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Article: "Adhesives for Increased Reliability in Medical Devices"
February 21, 2008 |Estimated reading time: 12 minutes
AbstractMedical devices require a greater degree of functionality in smaller spaces. While this is similar to the trends in mobile phone devices, there is a significant difference in the reliability required between these types of systems [1]. For many years underfill materials that have been used to increase the reliability of flip-chip devices in packages and on SMT boards. These materials are now being used more often to increase the reliability of surface mount devices such as chip scale packages (CSPs) and ball grid arrays (BGAs). This presentation will discuss the significant differences between the critical physical properties of flip-chip underfill versus board-level underfill, and how these properties affect the reliability of the underfilled device. In addition there exist several different "classes" of adhesives beyond the classic capillary flow materials, such as cornerbond or edgebond materials and encapsulants. In this paper we will discuss the differences between these approaches and their affect on the reliability of the electronics devices they are designed to enhance.
INTRODUCTION
Underfill materials are polymer systems (filled or unfilled) that are used to increase the reliability of a variety of area array packages used in high density electronic assemblies [2-8]. Underfill systems are typically epoxy based chemistries (e.g. bisphenol epoxies and cycloaliphatic epoxies) that sometimes have a filler added. There are two main reasons for using an underfill material. First, to relieve stress from the large CTE mismatch between a silicon device (e.g. flip-chips) and the substrate it is bonded to (e.g. FR-4). Second, to increase the reliability of the component, such as a chip scale package (CSP), with respect to physical shock and vibration. For medical devices both types of environments are possible.
Underfill materials are used to increase the reliability of a variety of component types including flip-chips, chipscale packages, ball grid arrays (BGAs), and micro BGAs. Most device manufacturers that use flip-chips on board (FCOB) underfill them due to the large CTE mismatch between the silicon die and the substrates typically used in manufacturing these products (either FR4 or Polyimide flex). Other packages may only be underfilled if there is either a perceived risk to the products (e.g. a cell phone may undergo frequent drops) or for a high reliability application (e.g. medical devices). The CTE matching of the underfill materials is typically achieved through the addition of silica or alumina based fillers.
This addition lowers the CTE in order to provide a gradient between the silicon chip and the substrate. Underfills used to provide physical protection to other packages typically have higher Tg, but they have a modulus that is better suited for that function. The need for a low CTE in an underfill is an often misunderstood concept. Not all underfills necessarily need to have a low CTE to provide reliability enhancement to all devices. The balance between CTE and Modulus should be examined in order to provide the highest reliability enhancement possible. A graphic showing the relationship between modulus, CTE, and two common failure modes is shown in Figure 1. As this figure illustrates understanding the failure mode of the device is critical in choosing the material with the right properties. Also, note that using a material with a low CTE does not guarantee an increase in reliability if the device is susceptible to solder fatigue cracking.
Figure 1. Relationship between filler and physical properties of underfills
Typically, in medical devices there is a greater concern for long term reliability so most underfills utilized have some amount of filler in them in order to increase the thermal cycle fatigue life. The exception would be for small hand held devices that have a shorter lifetime, e.g. monitoring devices.
Capillary Underfill
These low viscosity liquids are designed to flow under a component by capillary action, wetting to the chip and substrate surfaces and encapsulating the solder joints. Underfills designed for assembly-level flip chip generally cure in five minutes or less at 150oC to 165*C to form a hard seal with high adhesion to both the component (flip chip or CSP) and the substrate. Underfills designed for package-level assemblies offer improved assembly reliability but generally require more time to flow under the die and to cure. Faulty or skewed components must be detected prior to underfill cure, since most capillary flow underfills are permanent. There are reworkable systems available in the market, but typically the reliability requirements for medical devices takes precedence and these materials are not applicable.
Underfill is applied close to the edge of a flip chip or CSP to enable capillary forces to encapsulate the gap between the component and the board. Dispensing of capillary underfill materials requires specialized equipment to achieve the accuracy and precision required for high volume assembly. In most implantable medical devices space is at a premium, therefore it is important to get the smallest fillet possible. At a minimum, the dispenser must reproducibly position successive assemblies and apply a pre-determined volume of underfill to the edge of the component. Heating the substrate is a required secondary requirement that accelerates capillary flow. Cure is usually accomplished in belt-style reflow or batch curing ovens.
