Two metal layer flex
03/01/2001
ITS USE IN TBGA AND CSP APPLICATIONS PROVIDES MORE I/O CONNECTIONS
BY TIMOTHY G. LENIHAN
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Typical flex-based packages can be made of pliable base materials, like polyimide or Mylar, onto which conductive metals are deposited and patterned. Such packages are used in many applications that require electrical connections between printed circuit boards (PCBs) containing die or semiconductor chips. Examples of these applications include portable computer screens, processor boards, cellular phones that open during operation, high-performance graphics cards and engine control units in cars. In fact, the world's largest flex circuits - measuring 150 feet long - can be found on the solar panels of the international space station currently under construction.
While there are many combinations of pliable base materials and electrical conductors available, including copper, gold and silver, this article focuses on polyimide base materials with copper conductors. Such conductors have nickel and gold plated on top of them to allow wire bond and solder ball attachments.
Flex Packaging
While flex-based electronic packages can be very large, they can also be very small. This article focuses on the small variety, where the die or semiconductor chip is attached directly to the flex package.
Single metal layer (SML) flex or tape electronic packages have been available for years. One common application for SMLs is the smart card, because smart cards are thin, lightweight and low in cost compared with traditional PCB technologies. However, as semiconductor chips, die-like microprocessors, memory, application-specific integrated circuits (ASICs) and digital signal processors (DSPs) increase in function, the number of input and output (I/O) connections to them also increases. This requires bringing smaller connecting lines closer to the die on the electronic package or increasing wiring density. Connections are typically made using wire bonding with thin gold wires, or with solder ball connections using low melting lead/tin balls.
 Figure 1. Flex-based packaging types. |
The need to bring the package connections closer to the die is driven by system performance. Long connections can slow system performance in high-speed applications, and this can be problematic because there is a continuing need to reduce the overall system size, weight and cost in many new applications.
There are limits to the number of lines that can be patterned on a single side of a flex-based electronic package. This is driven primarily by cost, as the capital investment for semiconductor processing equipment is higher than for traditional PCB processing equipment. Yield is another factor that limits the number of lines or wiring density. When wiring or trace dimensions, line width and line spacing become smaller, the process yield is affected by smaller and smaller particles in the air. To address this problem, cleanroom facilities are often required.
One solution to this is to use both sides of the flex-based package for wiring, allowing larger line widths and line spacing. This technology is referred to as two metal layer tape (2ML tape). 2ML tape, where the die is directly attached to a cavity in the middle of the top of the flex or tape and where solder ball connections are on the same side as wirebond connections, is referred to as a tape ball grid array (TBGA). Die can be attached via wire bond connections - cavity-up TBGA or flip chip (FC) connections - FC TBGA.
 Figure 2. Cross-section of a typical flex TBGA. |
When the overall electronic package size is less than 20 percent greater than the die size, these packages are referred to as chip scale packages (CSPs). The definition of CSP is under review but this has been used commonly in the electronic packaging industry for some time. Both wirebond and flip chip CSP packages are available on the market today. Figure 1 outlines the different types of flex-based packages.
While 2ML or TBGAs may seem to be a good solution for improving densities, there are a few obstacles in making this type of package. First, the SML tape industry evolved around using 35 mm-wide base materials, in part because processing equipment from the photographic industry was readily available. SML manufacturers now also use 48 mm-, 75 mm- and soon 150 mm-wide base materials. To reduce manufacturing costs and be competitive with PCB manufacturers that process much larger areas, the trend toward larger tape processing will continue. New tooling will be required to support such processes, especially to align the top of the tape to the bottom.
2ML tape also requires the ability to make connections between the top and the bottom of the tape; some SML tape manufacturers accomplish this today by etching away the base flex material and making solder ball connections from the bottom. Because the bottom of the tape is used for solder ball connections and not wiring, this is not necessarily a good method. A better solution is to use small vias and free up more area for wiring.
Primary Advantages
There are many advantages to using 2ML tape because of its light weight, good electrical performance at high frequencies (because of the thin flex base or dielectric material with a solid ground plane), small vertical profile and high-density wiring that doesn't require using multiple levels of packaging for wiring connections.
Unlike SML tape, 2ML tape technology is still on the cutting edge and evolving. Most 2ML tape manufacturers either process in large panels or in roll-to-roll format. They also manufacture small vias using punching or lasers and by two-sided processing.
For Example...
To be more specific regarding the use of 2ML tape, a cross- sectional diagram of a typical TBGA is shown in Figure 2. Such packages are sold to assemblers and self-contained electronic manufacturers that have the ability to manufacture complete subassemblies to end-users. This particular example uses roll-to-roll processing with small vias (25 to 30 µm exit width) to connect the top of the tape package to the bottom. Laser technology makes the small vias. These vias are plated with copper to make the electrical connections from top to bottom. The signal or trace lines are made with deposited copper and patterned using semiconductor type processes. Typical line widths range from 30 to 50 µm and line spaces from 30 to 37.5 µm; this example uses a 330 mm wide format (Figure 3).
 Figure 3. 330 mm-wide processing. |
Vacuum metalization and additive plating build up the signal lines. Conductor thickness controls with tight line and space rules are achievable using thin film manufacturing technology. Smaller vias that can be made using punching or drilling technologies are produced using high-speed lasers. Die size reduction capability is a key advantage for 2ML tape users. Smaller die reduces die cost and increases the capacity of semiconductor manufacturers as more dies can be manufactured using the same capacity. The effect of die size reduction can be seen using physical layouts of a 40-mm, 432-lead cavity-down super BGA and the same size package and I/O count using a TBGA. An assembler can take advantage of continuing die shrinkage while using the same flex-based package. The die shrinkage would be transparent to the next level of assembly. This can significantly reduce cost conversion costs and qualification time.
What's next after 2ML tape?
The same limitations described earlier for SML tape will eventually catch up to 2ML tape as wiring density and performance requirements increase. Research has shown that 2ML tape can be multilayered like PCBs, embedding passive devices within the electronic package rather than mounting them on the surface of the package.1 This has been done by depositing and patterning thin film materials, in a roll-to-roll process, onto a polyimide based flex or tape. Chromium oxide was chosen for a resistor material and tantalum oxide for a capacitor material. Patterning the metal lines into spiral or square shape makes an inductor. The requirement to make a center connection for the inductor is accomplished by using vias and a metal trace on the bottom side of the tape.
One advantage of the demonstrated technology extensions for 2ML tape is that the number of I/O connections can be increased without using smaller lines and spaces by stacking or multilayering layers of 2ML tape. Also, by placing passive devices (resistors, capacitors and inductors) within the electronic package, the top surface is freed up for more dies and thereby keeps the overall system package small. This is critical for portable applications that require increased functionality but must maintain a small system size, weight and low cost. One such example would be to access your computer system via a cell phone, while at the same time keeping all of your current cell phone functions, size and price. A number of universities are working on making these types of packages a reality.
Reference
- Tim Lenihan, et.al., "A Novel Approach to Create an Integrated Flex Package," IPC National Conference Flexible Circuits Proceedings, pp. 89-96, 1998.
TIMOTHY G. LENIHAN, director of marketing and sales, applications engineering, product engineering and business planning for the Micro Products Division of Sheldahl, Inc. and adjunct professor in the Electrical Engineering Department and the High Density Electronic Packaging Center at the University of Arkansas, can be contacted at Sheldahl Micro Products, 1285 South Fordham Street, Longmont, CO 80503; 303-684-7154; Fax: 303-651-2265; E-mail: [email protected].