Curing: A Vital Step in IC Package Manufacturing
Historically, semiconductor manufacturers have favored large, benchtop, forced-convection industrial ovens for their curing processes. The most popular batch ovens have an interior volume of 4 to 6 ft3 with two or three wire-rack shelves. An electric fan circulates air across resistive heating elements and directs it into the cure chamber. Most come with a programmable temperature controller with overtemperature protection, an interior feedback thermocouple, an alarm, an exhaust port and an optional circular chart recorder. They are loaded and unloaded manually, often while the chamber is hot, by opening and closing a latched, hinged door on the front of the equipment.
Horizontal belt, solder reflow ovens have been used occasionally, but only for short-profile curing applications. Longer cure profiles require longer ovens that incur higher capital costs and consume more energy, gases and floor space. These ovens are more suitable for the inline, integrated manufacturing lines used by many printed circuit board assemblers.
Introduced more than 10 years ago, vertical ovens (figure 1) are gaining acceptance. Unlike horizontal belt ovens, vertical ovens are capable of processing long cure profiles in considerably less floor space. This is accomplished by raising and lowering the product through vertically arranged cure chambers or zones. Vertical ovens are designed for automation and have many safeguards against misprocessing.
Continuous ImprovementProcess engineers must optimize their processes, materials and equipment to obtain the highest quality, best manufacturability and lowest cost in a safe working environment without sacrificing product performance. These objectives often demand higher yields, less inspection and rework, tighter process control, lower direct and indirect labor costs, shorter queue and process times, less equipment downtime, smaller floor space, lower energy and other consumable usage, lower depreciation, lower direct material costs, better product reliability and improved safety precautions. The curing process requires the same attention and optimization as is given to any of the other assembly process steps. Typical quality issues include:
Contamination. Out-gassing of volatile species from organic materials in the product during heating can settle or deposit on clean surfaces of the subassemblies within the cure chamber. This issue can be reduced or eliminated by the selection of low-outgassing encapsulants or adhesives and by increasing the frequency and/or the volume of atmosphere exchanged and exhausted within the curing chamber. Look for equipment that offers an active or programmable exhaust feature. Purging and exhausting contaminated atmospheres can be accomplished faster and more frequently using equipment having smaller cure chambers.
Interrupting the cure process to load and unload small lots of product may contaminate product stored nearby or pose a health safety risk to operators working nearby. Product nearby should be stored in sealed dessicator cabinets. The area surrounding the cure equipment should be ventilated sufficiently. The cure chambers should never be opened while curing is still in progress. Chamber door seals should be inspected frequently for any signs of leaking gases and replaced, if necessary.
Oxidation. Heating processes such as curing or drying may oxidize metallized areas or pads used for electrical interconnection on an IC die or substrate. Copper is increasingly being used within IC packages as the preferred metallization for electrical interconnects. One of the disadvantages of copper is that it readily forms a tenacious oxide that is difficult to remove or break through, so care must be taken to prevent its formation. Oxide thickness can be controlled during the cure process by purging or backfilling the cure chamber(s) with nitrogen or a reducing atmosphere. Although curing in an air atmosphere is usually preferred, the cost of using alternative gases is often less than the cost of subsequent yield loss or rework of poor connections resulting from over-oxidized contact surfaces. Specify equipment that automatically opens and closes valves that control the flow of various gases at specific points in the cure process in order to minimize gas consumption costs and better control oxide formation.
Stress. Large differences in thermal mechanical properties between adjoining materials in a package can result in high stress gradients that can lead to interfacial delamination and ultimately to package cracking. These conditions may lead to lifted or severed electrical interconnects and mechanical damage to the IC die itself. Stress levels increase with IC die size. They can be controlled by the proper selection of encapsulants or adhesives and by specifying a proper cure profile to optimize their cured properties. A two-step cure profile (figure 2) may be required with very tightly controlled heating and cooling rates. Select equipment capable of programming multiple temperature setpoints for various durations. Profile the cure chamber to ensure temperature uniformity. Don't overlook the importance of controlling the cooling rate.
Incomplete Cure. As mentioned previously, cured properties of adhesives and encapsulants are highly dependent upon the material receiving the proper amount of energy (i.e., temperature) for the proper amount of time. Incomplete cure is occasionally caused by interruptions to the cure profile from power outages or equipment failure. This can be avoided by having backup power available and executing a good preventive maintenance schedule on the equipment.
More commonly, incomplete cure occurs with batch ovens and is caused by operators opening the cure chamber, interfering with the cure process, to add or remove product. Usually this occurs when the cure chamber size is too large and there is insufficient capacity of smaller cure chambers to keep pace with an upstream dispense process. When the chamber is opened, heat escapes, the product temperature drops, and it takes considerable time for the temperature to recover to its programmed value once the chamber is closed.