Several dispensing patterns are used for applying underfill to die and packages. The simplest pattern involves applying underfill to a single side and maintaining a reservoir as material wicks between the package and substrate. Previously, the underfill was applied to the remaining three sides using a "U" pattern to form a fillet. With newer generation materials the filleting step can be eliminated. The dispensing of a single line, or "I" pattern is the most common for smaller packages. Another common flow pattern recommended for fast processing of smaller or simpler devices involves dispensing an "L" of material (two sides), waiting for flow out and then dispensing an inverted "L" to complete the fillet, if needed. In most medical devices since the space is at a premium, the size of the fillet is a concern. The dispense method used can affect the size of the fillet, so when fillet size is critical it is important to consider the dispensability of the material for the application. Fluxing, or so-called "no-flow" underfills are an attempt to make underfill processing more compatible with conventional surface mount assembly processes by eliminating the dedicated oven required for cure. This class of material can also result in a much smaller fillet. In these materials fluxing function incorporated into the underfill combines the component attachment and underfill cure processes [9-11].
Fluxing underfill is applied directly to the attach site on the substrate prior to component placement. The flip chip or CSP is then placed into the underfill. During reflow, the underfill acts as the flux, enabling interconnect formation and selfcentering prior to curing of the underfill. Ideally, underfill cure is completed in the reflow oven. Otherwise, subsequent reflow or deliberate heat treatment is required to complete underfill cure.
Fluxing underfills are different from capillary flow materials in several ways. Viscosities are typically much higher than that of capillary underfills. Since inorganic fillers in the underfill will generally impede essential contact between the CSP solder balls and the attach pads, fluxing underfills are unfilled, which results in a higher CTE. The entire board must be discarded if defective or skewed components are detected, if the underfill is not removable.
Cornerbond and Edgebond
An alternative approach to complete underfilling of chip scale package devices for increased reliability with respect to shock and drop is to bond the corners or edges of a CSP, see Figure 2. With this technology, lines of underfill material are dispensed at the four corners of the CSP or BGA pad site prior to component placement, allowing in-line processing using standard equipment with curing taking place during normal solder reflow. But, like any process, there are some very important considerations - especially in the age of lead-free-- that must be evaluated and understood before applying the technology in process [12-13].
Figure 2. Image of Cornerbond material; on the left, before CSP placement on the right after placement and reflow.
When using Cornerbond technology, one must realize that the aterial will be cured during normal reflow and not postassembly as is the case with traditional capillary flow underfills. Therefore, the materials characteristics of the Cornerbond must be compatible with normal assembly flow and, in the case of lead-free, the higher temperatures of the Pbfree process. In order for the BGA solder reflow process to perform as it should, the solder balls must be able to collapse and self center so that proper interconnections between the device and the board can be formed. When manufacturers attempt to use standard Chipbonder or surface mount adhesive (SMA) materials in a lead-free process to achieve Cornerbond underfill-type reliability enhancement, the results can be catastrophic. SMA materials are not designed for this purpose and, thus, can be quite detrimental. Also, typical SMA adhesives utilize resins that have a higher ionic content than typical underfill adhesives, and may not be suitable for a sealed implantable device.
Some materials can be applied after the device is soldered to the substrate; these are known as Edgebond materials,
Figure 3. These materials can be cured via a thermal process (e.g. ovens) or UV light. The advantage to the Edgebond materials is that since the material is applied post-reflow there is no chance of interfering with the soldering process. In addition, the fillets can be well controlled since these materials tend to be much higher in viscosity.Figure 3. Image of Edgebonded CSP (UV cure material).
In addition to underfill materials for CSP and BGA devices used in medical electronic devices, there are also many applications for encapsulant adhesives. These are materials designed to protect wirebonded silicon die from thermal stresses and physical damage. These materials can also play a role as a "potting" material in small devices. Since these are typically high-grade (low ionics, low stress) materials, they can provide additional protection and reliability to medical electronic devices. These encapsulants can also be used to reinforce the packaging used to seal and protect the PCB inside the medical device.
RELIABILITY TESTING
There is already a large body of work showing that filled underfill systems provide a significant increase in thermal cycling reliability for flip-chip devices [2-7]. Newer materials are being used to improve shock/drop resistance. Drop testing was conducted as per the JEDEC J22-B111 specification [13] using a Landsmont model 15-D shock tester and the board design as specified by the standard. Test boards are placed component side down on the fixture and dropped such that they experience a pulse of 2900g with a pulse width of 0.3 ms, Figure 4.Figure 4a. Sketch of drop test and pulse.Figure 4b. Drop test vehicle as per JESD22-B104.
Different underfill systems provide different levels of reliability. Figures 5 and 6 illustrate the different levels of reliability that can be achieved (in shock/drop) with different underfill materials.
Figure 5. Wiebul plot illustrating the increasing shock/drop reliability with underfill material.Figure 6. Comparison of underfill systems and the effect of drop test performance.
