March 1998
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The µBGA® Package Leads the Way to Smaller Consumer Electronic Products
Ball grid array formats. such as the µBGA® package. contribute to overall system cost reductions by offering improved manufacturability.By Ron Bauer, Intel Corp.,
Folsom, California
Mobile communications and computing applications are getting smaller in size and lighter in weight with more features than ever before. These shrinking consumer products are fueling the growth of chip-scale packages (CSP) to meet portability needs. Additionally, many of these applications also have substantial cost reduction requirements.Customer Requirements
During the past few years, Intel's Flash Memory Components Division has conducted research on next generation chip-size packaging for flash memory. Our customers, in applications such as handheld communications (cellular phones, PCS, pagers) and computing, PCMCIA I/O cards and other small form factor systems, were requesting more compact IC packaging to meet their system miniaturization goals. Not only did customers require the smallest and lightest possible package, they required that the next generation package be aggressively priced, contribute to lower overall manufacturing costs and meet their product reliability needs.The market for shrinking consumer products is experiencing explosive growth and fierce competition. Just about every handheld consumer product seems to be shrinking at a very rapid rate. The race to "Dick Tracy" type products is on, and it is being judged in millimeters, grams and additional functionality. At the same time, strong competitive forces are putting pressure on manufacturers to reduce the cost of these products.
In September 1997, at Surface Mount International, Mike Campbell of the BPA Group Ltd. predicted the market for mobile communications will grow from about 100 million units per year in 1996, to around 375 million units per year by 2001, with even more designs that include the functionality of a phone combined with data access (Internet) or palm-top computing.
BPA Group Ltd. also forecast a strong reduction in the cost to produce mobile phones, showing that the average cost to produce a GSM handset today is about $130 to $140 per unit. This cost will be reduced to about $40 per unit in 2002.1
Design Cycles
Newly designed, or redesigned consumer products are being introduced almost daily, and product design cycles are decreasing to meet these accelerated introduction cycles. In the past, aggressive companies may have introduced both a fall and spring model, but in this new environment, it is not uncommon to see new models being introduced every quarter. The entire market for CSP solutions is seeing the ripple effect of this environment, and suppliers are seeing strong increases in demand for these components. Because of this demand, the opportunities for manufacturers and suppliers are great.
In response to these needs, Intel has added the µBGA package to its family of package options for flash memory devices. The µBGA package meets all the customer requirements for size, weight, product and manufacturing costs, ease of use and reliability.
The µBGA package is a true die-sized package. In some cases, the equivalent µBGA package is 80% smaller then the TSOP package alternatives. In addition, the use of a matrix of solder balls for mounting the component to the PC board allows for routing to occur underneath the package, using standard PC board trace/space design rules. Thus, the total footprint for the package on the board is as small or smaller than any other CSP solution. In addition, this routing technique can often result in a smaller footprint than even a bare die solution.
The customer's need for a cost effective packaging solution is a major consideration. The µBGA package is a cost effective solution from both pure package cost and total system cost points of view. Intel also strongly considered the cost reduction potential of the package technology and believes that the potential to reduce the cost of the assembly process for the µBGA package will quickly make it as cost-effective as the TSOP.
A Key to Cost Reduction
Ball grid array packages, including the µBGA package, are seen as one of the keys to overall system cost reduction. The self-alignment features, and lack of coplanarity issues, drive production defect rates to very low levels. One of the early users of the µ1BGA package, XeTel Corp., Austin, Texas, is experiencing very high first-pass yields with these very small packages. Dr. Julian Partridge, XeTel's director of research, believes that the long term process yields will be comparable to full-size BGA packages (i.e. defect levels below 10 ppm).The 0.75 mm ball pitch of Intel's µBGA package allows customers to rapidly implement this new package using existing surface mount equipment and conventional 5 mil trace and space PC board technology.
The ease of implementation is a key to rapid adoption of this technology since it directly relates to the customer requirement for ease of use and lower manufacturing costs. CSPs with pitches less than 0.75 mm will become more common in the coming years. The widespread availability of high density PC boards is needed to take consumer products to the next level of product size and cost reduction.
Another manufacturing benefit of converting to a BGA-type package is increased placement speed. The selfalignment feature of the package allows relaxed placement tolerances, which has allowed at least one manufacturer to use a high speed chip shooter to place flash memory chips on the PC board. The chip shooter was able to place µBGA packages with a 100% assembly yield at a rate of 5 parts per second. This is five times faster than placing TSOP package using the slower, and more accurate fine pitch placement system. This assumes that the CSP edge-to-array tolerances, machine placement accuracy and PC board tolerances have been established.2
Unique Construction
The unique construction of the µBGA package also leads to lower manufacturing costs for users. The package is constructed with a low modulus elastomer layer that decouples the coefficient of thermal expansion difference between the PC board and the silicon die. Because there is virtually no stress on the solder joints, the µBGA package does not require the additional processing and associated cost of adding underfill to improve interconnect reliability. (Figure 1)
Figure 1 - Typical Solder Joint Reliability Stress Points
In addition, the µBGA package construction has excellent resistance to moisture. Components are judged for their moisture sensitivity against a scale of 1 to 6 ( 1 is not moisture sensitive, to 6„extremely moisture sensitive) as outlined in both the IPC and JEDEC specs. Intel has qualified the µBGA package at IPC/JEDEC moisture level 2 (limited moisture sensitivity; the product can be surface mounted anytime within 1 year of when the shipping bag is opened with no special precautions and no prebake).
