As portable and high-performance products such as hand-held PCs and wireless phones continue to shrink in size, the demand for more complex devices in higher- density packages increases. More and more manufacturers are assembling bare dice into multiple device packages, such as memory modules (DIMMs and SIMMs) and multichip modules (MCMs).
Bare dice provide improved performance, reduced package size, and lower package cost over individually packaged devices. However, the cost of detecting and eliminating bad devices in a module can be very expensive since a single bad die will result in a defective module. This has led IC suppliers and module manufacturers to explore the use of performing die-level test and burn-in.
Companies have investigated various methods to test and burn-in bare dice, and many strategies have been ruled out as impractical or cost-prohibitive. Bare dice vary greatly in size, contact type, and contact location, making it impractical to use conventional burn-in or test sockets.
Alternatively, temporary die carriers have emerged that offer a reliable solution for providing known good die (KGD) (Figure 1). This technology allows a bare die to be processed like a packaged device in a typical semiconductor back-end test and burn-in process flow.
The Process
A KGD is processed like normal packaged dice through wafer-saw. After wafer-saw, the wafer frames are sent to the carrier loader/unloader (CLU). At the CLU, the dice are loaded into carriers that are placed into handling trays. These trays can be processed like standard packaged devices. Usually, this includes pre-burn-in test, burn-in or test-during-burn-in, and final test. A KGD is a die that has passed all tests.
KGD Processing Requirements
The following are required to process KGD:
Carriers.
CLUs.
Carrier handling trays.
Burn-in boards (BIBs).
Test contactors.
Dice handling trays/tape.
Carriers
All major carriers, including the DiePak® from Aehr Test Systems, are composed of three major components or assemblies: the substrate, the base, and the carrier lid (Figure 2). Together these components make electrical contact with the die and route signals, power, and ground to the outside of the carrier. Then, the device can be electrically connected to test contactors and burn-in sockets.
The substrate is a flexible polyimide film with copper traces that connect to contacts that mate with the die. These contacts are bumps that contact wire bond pads or lands with solder bumps, plated bumps, stud-bumps, or microsprings.
The other end of the trace connects with lands that contact either burn-in sockets or test-contactors. This substrate is similar in appearance to a flex-circuit. The substrate can be configured in one or multiple metal layers to accommodate different performance requirements.
The base is a rigid assembly that supports the entire carrier, especially the substrate and die. The base includes a plate which supports the substrate and mechanically aligns the external contacts, an alignment plate which secures the substrate to the support plate, and an elastomer support which helps to planarize the die and provides compliance beneath the internal contacts.
The carrier lid is composed of a spring-loaded pressure foot that holds the die in place and provides the contact force necessary to maintain electrical contact and the hinged lid, which connects the pressure foot to the rest of the carrier assembly. When the carrier lid is open, the die can be loaded and unloaded.
CLUs
Several manufacturers provide CLUs. This equipment represents the only capital equipment investment required to modify the manufacturing flow to process KGD. Typically, this equipment is a modified flip-chip bonder with the capability to:
Pick the die off the wafer frame.
Flip it over.
Align the die contacts to the substrate contacts.
Place the die.
Provide vacuum to hold the die while closing the carrier lid.
Load the carrier into JEDEC-type carrier handling trays for further processing. Equipment available today can load more than 500 carriers per hour. The CLU also unloads the carriers. Carriers can be unloaded approximately twice as fast as they are loaded.
Carrier Handling Trays
Trays also are needed for die carriers. The trays must be designed to hold the carriers either live-bug or dead-bug because the carriers must be presented in the upright position to the CLU and the burn-in board loader/unloader. But since the substrate contacts are on the top of the carrier, the carrier must be presented to the test handler upside down.
Burn-In Boards
There is virtually no difference between BIBs designed for packaged devices and BIBs designed for carriers. The BIB sockets for carriers are very similar to a typical open-top QFP or PLCC socket. Although the carrier is designed for each device, the carrier form-factor and sockets are standardized. This allows the use of generic BIBs, resulting in smaller BIB inventory and fewer new BIB designs.
Test Contactors
Test contactors help automate high-speed testing of the dice. Several test contactors available today are designed specifically for KGD carriers. They are very similar in design and function to contactors used for packaged parts.