In Figure 5 we can see a significant increase in the life of an underfilled device compared to a non-underfilled device with respect to drop and shock. In Figure 6 we show that the underfill formulation has an effect on the performance of the materials, although both underfills tested offer significant increase in reliability, there is a difference in performance.
In addition to shock and drop another common failure is due to bending of the circuit board during assembly or handling. For these situations a Cornerbond or Edgebond material is typically used to reinforce the device. For bend testing a drop test PCB (Figure 4b) that is partially populated was used. Only the three components along the center (top to bottom) are placed and the entire PCB is flexed, and the resistance measured. Figure 7 shows the results between non-Edgebonded components and Edgebonded components. With an Edgebonded component we can see an increase in the amount of deflection before a failure occurs. Without Edgebonding we can deflect the board approximately 6mm, with an Edgebonded device the board can be deflected up to 15mm, before there is a failure of the solder joints.
Figure 7. Bend Test of non-Edgebonded (Board #7, top) and Edgebonded (Board #9, bottom) components.
CONCLUSIONS
For medical electronic devices there are several adhesive materials that can offer reliability enhancements. Underfill and encapsulant materials provide increased reliability and can still meet the stringent requirements often needed for medical devices, such as low ionic content and low outgassing. These materials allow for extended lifetimes of the finished product. As the electronics industry develops newer technology, smaller packages and newer assembly methods, there will be a parallel path were advanced adhesives will be developed to make these technologies viable for high-reliability applications [15-16].
REFERENCES
1. A. Primavera and J. Hoffpauir, "Reliability Considerations for Implanted Medical Electronics", SMTA Journal, Vol. 18-4, 2005.2. Chip Scale Package, J.H. Lau and S.W. Ricky Lee, McGraw-Hill, 1999 and Flip Chip Technologies, J.H. Lau, McGraw-Hill, 1996.3. Underfill: The Enabling Technology for Flip-Chip Packaging, S.L. Buchwalter, M.E. Edwards, D. Gamota, M.A. Gaynes, S.K. Tran, in Area Array Interconnection Handbook, K. Puttlitz and P.A. Totta Eds., Kluwer Academic Publishers, 2001.4. A. Babiarz, H. Quinones, "Advances in Fast Underfill of Flip Chips", ICEP/Microelectronics, Tokyo, Japan, February 2001.5. J. Liu, R.W. Johnson, E. Yaeger, M. Konarski, and L. Crane, "CSP Underfill, Processing and Reliability", Proceedings from the Technical Program, APEX 2002.6. B. Toleno, "Underfills in Pb-Free Assemblies", Circuits Assembly, June 2005.7. G. Carson and M.E. Edwards, "Factors Affecting Voiding in Underfilled Flip Chip Assemblies", Proceedings from the Technical Program, SMTA International, 2001.8. E. Yaeger, Z.A. Szczepaniak, M. Konarski, L. Crane, Z. Hou, G. Tian, and R.W. Johnson, "Underfill Materials, Processing and Reliability for Fine Pitch Flip Chip on Laminate Assembly", Proceedings from the Technical Program, APEX 2002.9. P.N. Houston, B.A. Smith, and D.F. Baldwin, "Implementation of No Flow Underfill Material for Low Cost Flip Chip Assembly", Proceedings from the Technical Program, SMTA International, 2001.10. J. Liu, R.W. Johnson, E. Yaeger, M. Konarski, and L. Crane, "CSP Underfill, Processing and Reliability", Proceedings from the Technical Program, APEX 2002.11. D. Baldwin and M. Colella, "Reliability of a Void Free Hybrid No-Flow Underfill Process", International Wafer Level Packaging Conference, 2007.12. B. Toleno and J. Schnieder, "Processing and Reliability of Corner Bonded CSPs", Proceedings from the IEMT, SEMIcon West, San Jose, 2003.13. M. Kokonowski and B. Toleno, "Improved BGA Shock and Bend Performance using Corner Glue Epoxies", SMTA International, 2006.14. JEDEC Standard No. 22-B111, JEDEC Solid State Technology Association, July 2003. 15. B. Toleno and D. Maslyk, "Process and Assembly Methods for Increased Yield of Package on Package Devices", SMTA Pan Pac Microelectronics Symposium, 2008.16. S. Rajagopalan, K. Desai, M. Todd, G. Carson, "Underfill for low-k silicon technology", IEEE CPMT, July 2004.This paper was presented at the 2008 SMTA Medical Electronics Symposium (Anaheim, CA) and is published here with permission.<?xml:namespace prefix = o />