Filler Settling, Resin Bleed and Moisture Absorption. After dispense, a number of issues may arise if dispensed epoxies or adhesives do not start the cure process promptly. In some cases, filler particles separate from the resin material and settle to the bottom of the bond line. In other cases, low-viscosity resin, given sufficient time, “bleeds” outward from the bond line and wicks along the substrate surface, contaminating nearby regions. Epoxies and adhesives are generally hygroscopic. Absorbed moisture, combined with certain impurities, can form corrosive compounds that may dissolve IC die interconnects and bond pads. The moisture may vaporize during cure, leaving voids and causing poor adhesion.
For the reasons just stated, it is very important to minimize the queue time between dispense and cure. The best solution is to minimize the batch size. Instead of processing one or more magazines of lead frames or process carriers through dispense and cure, it is much better to process each frame or carrier as if it were a single lot. This may be impractical with batch ovens, but it is easily accomplished with an integrated, inline horizontal or vertical oven system.
Misalignment and Leveling. In some packages, alignment or positional placement of certain components or flatness of the encapsulant is critical. Until an epoxy or adhesive is cured, a “placed” component is free to rotate, slide or tilt in any direction. This movement during cure may be caused by rough or careless handling and loading of the cure chamber. It may be caused by placing the product on non-level shelves or platforms in the cure chamber. It may even be caused by placing the product directly in the path of the airflow, if the air velocity is too high.
Safety. The primary safety concern of the cure process is to prevent operators from accidentally touching hot surfaces and receiving severe burns. Operators of batch ovens should wear protective gloves when loading product into a hot cure chamber and unloading hot product. Further insurance would come from using cure equipment that “actively” cools the cure chamber and product to 122oF (50oC) or less before the chamber door is allowed to open. Other equipment safety concerns may be avoided by specifying equipment that is CE and/or SEMI S2-93 compliant.
Reliability Performance. Much of the variability encountered in long-term product reliability may be a direct result of inconsistent staging and curing within a given lot or from lot to lot. With batch processing, each package in a lot receives a slightly different cure, although it was loaded and unloaded from the cure chamber at the same time as the others and programmed to receive exactly the same cure conditions.
If a mix of packages is cured together in the same chamber, then the temperature of some package types will ramp up and down faster than other types. Temperature ramp rates depend on the size of the load or thermal mass placed in the cure chamber. A larger load will heat up and cool down more slowly. It is possible to overload a chamber such that one or more packages never reach the desired package temperature in the time specified. Those packages located near the outside of a load will ramp faster than those “buried” deep within the load. Those packages stacked closer together will ramp slower than those spaced further apart and exposed to more airflow. Those packages located upstream closer to the fan in forced-convection ovens will ramp faster than those “shadowed” downstream. It is important to profile (figure 3) a cure process by placing thermocouples on actual product at several locations within the cure chamber for different product types, load configurations and load sizes to ensure each and every product receives the minimum required cure time and temperature.
Manufacturability IssuesManufacturing consistency comes from tightly controlled processes. Critical variables and performance parameters need to be monitored constantly during the cure process. The equipment should have safeguards and interlocks that prevent dangerous situations -- both to the operator and to the product. Look for equipment that provides jam-detection during product transport or motion, real-time alarms to signal the temperature is outside of allowable control limits and remote monitoring, programming and diagnostics via modem connection.
Equipment Utilization. Excessive equipment-related downtime cannot be tolerated. Unexpected downtime can create a bottleneck in the flow of a balanced assembly line, slowing throughput and causing product to be staged until the equipment is repaired. Excessive queue time between dispense and cure, as discussed above, is intolerable. Specify reliable equipment with good mean time between failure (MTBF) or mean time between assist (MTBA) ratings. Modern equipment offers event datalogging that is useful in establishing an objective and accurate record of all alarms requiring assistance and time stamps to calculate actual uptime or downtime. It is important to follow the equipment manufacturer's recommended preventive maintenance schedules.
Throughput and Capacity. The throughput generally is governed by the time required to cure the epoxy or adhesive material and any loading and unloading time. Integrating the cure equipment to the dispense equipment provides the fastest throughput because it eliminates any potential delay in loading and unloading by inattentive operators.
As discussed previously, batch processing can be successful if the batch or lot sizes are kept small and cure chambers are optimized for small batches or lots. Interrupting the cure process to load a batch to a large chamber that is in the midst of processing another batch may have serious quality and reliability implications and should be avoided. Thus, many small batch ovens are required to provide sufficient cure capacity to balance the dispense capacity. For inline cure equipment, it is simply necessary to balance the product delivery rate to the cure equipment transport speed established by the cure conditions and provide sufficient buffering of upstream product.
Assembly Yield. It is often difficult to attribute yield losses directly to the cure process. Evidence of miscured product usually does not appear until late in the assembly flow or in subsequent reliability monitoring. There may be an occasional yield loss due to a “missing” or “lost” unit due to careless handling or miscounting the units “in” or “out” of the operation.
