT) using a "bed-of-nails" can also compress the PTHs. Circumferential cracking of the copper plating that forms the PTH wall can also occur which is driven by the differential expansion between the copper plating (~17 ppm) and the out-of-plane CTE of the printed board (~70 ppm). Figure 3 illustrates these three failure mechanisms. Figure 3 Plated Through Hole Failure Mechanisms: voids (left), etch pits (center) and barrel cracking from fatigue (right) Wouldn t it be an ideal scenario if you could determine during the design phase of your product whether it will survive the intended environments for its projected lifetime 5

6 without incurring any of the noted failure mechanisms? You can accomplish this with DfRs Automated Design Analysis Tool, Sherlock. Solder Fatigue Failures Thermo-Mechanical Fatigue of solder joints is one of the primary wear-out mechanisms in electronic products. This is especially true in products used outside of commercial/ consumer environments where a longer lifetime is required and more severe operating conditions exist. Sherlock assesses the fatigue of the solder joints as a function of the stresses applied during its lifetime and provides insight into whether joints are susceptible to failure. Figure 4 is an example of a fatigued solder joint on a Ball Grid Array (BGA) package. Figure 4 Example of Solder Joint Fatigue on BGA Package Sherlock Automated Design Analysis There are several high levels steps involved in running the Sherlock software. They are: Define Reliability Goals Define Environments Add Circuit Cards Import Files Generate Inputs Perform Analysis Interpret Results Reliability Goals Desired lifetime and product performance metrics must be identified and documented. The desired lifetime might be defined as the warranty period or by the expectations of the customer. Some companies set reliability goals based on survivability which is often bounded by confidence levels such as 95% reliability with 90% confidence over 15 years. The advantages of using survivability are that it helps set bounds on test time and sample size and does not assume a failure rate behavior (decreasing, increasing, steady-state). 6

7 Defining Environments Meaningful reliability predictions must take into account the environment in which the product is used. There are several commonly used approaches to identifying the environment. Approach 1 involves the use of industry/military specifications such as MIL-STD-810, MIL-HDBK-310, SAE J1211, IPC-SM-785, Telcordia GR3108, and IEC The advantages of this approach include the low cost of the standards, their comprehensive nature, and agreement throughout the industry. If information is missing from a given industry, simply consider standards from other industries. The disadvantages include the age of the standards, some are more than 20 years old, and the lack of validation against current usage. The standards both overestimate and underestimate reliability by an unknown margin. Another approach to identifying the field environment is based on actual measurements of similar products in similar environments. This gives the ability to determine both average and realistic worst-case scenarios. All failure-inducing loads can be identified and all environments, manufacturing, transportation, storage, and field, can be included. In addition to thermal cycle environments, the Sherlock software accepts vibration and shock input as well. Figure 5 shows representation of this input. Vibration loads can be very complex and may consist of sinusoidal (g as function of frequency), random (g2/hz as a function of frequency) and sine over/on random. Vibration loads can be multi-axis and damped or amplified depending upon the chassis/housing. Figure 5 Typical stresses that can be addressed in Sherlock 7

8 Import Files The software is designed to accept ODB++ files which contain all the data for the PCB, the components, and their locations. The data can also be imported with Gerber files and an individual bill of materials. Figure 6 shows an example of a PCB stack-up and relevant data for reliability modeling. The software automatically generates the PCB stack-up, and its embedded database of over 400 laminate materials having 48 different properties provides input to the models based on the materials selected. Parts List Individual component data is part of the ODB++ file; however, modifications to the data can be made manually to ensure the physical characteristics of all the components are accurate. Figure 7(a) shows two examples of the component editor, illustrating the images of a C-Bend package and a QFN to facilitate selecting the correct package during an analysis, while Figure 7(b) shows the components laid out on the board. Figure 6 PCB Stackup with Materials Database 8

9 Figure 7 Illustration of Component Editor (a) on top and Board layout (b) on bottom The software can also establish 3D models by creating a mesh structure and the model from the data input to the analysis. Figure 8 illustrates the mesh construction and Figure 9 a typical 3D image obtained. 9

10 Figure 8 Mesh Configuration Figure 9-3D Image obtained as a Result of Analysis This information is then processed to obtain the outputs of reliability information from Sherlock. 10

