March - April 1999
eMail the Editor
Flex-Frame Standards Emerge for CSP Assembly
By Vern Solberg, Contributing Editor
As more companies move into the smaller flex-film-based chip-scale and chip-size packaging arena, they realize the efficiencies gained when the device sites are furnished in a uniform array format.
The high volume suppliers of flex-based IC packaging material for CSPs typically process the materials in a reel-to-reel format. That is, flexible film, using non-reinforced polyimide and copper foils, are laminated together in a continuous, narrow strip, which is typically processed in standard widths of 35, 48 and 70 mm.
The polyimide film is pre-punched before copper foil lamination with features at the edge to enable processing. In addition, all holes and openings required for package assembly are provided in the film as well. The copper foil is then photo-imaged and chemically etched to define the circuit pattern, followed by electroplating gold over the remaining exposed copper. This etched copper on polyimide flex-film, as well as the finish plating, is accomplished very efficiently in a series of reel-to-reel process steps.
Direct Transfer
Because the flex-film material is supplied in a continuous reel format, it can be transferred directly onto the package assembly line for processing; however, many companies are not equipped to process CSPs in a reel-to-reel configuration.Most package assembly operations are configured to handle a smaller format, but the flexible materials are difficult to handle. To enable easier handling of the polyimide film-based materials during the progressive stages of assembly, the flex-film can be attached to a rigid carrier frame.
Flexible polymide film-based material furnishes several
CSP sites in an array format and is mounted to a
rigid composite frame carrier for efficient handling
throughout all package assembly processes.
The magazines are furnished in a wide variety of physical configurations, allowing the user many choices in accommodating any number of flex-carrier widths and lengths.
The flex-film carrier format (as shown in the photo) has proven ideal for IC assembly processes. Although some companies may choose to develop a way of completing the entire assembly while in the reel-to-reel format, most prefer the more familiar format typical of the leadframe strip carrier.
The flexible circuit support frame (flex-frame carrier) has evolved from a rather expensive stainless steel carrier that costs more than $5.00 each to a plastic-based material that costs less than $0.25 each. The thin, stainless material is very rugged and it can conceivably be used over and over again; cleaning and reclaiming the metal carriers, however, is rather labor intensive.
Alternative Flex-Frame Materials
Working closely with material and fabrication suppliers, several alternative materials were studied. Following a series of informal meeting with users, the development of a specification for the flex-frame evolved-not only to define the physical features needed for machine transfer-but to describe the serialization methodology, as well.The laminate carrier frame retains most of the mechanical features of the earlier stainless steel version, including tooling holes. The tooling holes are necessary for precise alignment while attaching the flex-tape to the carrier frame. A larger pattern of holes is utilized when transferring and holding the carrier frame at various assembly stages.
Many organic laminate materials have been evaluated for the flex-film carrier frame, and although several suppliers will likely emerge with suitable alternatives, the laminate selected must remain stable throughout several stages of the assembly process.
Temperature Variations
In addition to the curing processes of encapsulation or plastic over-molding, the most severe environment is probably the solder ball attachment process. The ball contact attachment temperature can vary somewhat, depending on the specific ball alloy, from 230°C to 260°C (or even higher with some high-lead alloy materials).The organic laminate frame carrier currently in production for µBGA‰ package assembly is a relatively low-cost epoxy cellulose core CEM-1 composite material faced with a woven glass reinforcement on both outer surfaces (see the table).
After extensive testing and evaluation, the reinforced CEM-1 composite combination exhibited excellent mechanical and thermal properties though all processes. The material can be drilled, milled or punched, and the surface has proven to be ideal for rapid laser or ink marking.
This composite has achieved UL, BSI and CSA industry approval and can be furnished in the standard sheet sizes commonly used in the PC board industry. Although the laminate material is typically furnished with copper foil bonded to its surface for etched circuit applications, it is also available without copper cladding.
Materials can be supplied in thicknesses between 0.70 mm and 1.6 mm; however, the thickness that appears most efficient for µBGA package assembly is nominally 0.71 mm.
Post-Package Assembly Considerations
Because the flex-film carrier frame will be an integral part of the array of devices through all process steps, marking and serialization must be considered. Material and process control is always a priority when processing ICs in large volume, but, because of the numerous methods for serialization and coding, it may be difficult to achieve consensus.SEMI has approved several coding systems for monitoring IC manufacturing and has recommended that specific zones within the carrier frame boarder be reserved for serialization to accommodate the eventual standardization of the flex-film carrier frame.
Many processes, such as testing, inspection, printing, ink marking and laser marking, can be performed while the devices are still in their carrier frames. The carrier is ideal for most singulation operations, as well.
Techniques for singulation include wafer sawing, die punching and the use of rotary knives. Some companies are adapting water-jet or laser processes for singulation, and at least one Switzerland-based company has developed a combined laser and water process.
Although tooling costs and tool maintenance must be considered, die punching is highly efficient for companies processing devices in very high volumes. Programmable CSP singulation may be preferred, as well, and the systems are efficient and cost effective.
Sawing to separate completed CSP devices mounted on a metal carrier is possible, too. This method, however, has proven to be less efficient than the use of composite frames, because the epoxy-based carrier frame is organic allowing the saw to pass through the frame and flex-tape simultaneously.
We would like to hear from you. To contribute to Application Notes or to comment, please contact Vern Solberg at Tessera by e-mail at vern@tessera.com or phone 408.383-3614.
| Flex-Tape Carrier Frame Material | |||
|---|---|---|---|
| Material Property | Units | Value | Conditioning |
| Tg (DSC) | °C | 95 | - |
| CTE X-axis | ppm/°C | 13 | Ambient to Tg |
| CTE Y-axis | ppm/°C | 16 | - |
| Volume Resistively | min. megohms-cm | 1 x 108 1 x 107 C-96-40/90 | E-24/125 |
| Surface Resistively | min. megohms | 5 x 106 1 x 108 | C-96-35/90 E14-125 |
| Permittivity, @ 1 MHz | - | 4.5 | C-40/23/50 |
| Loss Tangent, @ 1 MHz | - | 0.025 | C40/23/50 |
| Arc Resistance | min. seconds | 90 | - |
| Flammability | - | V-0 | - |
| Water Absorption | % | 0.17 | D-24/23 |
| Dielectric Breakdown | kV | 60 | D48/50 |
| Shear Strength | max. psi | 15,000 | Condition A |
| Flexural Strength-LW CW | psi psi | 25,000 45,000 | Condition A Condition A |
Source: AlliedSignal Laminate Systems