Dice Handling Trays/Tape
After processing, the dice must be stored and handled in trays designed for bare dice or mounted in a tape-and-reel format. This will keep the dice clean and undamaged until ready for assembly.
Challenges
There are two major challenges in producing KGD. First is establishing and maintaining a low-resistance electrical connection to the die. This must be done continuously at test speeds of greater than 100 MHz, temperature extremes of -50° C to 150° C, and after the mechanical jarring associated with normal handling. Second is loading the dice into the temporary carriers, handling the carriers as they are processed through the manufacturing flow, and unloading the dice from the carriers when the test process is complete.
Electrical Contact
Establishing contact with the die is difficult because most contact metalization is composed of nonprecious metal. Standard aluminum and solder-based contacts will form a metal-oxide film. Since it is not practical to use a wiping-type contact mechanism, the substrate contact must break the metal-oxide on the die.
Maintaining contact through the wide temperature excursions of burn-in and test is a challenge. The difference in the coefficient of thermal expansion (CTE) between the polyimide substrate and the silicon die and the mechanical jarring of the carrier that occurs during normal handling can cause problems.
To minimize this effect, low CTE materials are used to constrain the expansion of the substrate at elevated temperatures. With this design, the carrier can be used with large dice (<500 mil). It also is necessary to have proper alignment of the die onto the substrate and adequate contact force to minimize movement between the substrate and the die during handling.Normally, the simplest substrate can be used to test die up to 100 MHz. Designing more complex substrates with multiple metal layers with power and ground planes can improve the substrate performance to speeds greater than 250 MHz.
Loading, Unloading, and Handling the Carrier
The main issue with loading, unloading, and handling the carriers is verifying that the CLU can accurately place the die in the carrier and close the carrier so the die contacts are properly mated with the carrier substrate contacts. This is addressed in two ways:
1) By specifying a CLU that has a placement accuracy of less than ± 10 microns so the substrate contacts can be reliably placed on wire-bond pads as small as 75 microns.
2) By designing a substrate optimized for optical placement systems. This is achieved by creating features on the substrate that are easily captured by automated vision systems.
Maintaining a high contrast between the contacts and the substrate surface makes the contact positions easy to define.
Other Considerations
Just as a wafer-probe will leave marks on the die contacts, so will the KGD process. The substrate will leave bump marks on the wire-bond pads or cause bump deformation on bumped dice (Figure 3). Although they are quite visible, bump marks on wire-bond pads are less damaging than those left by wafer probe needles and have been shown to have no effect on wire-bond performance.1
The amount of bump deformation on bumped dice will depend on many factors, such as initial bump height coplanarity, bump volume, burn-in temperature and duration, bump composition, total bump count, and die size. Normally, deformation will range from 10% to 30%.
It is necessary to determine the acceptable amount of deformation and whether more processing will be required. Generally, the acceptable criterion is determined by how much the bump can be deformed and still be reliably reflowed during the assembly process.
Typically, the bump coplanarity is improved after being processed in the carrier. As a result, the bump deformation normally does not have a negative impact on the reflow process.
Summary
The use of KGD provides a cost-effective means to produce fully tested memory and multichip modules. With this process, the cost of producing KGD will be less than the cost of producing packaged parts because the cost per use of the carrier is less than typical packaging costs.
Bare dice can be fully tested using many of the same processes used to test packaged devices. While there are challenges, there also are viable solutions available today that enable the manufacture of KGD. KGD provides unique benefits that will be critical in many new packaging applications.
Reference
1. Kost, D., et al., “Motorola Fast Static RAM Known Good Die Manufacturing Process,” 1997 International Conference on Multichip Modules, pp. 245-249.
About the Author
Warren Murray is the sales and service manager for the Device Interconnect Group at Aehr Test Systems. He has worked in the electronics/packaging industry for 14 years and been involved with the development of KGD technologies for the past six years. Mr. Murray graduated from University of Redlands with a B.S. in chemistry.
Aehr Test Systems, 1667 Plymouth St., Mountain View, CA 94043, (650) 691-9400, e-mail: [email protected].
Copyright 1999 Nelson Publishing Inc.
March 1999