Die attachment or component attachment consists of attaching a die, capacitor, resistor, onto a substrate, carrier or housing; attachment medias are epoxy (conductive and non-conductive) or eutectic attach. The designer will choose the best attachment media based on mechanical, thermal and electrical requirements.
Conductive epoxy paste attachment uses silver flakes mixed in with the binders, and resins of the epoxy; the use of silver flakes allows the epoxy to become electrically and thermally conductive. Several types of conductive paste are available and are tailored toward specific electrical, thermal, and mechanical requirements.
Figure 1: IC placed in silver epoxy view at 45 degree angle (photo courtesy of TJ Green Associates LLC)
Figure 1 depicts an IC die nicely placed in silver epoxy with a fillet of epoxy visible around 100% of the perimeter. All hybrids are manufactured in accordance with the Mil-Std-883 TM 2017 visual inspection standard.
These pastes are applied through several different means which include:
- Stamping or daubing (Uses a dish to "pick" (Daub) the epoxy onto the end of a tool and placing it in the desired location.)
- Positive displacement dispensers (Uses an auger to feed the epoxy through the needle for precise control.)
- Air powered dispensers (Air pressure and time adjustments allow a plunger to push the epoxy through the syringe and needle.)
The type of epoxy specified for the design will dictate how the epoxy is applied; many pastes are dispensable through the use of positive displacement pumps, which allows for precise dot diameters and volumetric control and this method is generally the industry standard for automated epoxy dispensing.
Non-conductive epoxy is generally used for structural support, and as the name implies to isolate conductive traces or elements. This paste can also be applied by the same methods as described under the conductive epoxy section.
Curing of these epoxy are required after the components have been placed. Many of the epoxies rely on a thermal cure as set by the manufactures data sheet for the specific epoxy. Great care must be taken to ensure the materials in the design can accommodate these temperatures, as the thermal range can be as high as 175 degrees Celsius.
A eutectic attachment is a mixture of two metal alloys' that when combined have a lower melting point than the original materials. These eutectic attachments go from a solid to liquid state and bypass the "plastic" state. This type of attachment has several advantages including low ohmantic contact, excellent thermal characteristics and quick processing times. Industry standard eutectic attachment methods are:
- 80/20 AuSn (Gold-Tin) attachment. (Has a melting point of 280°C.)
- 88/12 AuGe(Gold-Germanium) attachment.(Has a melting point of 361°C.)
- 97/3 AuSi * (Gold-silicon) attachment (Has a melting point of 370°C.)
As shown above the processing temperatures are very high, and as such the design needs to be able to accommodate these high temperatures. Typical applications use the alloy preform between the die onto an Au plated shim, carrier or substrate that can accommodate the high processing temperatures. The only exception to the above is the 97/3 AuSi attachment; this attachment can be directly scrubbed onto the substrate. However, the metallization schemes on the die and substrates must have the correct thickness.
The eutectic process can be performed manually, semi-automatically or fully automatic; volume and design usually dictates which process will be used. Since, many different size die are available, the tooling required is typically custom made. This tooling can consist of pickup tool, collets, and the use of custom machined graphite boats; as such the price range for the tooling ranges from several dollars to thousands of dollars.
Substrates including alumina and soft substrates (Duriod) can be attached with paste epoxy, sheet epoxy, or eutectic attachment.
Substrate attachment also falls under the general category of die attachment. Soft substrates (Duriod) and hard substrates (Alumina) are typically attached with sheet epoxy, as the aspect ratios of these substrates can be rather large and the TCE mismatch between the materials could cause failures with other forms of attachment. Sheet epoxy comes in varying thicknesses and can be cut to shape with a die, laser, or an X-acto knife. The sheet epoxy requires pressure to be applied during thermal cure; therefore, special tooling is required for this type of attachment.
Eutectic attachment of hard substrates is very similar to eutectic die attachment. Typical design have the substrate attached to an Au plated shim, or carrier. Either AuGe or AuSn can be used, but size consideration must be taken into account when using this design; typical designs have substrates dimensions less than 1" squared. Soft substrate attachment using eutectic attachment is possible, but should be avoided as the processing temperatures can be detrimental to the dielectric of the soft substrate.
Attachment using paste epoxies on hard and soft substrates also have design constraints, which are dictated by the type of epoxy being used. Therefore, the design needs to evaluate for TCE mismatch between the materials.
Typical applications have the paste dispensed through a needle, either through positive displacement pumping, or through air pressure. While, other methods (Daubing, and stamping,) are used to place epoxy, the positive displacement pumping allows for precise dot diameters and volumetric control and this method is generally the industry standard for epoxy dispensing.