On-bonder curing improves packaging productivity
09/01/2001
Advances in die attach change the face of assembly
BY RICK WEAVER
cover story
Die attach (DA) curing - either on a step-cure oven or in a box oven - is the least efficient step in the total assembly manufacturing process for wirebonded integrated circuit (IC) packages. Delays between die placement and die attach cure can exacerbate resin bleed, and allow die shift, as well as subsequent die misalignment and wirebond failures. This article will describe curing processes that can be used to improve yield and productivity.
Early Die Attach Adhesives
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In the early 1970s, the microelectronics industry was looking for alternative materials that would allow attachment of electronic components at temperatures lower than those required for traditional eutectic solders. The search led in the direction of organic polymers and focused on epoxy resin. Epoxy compounds can be categorized as having a step-addition cure, where a reaction initiator is required to convert an epoxy resin into a thermoset solid.
The early epoxy component attach adhesives, although highly reactive, did not completely react with epoxy resin. The unreacted curing agent then combined with moisture, which hydrolyzed the free chloride ions and created corrosion on the metallic surfaces within the device or package, causing catastrophic electrical failures. As early problems slowed the potential implementation of epoxy component attach adhesives in the microelectronics industry, other polymers were evaluated for this purpose.
Pre-imidized polyimide, with a low level of hydrolizable chloride, had a distinct advantage over epoxy adhesives for microelectronic devices that were susceptible to corrosion. This type of polyimide adhesive was used for semiconductor die attach beginning in the late 1970s and is still being used in some applications. However, the requirement for a long (more than one hour), two-step cure profile to evolve the high level of solvent contained in the polyimide materials reduces productivity. Polyimide die attach adhesives typically have a high modulus and require high-temperature processing and require high-temperature processing, typically 270°C, while epoxy adhesives typically cure at 150°C.
 Figure 1. A whole set of equipment (curing ovens) can be removed from the factory when on-bonder curing is used. |
By the mid-1980s, the difficulties with epoxide systems were being resolved by the use of "cleaner" constituents with lower chloride levels and subsequent reduction of component or package corrosion. This second generation of microelectronic adhesives was implemented in the production of military hybrids and in plastic dual inline package (PDIP) semiconductors. At the end of the 1980s, an increasing volume of semiconductors were placed into plastic packages and silver-filled epoxy became the primary die attachment adhesive for the industry. The emphasis became high productivity rates, which challenged the epoxy die attach manufacturers to develop better rheology products that would not exhibit stringing or tailing of material during the very high-speed dispensing processes.
Options to Enhance Productivity
Throughout the following decade, managers of semiconductor packaging facilities worldwide began viewing productivity improvement as a means of cost reduction and an increasingly important factor for improving profitability and competitiveness. In the early 1990s, as the quality and reliability of epoxy die attach became more consistent, interest was expressed about reducing the cure time for the epoxy from the traditional one-hour minimum. The term for this concept was "snap-cure," and is generally accepted to mean a cure time of less than one minute in duration.
 Figure 2. Time of reaction for a commercial BMI-based die attach adhesive. |
Although epoxy die attach compounds have provided good overall results for semiconductor die attach during the past 20 years as modifications were attempted to reduce moisture absorption, decrease modulus and accelerate cure reaction time, the results are now falling short of advanced processing requirements. Epoxy polymerization reactions can potentially be accelerated to a few seconds, but the dynamics of die attach rheology then intervene. To ensure optimum dispensability with single needle, multiple needle or pattern write technology, die attach adhesives are typically formulated to a viscosity between 6,000 to 10,000 cP and a thixotropic index (TI) of 3.5 to 6.0. To meet these criteria, epoxy die attach formulations typically included diluents to lower the viscosity to the target. Although current technology epoxy die attach adhesives use diluents that react into the crosslink matrix rather than evolve during cure, they must have sufficient time to react to produce a void-free bond.