With inline cure equipment, contingency plans should be established if the transport system should halt in the middle of the cure process. In the event this happens, most of the product, if not all, can be salvaged by manually removing it from the stopped system and completing the cure with other equipment. Some of the more advanced inline cure equipment provides a recovery recipe or program that can be invoked to complete the cure of work-in-progress if it appears that it may take too long to assist or repair the equipment.
SummaryThe curing process is definitely a vital step in IC package manufacturing. It must be controlled and monitored as diligently as any other process step. The effects of improper curing are subtle and often attributed to inconsistencies in the materials themselves or in the dispensing and/or placement operations. The key to overcoming the quality and manufacturability issues described previously is to avoid variability in and minimize the queue time between dispense and cure. Curing small batches of product is impractical and not very cost effective unless it is done with inline vertical oven technology. Although the inline vertical cure equipment carries a relatively high, initial capital cost, total cost-of-ownership calculations have repeatedly shown this solution to have the best return on investment. PH
Sidebar: Common Curing ApplicationsA typical IC package assembly flow consists of many process steps, and a number of them require drying or curing. These cure steps are typically performed in large, forced-air, convection ovens at various temperatures ranging from 257 to 347oF (125 to 175oC) for periods ranging from 1 to 6 hr. In some cases, a two-step cure profile is required.
Substrate Bake-Out. One of the first steps in the assembly process is a curing step used to bake-out latent moisture in the organic substrates for pin grid array (PGA), ball grid array (BGA) and chip scale (CSP) packages. There is no need to bake-out metal leadframes used for more mature plastic package styles.
Underfill Cure. After flip chip (FC) dice are soldered to substrates, the gap between the face of the IC die and the substrate is “underfilled” with a low viscosity, silica-filled epoxy to reduce the mismatch in coefficient of thermal expansion between the die (~3 ppm per oC) and the laminate substrate (~16 ppm per oC). The underfill epoxy helps prevent thermal fatigue and cracking of the solder joints.
Heatsink Cure. Increasing power dissipation and decreasing die sizes are driving the increasing need to attach heatsinks to packages and the backside of FC dice. Many different adhesive materials, supplied in liquid (or paste) and film (or preform) formats, are used for this application. These adhesives contain high thermal conductivity filler particles, and it is critical to maintain a uniform, thin, void-free bond line for lowest thermal resistance.
Die Bond Cure. In most wire bond (WB) applications, the IC die is adhesively bonded to either a metal leadframe or laminate substrate. The adhesive contains filler particles that make it electrically conductive or insulative, depending upon whether or not the die backside requires an electrical bias or thermal connection. In multichip and stacked-die applications, the die bond and curing operations may be repeated two to five times. As with heatsink attach, it is critical to prevent filler settling and achieve a uniform bondline with strong adhesion.
Coating Cure. Although junction coating is rarely used today to protect the IC die from the corrosive effects of ionic impurities in molding compounds, new coating applications are being developed. Processes for coating long, fine-pitch wire bonds to prevent wire sweep and electrical shorting at mold; processes for spin coating resists; and encapsulants on IC wafers in assembly for wafer-scale packaging are under development. It will be critical to achieve strong adhesion.
Encapsulation Cure. Chip-on-board (COB) applications, having either WB or FC interconnects, are generally encapsulated with liquid, silica-filled epoxy materials dispensed from syringes. In the “dam and fill” process a thixotropic barrier, or dam, is dispensed on the substrate around the IC die and interconnects. Then, the dammed region is filled with a lower viscosity, self-leveling epoxy to cover and protect the die. It is critical for the fill material to achieve good, void-free, adhesion to the dam material and underlying components. It is essential that the fill material remain level with the top of the dam material throughout the cure process.
Potting Cure. Liquid encapsulants also are used for potting optical components such as LED lamps and displays as well as many large electrical component housings. In optoelectronic applications, any voids or bubbles in the optical path will distort or disrupt the light transmission.
Lid Seal Cure. Epoxies are occasionally used to “glue” a lid to a substrate or case, when hermeticity is not required, to enclose and protect dice and wire bonds from contamination, moisture and mechanical damage. Good, void-free, adhesion is essential.
Post-Mold Cure. The vast majority of packages manufactured today contain a single IC die and WB interconnects that are subsequently transfer-molded using a thermoset epoxy mold compound. The pelletized compound is injected into a hot mold die to form the body around the IC die and cured in the mold at 357oF (175oC) for 1 to 2 min. After ejection from the mold, the packages require an additional curing process, called post-mold cure, to complete the chemical cross-linking of the material. Again, it is critical to achieve good adhesion.
Marking Cure. Most marking inks in use today are either cured with ultraviolet (UV) light or heat-cured. They are formulated for good adhesion and resistance to solvents. Mark permanency testing will fail if cured improperly.
Dry Pack Bake-Out. Lastly, before moisture-sensitive IC packages are shipped, they are dried, or baked-out, thoroughly and sealed in air-tight bags at an operation called dry pack. If the packages are allowed to absorb atmospheric moisture, they are likely to suffer package cracking, if not internal delamination, during subsequent board assembly processing.