Die Attach

A die mount or die bond step where a rectangular slice of semiconductor (die or chip) is mounted on the support structure of the package, for example, a leadframe die pad, cavity, or substrate using conductive adhesive. Depending on the die-attach adhesive paste, rapid cure can occur in seconds at 80-450C. Silver-based ide attach adjesives have been traditionally used to assemble ledframe devices.  Pure solder wire drawn onto the die for flux-free chip soldering in an inert environment is another commonly used method.  New methods and materials include a wide range of lead and lead-free solder alloys by Henkle and others, cured at 350 to 400C and above to prevent void formation.

Encapsulation and Sealing

Encapsulation and sealing are used to protect devices from adverse environmental and mechanical influences.  Epoxy polymerization, a fast clean encapsulation methodology most commonly involves an anhydride epoxy curing agent which typically requires curing at 140-180C after a lower temperature (120-130C) drying stage. 

Hermetic sealing then takes place either by metal fusion or glass sealing. Metal packages are often sealed by metal fusion either welding, brazing or soldering.   

Glass sealing is used for ceramic dual in-line packages (CERDIP) where an epoxy or grout is used to hermetically seal the two halves together, provides an air and moisture tight seal to protect the IC die inside. Glass sealing commonly cured in convection and infrared furnaces in a nitrogen atmosphere at 350-420C for soft glass seals, 400-550C for standard sealing glasses and greater than 850-1000C for high temperature sealing glass. After lids and ceramic are coated with sealing glass, an AG-series IR furnace is designed to manage the CERDIP sealing process through each critical stage: organic burnout and offgassing (200-250C, softening of the glass, glass profile formation at 350C, chemical bonding at 425-500C, annealing, and controlled cooldown).  Tummala, Rymaszewski and Klopfenstein observed in the Microelectroinics Packaging Handbook that moisture removal has been shown to occur in at least three distinct phases (offgassing at 90-210C, absorbed moisture at 365-375C, and chemically combined water at 470-480C).

Adhesive Challanges

Without tight process control, some of the following processing steps can present difficult problems for adhesive:

•Wire-bonding temperature; performed from 125-250C with ultrasonic energy. Flexible adhesive may absorb too much energy or move excessively for small die.

•Molding temperature and forces; performed at typically 175C, the movement of molten epoxy molding compound may destroy the die-attach joint if not properly controlled.

•Soldering; performed at 220-280C, the adhesive must be able to maintain its thermal stability and position during this processing step of device assembly.

Curing Considerations

Heat cures offer higher throughput and faster processing times. Heat cure times typically are shorter with increasing temperatures.

Heat Curing Heat cured adhesives can be either one-part or two-part products. Typically, cure and the development of adhesion are not achieved until the material is heated above 100C. Newly developed formulations cure more quickly at temperatures as low as 90C. At these temperatures, the adhesives will generally cure in about an hour. For all the heat cure adhesives higher temperatures will result in faster cure.

Limitations on cure speed are generally dependent on the temperatures that the unit and components are able to withstand. Heat curing can be done in a batch or conveyor oven. Heat cured adhesives will cure in either thin or thick section and can cure when confined.

The heat cure adhesives can have their cure poisoned (inhibited) by certain materials due to the type of catalyst used, resulting in poor or incomplete cure. With a mild case of inhibition, the adhesive can appear to be cured normally, but adhesion is poor. If you have problems with poor cure or adhesion, contact Dow Corning’s technical experts in the Application Center.

Profiling for Solder Fusion, Die Attach and Sealing

Profiling for these operations are initially determined through results of preliminary trial because of the great number of variables affecting thermal response of the furnace and the system being processed.  Boats, fixtures and other processing aids should have a thermal mass as small a practical, consistent with good manufacturing practice and requirements.

Conveyor speed should be adjusted as high as practical.  Zone 1 (and 2) temperature should be set to a temperature which will bring the system being processed to an internal temperature substantially below the fusion temperature.

Spike zone (zone 2 or 3 depending on model) should be set such that the internal temperature achieved by the system will insure that fusion is complete, without exceeding limits imposed by the separate components of the system. Generally, maintaining as large as possible temperature differential between the heating zones and the spike zone, enables the furnace to generate the high intensity spike.

Dummy loading ahead of the systems being processed, to bring the furnace to a stable condition is most desirable and is also determined by initial testing.

Atmosphereic requirements are process dependent, and should be adjusted to provide proper flow direction for volatile removal (if any), and for atmosphere composition to control any acverse of atmosphere on the product at temperature.

S-Series and AG-Series IR Furnaces

Infrared furnaces offer precise control of parameters critical to thick film production processes. S-Series, S-Series Hydrogen and AG-Series deliver 1000C high temperature processing, stabilize quickly, and allow operator to control the atmosphere and flow of process gas at multiple stages. Proprietary TPS furnace control softwareincludes sophisticated PID temperature control and power management by zone and can integrate predictive profiling software from KIC, dataPaq and others. Process engineers can save an almost infinite number of recipes, as many needed for each paste or substrate system. Changing recipes is quick and easy. Once recipes are loaded the furnace stabilizes to the new settings within minutes (often less than 10 minutes).

AG-Series furnaces include essentially all the features of S-series models, but AG-Series adds better zone isolation and the ability to exhaust process gas at more frequent intervals, key for taking advantage of the benefits of some of the newer materials and processes.

 S-Series, S-Series Hydrogen and AG-Series are built-to-order and configured to perform at the highest levels for its destined production room environment.

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