 Figure 3. Die attach bonder. |
The solution for overreacting an epoxy snap-cure die attach adhesive, and generating voids, is to heat the material in a ramping, or stepped cure, operation. This type of processing requires the installation of "stepped cure" ovens into the factory as stand-alone units. While this process slowly evolved in the semiconductor packaging industry, more advanced solutions were being explored.
For example, the process of dispensing die attach adhesive onto the substrate, placing the die and curing the die attach adhesive can be combined in a continuous process self-contained within the die bonder unit without the loss of die bonding speed. Using this type of process, the cure of the die attach material would no longer require stand-alone curing equipment that takes cleanroom floor space. Also, the process typically would not slow the die bonder units-per-hour (UPH) rate. Therefore, with no floor space consumed by curing equipment and no additional time consumed for curing, the cure process can become transparent in the assembly operation (Figure 1).
Materials for In-line Curing
To cure on-bonder without reducing throughput, the die attach material must be capable of completing a significant portion of its cross-linking reaction within 10 seconds and develop a void-free bond line. Traditional die attach adhesives have been based on step-addition reactions that have been unable to meet this objective. Therefore, alternate polymer chemistries have developed in recent years, including systems containing acrylate and liquid bismaleimide (BMI). These new systems, which employ monomers that will co-polymerize, are replacing traditional epoxies because they possess several unique characteristics. The greatest difference between BMI systems and epoxies is a curing mechanism that is free-radical initiated rather than a step-addition mechanism. A free-radical chain reaction, once initiated, proceeds at an extremely rapid rate. In Figure 2, a series of time-of-reaction kinetics curves are presented at temperatures from 110 to 170°C for a commercial BMI-based die attach adhesive.
 Figure 4. Cross-sectioned bondline, BMI-based on-bonder die attach (magnification 500X). |
The free-radical chain reaction of advanced polymer systems, in which all the chemical constituents co-polymerize, supports extremely rapid reactions in which the majority of cured adhesive characteristics are attained in a period of time as short as 5 seconds. These free-radical curing systems typically contain 100-percent solids, which reduces the weight loss during cure to less than 0.2 percent compared to a commercial epoxy snap-cure die attach adhesive that reports a weight loss of 2.1 percent during a 60-second cure at 200°C.
Flex Tape Applications
The packaging process of thin flexible polyimide (flex) tape devices can be challenging. Die bonding on flex tape is difficult because of the nature of the polyimide and its tendency to curl. To resolve this, copper rails have been attached to some designs to improve rigidity, while bare tape is used in others. However, the die bonding, handling and transportation of flex tape from the time the die is placed until it is cured can cause bondline shift and die movement, resulting in misalignment and die tilt. The delay between placement and cure also allows time for resinous bleed to escape from the die attach materials and contaminate wirebond pads on the substrate. Each of these phenomena - bond line shift, die movement and bleed - can create the need for additional processing steps, such as cleaning, and may also create yield loss. Using on-bonder curing for this type of application can solve these problems.
Experiment on Flex Tape Array
An experiment was run to demonstrate the on-bonder curing method for a flex tape application. For comparison, die attach was also done using a traditionally cured, high-volume silver-filled epoxy die attach adhesive and quick-cure silver-filled epoxy adhesive. All samples consisted of 12.5 mm x 5 mm x 355 µm devices mounted with adhesive onto a 79-µm thick polyimide 4 x 4 matrix flex ball grid array (BGA) substrate.
 Figure 5. Cross-sectioned bondline, traditional epoxy die attach (magnification 500X). |
Parts for the on-bonder method were assembled with a silver-filled BMI-based adhesive on a die bonder retrofitted with a heater block assembly to allow on-bonder curing and cured at 160°C for 18 seconds. The vacuum holddown used during die attach dispense, die placement and die adhesive cure maintains the planarity and stability of the polyimide flex tape during all of the critical die attach functions. The bonder is shown in Figure 3.