Trapped moisture is a concern in other packages, such as plastic encapsulated types, because during the solder reflow process, moisture that is trapped in the package turns to steam, builds up pressure within the package and can rupture the interior structure or form cracks that can propagate to the exterior. (Figure 2)
Figure 2 - Moisture Sensitivity Comparison.
The internal gap that is formed in this process is referred to as delamination or "popcorning" and can be detected with C-mode scanning acoustic microscopy
(CSAM). The delamination can stretch or break interior bond wires, crack the die or delaminate the die from the mounting pad. This can render the component useless, or at best marginally reliable.
Simplified Production
Moisture sensitivity is a great concern for manufacturers since it adds complexity and cost to the surface mount process. Products that have low or no moisture sensitivity, like the µBGA package, allow manufacturers to simplify their production process. Components are loaded onto a machine and left there until they are used, without having to take the remaining units off the machine at the end of the day and put them into a dry box (or nitrogen cabinet) until the next morning.This process, required for moisture sensitive parts, adds cost and impacts the production flow. The purchase and maintenance of additional cabinets to store packages is also required. If all of the units are not used before the time limit, an additional bake step is also required.3 Under normal production conditions, the µBGA package does not need any of these additional processes.
Reliability
Last, but definitely not least, all package solutions must meet the reliability needs of the end product in which it will operate. A primary market for flash memory devices is handheld consumer products. At Surface Mount International, BPA's Campbell also reported that the typical life cycle for a mobile phone is very short, with most consumers replacing their handset every 2 years when they opt for smaller and lighter versions.Although reliability requirements do vary, the typical design requirement for telecommunications is a temperature range between -40 °C to +85 °C, for a
7-20 year life.4 Since it is very unlikely that a product is ever going to be used beyond these conditions, the reliability requirements for the packages should be in-line with these conditions. The µBGA package is also unique because the package construction can be tailored to meet various reliability needs.
By changing materials and processes, the package can be designed to operate in harsh environments such as underhood automotive or military avionics. Most consumer products do not need such extended reliability, and manufactures of typical consumer products are not willing to pay for performance that is not needed for their product.
The trend is to reduce costs, and maintain reliability limits needed for the specific product. These trends are continuing to drive the high level of research being done by Tessera and its many licensees to continue improving the cost and performance of the µBGA package.
Production Qualification
Intel has completed the full production qualification of the µBGA package for the 8 Mb Flash File™ and 8 Mb Advanced Bootblock products. TheBGA package has met or exceeded all of Intel's stringent quality and reliability requirements. The package qualification was completed very rapidly compared with other package qualifications done by Intel. One of the factors in Intel's choice to license the package concept instead of internally developing a new package technology from ground zero, was the ability to rapidly bring an excellent packaging solution to market that met all of our customers' needs.
The qualification for each of the products included package temperature cycling (-40 to +85 °C) of more than 500 units for over 1000 cycles with no valid package failures. The package was also mounted to an FR4 PC board, with very good results.
More than 1200 solder joints were temperature cycled at both -40 °C to +85 °C, and 0 °C to +100 °C with no valid failures. All the other classical reliability tests such as infant mortality, preconditioning IPC L-2 CSAM, high voltage dynamic life test (HVELT), high temperature storage (bake), steam (autoclave), temperature humidity bias (85/85) and bend and twist tests met or exceeded Intel's package certification Q&R requirements. The data from the tests is available from Intel.
Intel is now shipping production quantities of its flash memory components in the µBGA package, and is offering most new flash memory components in the this package. Over the past year, Intel has focused many resources to develop manufacturing capability for this package, and is currently ramping production in multiple factories worldwide with plans for more than 1 million units per month capability in the near future. We believe that the µBGA package is uniquely qualified to meet customers'needs in this era of shrinking consumer products.
µBGA is a registered trademark of Tessera, San Jose, CA. Other brands and names are the property of their respective owners.
References
- M. Campbell, "The Future of High Density Packaging in Personal Communicators," keynote presentation for Chip-Scale Package Symposium, September 8,1997, San Jose.
- J. Partridge and V. Solberg, "High Speed PC (Memory) Card Assembly with Chip Scale BOA," Proceed ings of Surface Mount International, September 7-11, 1997, San Jose, pp. 435-440.
- R. Bauer and S. Greathouse, "Advanced Chip Scale Packages Break Traditional Reliability Paradigms," Proceedings of SEMICON West, San Jose, July 18, 1997, pp. F1-F10.
- V. Solberg, "Environmental Qualification Testing of Chip-Scale and Chip-Size BGA Devices," Proceed ings of CHIPCON '97, February 1997, Sunnyvale, CA.
Mr. Bauer has spent 14 years with Intel Corp. During his first eight years, he developed manufacturing processes and equipment for new packages. He is currently the Technical Marketing Engineer for advanced packaging based at Intel's Folsom, California facility. Direct email to him at ronald_j_bauer@ccm.fm.intel.com or contact him by phone at 916.356.5216.