11 Analyses Sherlock can currently perform eight different analyses. They are: CAF Conductive Anodic Filament Formation Plated Through Hole Fatigue Solder Joint Fatigue Finite Element Simulations ICT Impact DFMEA Vibration Fatigue - Natural Frequencies Mechanical Shock The output information from Sherlock can be illustrated in several ways; as an unreliability curve, a parts list showing the discrepant parts, the physical location, the overall assessment and the relative rank ordering of the components. Figure 10 provides examples of the component location and unreliability curves outputs. 11

12 Figure 10 Failed component Locations (top) and Unreliability Curves (bottom) Sherlock can also identify areas where natural frequencies impact the product design, as shown in Figure 11. It can provide a pictorial illustration of the stress locations for each of the natural frequencies identified. The software can identify up to 10 natural frequencies. Figure 11 Natural Frequencies Identified by Sherlock (1 st -upper left), (2 nd -upper fight), (3 rd -lower left) and 4 th Lower Right) 12

13 In addition, Sherlock can provide data regarding the strains applied to the circuit board as a function of the vibration stress levels. Figure 12 illustrates this data in the XX direction for the 1 st image and YY in the second. Figure 12 XX (top) and YY (bottom) Images Illustrating Strain Levels 13

14 WHAT IF ANALYSIS EXAMPLE Figure 13 is a comparison of the solder joint fatigue characteristics for a circuit card assembly in an avionics application. The left image shows the results for the assembly with Sn/Pb solder while the right image illustrates the results for a SAC-305 solder composition. In the Tin/Lead analysis the solder fatigue (green curve) exceeds the 10 year life expectancy identified for the product. However, when a separate analysis was performed with SAC-305 the green curve shows the solder joint fatigue occurring prior to the 10 year mark. This comparison analysis was accomplished in less than 5 minutes and Demonstrated to avionics customer that transition to Pb-free would have a detrimental impact to product performance. Figure 13 Comparison of Sn/Pb and SAC-305 Solder Joint Fatigue in Avionics Application In another example, an Industrial controls customer was experiencing failures during vibration testing. Through Sherlock, DfR was able to rapidly identify optimum location of additional standoff that altered the natural frequency enough to eliminate the design issue. Figure 14 illustrates this difference. This what if was also done in less than 5 minutes. Figure 14 Comparison of Mount Point locations (left image is failing), (right image after addition of mount point-passing) 14

15 PRODUCT TEST PLANS Product test plans, also known as design verification, product qualification, and accelerated life testing (though, these are not the same thing), are critical to the successful launch of a new product or new technology into the marketplace. These test plans require sufficient stresses to bring out real design deficiencies or defects, but not excessive levels that induce non-representative product failure. Tests must be rapid enough to meet tight schedules, but not so accelerated as to produce excessive stresses. Every test must provide value and must demonstrate correlation to the eventual use environment (which includes screening, storage, transportation/shipping, installation, and operation). The critical first step is a good understanding of the use environment for the product. How well is the product protected during shipping (truck, ship, plane, parachute, storage, etc.)? How does temperature/humidity, thermal cycling, ambient temperature/operating temperature, salt, sulfur, dust, fluids, etc. as well as mechanical cycles (lid cycling, connector cycling, torsion, etc.) impact the product, particularly in an avionic application. Followed by the question - Do you have data or are you guessing? Product test plans are critical to the success of a new product or technology and must be stressful enough to identify defects and show correlation to a realistic environment. PoF Knowledge can be used to develop test plans and profiles that can be correlated to the field. This paper will provide insight into a process to develop viable test plans and a tool that facilitates the entire process so that minimal testing is performed, thus reducing costs and schedule impacts. Figure 15 is an example of how Sherlock was able to provide an appropriate test time and test condition based on field environment and likely failure mechanism as an input to the customer for a product qualification plan. 15

16 Figure 15 - Failures Curves on Top and Predicted Test Plan for Validating Data on Bottom Summary and Conclusions This paper has presented some of the issues associated with electronics reliability and a tool that can be used during the design phase of a project to mitigate those reliability issues. Knowing this information at the design stage permits the designer to make modifications to the layout such as hold down points, component placement, stiffeners etc. to assure compliance with the reliability requirements, prior to initiating a prototype manufacturing run, thus reducing both cost and risk, while simultaneously improving time to market. 16

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