Figure 4 is the cross sectioned bondline of one of the devices mounted with the BMI die attach The bond line thickness is consistently 21 µm at the center and each end. Figure 5 is the same substrate and die, with a traditionally cured epoxy die attach adhesive. The devices were assembled and staged for one hour, followed by an oven cure with a 30-minute ramp to 150°C for one hour.
 Figure 6. Cross-sectioned bondline, quick cure epoxy die attach (magnification 500X). |
The results for the devices in Figure 5 indicate the die attach adhesive has pulled back from the periphery of the die, creating an average bondline thickness at the center of the die of 27.4 µm and combined average of both ends of 14.5 µm. Also observable in this cross section are resin-rich areas in which the movement of the die, flex substrate, and die attach adhesive during handling and cure have displaced the silver filler in the adhesive and allowed the epoxy resin to pool in the voided areas.
Figure 6 shows a device assembled using the quick cure adhesive. The adhesive was dispensed and the devices placed and staged for one hour. They were then oven cured for 15 minutes at 150°C in a preheated oven without a ramp.
As with the traditional oven-cured epoxy, the quick-cure processing also allowed the polyimide flex to deflect upward around the die edges while maintaining a relatively constant adhesive bond thickness (29.0 µm) in the center of the die (Figure 6). The edges of the flex were pulled upward creating a substantial bow to the bottom of the package.
Equipment Requirements
It is possible to cure die attach adhesives designed for on-bonder curing elsewhere in the process, using existing in-line equipment (for example, preheaters on a wirebonder). However, to gain the full advantage of the BMI-based adhesives, including reduced die movement and bleed, the cure should take place just after die placement on the diebonder.
Stable bond line thickness, minimum die tilt, regulated fillet with 100-percent coverage, homogeneous adhesive layer (no voids), precise and regular dispense volume are criteria that must be considered to achieve the optimum quality level for the adhesive layer between the chip and substrate. To accomplish this, it is necessary to use a chip assembly machine that includes a precise adhesive dispense system and an integrated curing system that ensures precise control of time and temperature to produce a stable and reliable process.
Thermal Response of Substrates
To know how a die attach adhesive material reacts during on-bonder curing, it is necessary to know the thermal response of the substrate on which it will be placed. This allows estimation of the percentage of the curing reaction that may take place during the die attach process.
 Figure 7. Thermal transfer rates for various substrates. |
Figure 7 shows in detail the rate at which different substrate materials allow thermal transfer through their mass to the die bond site. To obtain these temperature profiles, it is important to have perfect thermal contact with the oven or heater surface. For processing the sensitive flex BGA materials, the cure station was switched from the fixed oven heater block station to a moveable shuttle oven heater block. This is done to minimize any movement of the substrate during the die attach process. The substrate was held in position until the full matrix was completed. During the transportation, the vacuum hold down was switched off briefly.
Because of the short time and distance required to transport the substrate to the curing position after adhesive dispense and die placement, the adhesive on a BGA flex polyimide substrate remains in place with minimal disturbance. When the substrate with cured die exits the die bonder, storage time in the magazine no longer has an influence on die movement, die tilt or adhesive resin bleed. This is particularly an advantage with thin polyimide flex film that may be only 50-µm thick and sags in the magazine.
Conclusion
At present, the liquid BMI and acrylate-based die attach systems have gained acceptance throughout the semiconductor packaging industry with an estimated 45 percent of the rigid and flex array die attach market in 2000. The experimental results demonstrate the ability to improve both the quality and manufacturing productivity of advanced semiconductor packages, such as tape and rigid arrays, by the implementation of both advanced free-radical curing die attach adhesives and advanced die bonding equipment that is capable of on-bonder curing. The combination of material and equipment is essential to drive the maximum capabilities and benefits of this advanced packaging process.
AP
Rick Weaver, technical services manager, can be contacted at Dexter Electronic Materials, 9938 Via Pasar, San Diego, CA 92126; 858-695-1716; Fax: 858-695-0951; E-mail: [email protected].