There are four primary substrate types used for ball grid array (BGA) packages: ceramic, plastic PCB materials, such as bismaleimide triazine (BT), metal and flexible tape. Of these, the most widely used and accepted are BT substrates and flexible tape. Recently the industry has debated which of these materials is best for BGA applications, even though either material is acceptable. As the debate heats up, I'd like to add some opinion on the side of flex tape.
Ball Pitch Technology
Tape allows for an easier migration to higher package density (ball pitch reductions from 1.0mm ' 0.8mm ' 0.65mm ' 0.5mm) without a change to the packaging format, manufacturing process or carriers. Today's standard design rules for tape allow for 30 micron metal lines and spaces (15 microns best case), which is significantly better than the design rules required for BT (100 microns standard, 50 microns best case). Because of the increased design density characteristics offered by flex tape, routing complexity/density can be achieved without adding additional metal/ dielectric layers to achieve stress balancing.
Material Performance Factors
The glass transition temperature for polyimide flex tape is around 240°C, as compared to 175°C for most BT substrates. This can be significant when considering coefficient of thermal expansion matching and package reliability performance. Also, the dielectric constant for tape is slightly lower, indicating that signal speed will be better. The tape structure allows for better impedance control by precisely matching lines and spaces, and it can further aid in transmission line structure.
Coplanarity
Typical coplanarity results for a properly designed and processed 27 x 27-mm. flex tape BGA are usually 75 to 100 microns. Compare this to the 100 to 125 micron level achieved for a typical 27 x 27-mm. BT substrate BGA. Also factor in that coplanarity will continue to increase with the number of additive layers applied to the BT structure to achieve density demands.
Cost Considerations
While many claim that BT is far more cost-effective than flex tape, this contention quickly dissipates as package density increases. BT substrates are manufactured in large panel sizes (e.g., 18-in. x 24-in. and 36-in. x 36-in.) that add to difficulties in stacked tolerances, registration and alignment. As such, due to BT substrate manufacturing methods, derating of product for speed and line impedance performance is required. This affects product yields and cost. Also note that this factor increases significantly with the addition of layers applied to the BT substrate. One could claim that flex tape requires the use of stiffeners that add to the cost of the package, but many very large format flex tape packages are made without the use of stiffeners.
Time to Market
One of the most contentious issues facing any semiconductor company is time to market. Many times, a large portion of a company's inability to bring its product to market on time is because of packaging. And considering that development efforts are underway to be able to provide certain flex tape packages in four weeks (after layout), this timetable will be significant when compared to the approximate three months it will continue to take for BT substrates. While such capabilities may not yet be available, at least the technology is.
While flex tape is not without its limitations, it does hold some significant benefits for packaging BGAs. So, why aren't there more tape BGAs in the market? One of the fundamental issues in the acceptance of a packaging material by the market is supply infrastructure. Today, there are very few companies that can provide flex tape for packaging applications – compare this to the vast number of suppliers of BT substrates. The supply chain for BT substrates is much more extensive so BT suppliers can be more responsive to individual customers and the overall market. In addition to supply limitations, the marketplace is not well-educated in the specification and design, wire bonding and molding of flex tapes and the handling systems required to ensure product flatness and coplanarity to manufacture a superior package.
Flex tape BGAs can be manufactured as effectively as BT substrate BGAs. The flex tape BGA's package quality and reliability can be made to meet or surpass the results of similar package types using a BT substrate. In many ways, the package performance benefits of flex tape exceed those using BT substrates. Coupling this with the added benefit of being able to easily migrate and increase package density, flex tape BGAs certainly have a viable place in the advanced packaging portfolio.
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MARK DiORIO, chief operating officer, can be contacted at MTBSolutions Inc., 2685 Marine Way, Suite 1220, Mountain View, CA 94043; 650-960-3230; E-mail: [email protected]