MEMSonics is capable of providing a full spectrum of semiconductor packaging services ranging from Wafer Saw, Die Attach, Wire Bonding, Encapsulation and other related processes, depending on customer requirements.
Wire Bonding
There are basically two forms of wire bonds, Wedge bonds and Ball bonds. Recent industry survey shows that about 90% of all electronic packages and assemblies are produced using ball bonds and about 10% are produced with wedge bonds. An increase in the use of ball bonding is expected as semiconductor devices increase in functionality and decrease in size causing smaller bond pads and closer bond pad spacing.
The most widely used wire materials are Gold (Au) and Aluminum (Al) however, Silver (Ag) and Copper (Cu) are also used. Copper wire (ball bonding) has gained considerable attention due to its economic advantage and strong resistance to sweeping. Gold was the original material used when wire bonding technology was developing. Aluminum has become popular due to its good electrical performance and lower cost. Gold wire can be bonded in the shape of a wedge or a ball. Ball bonds can be used in very tight spacing. Aluminum wire can only be wedge bonded and so is limited when spacing is tight.
There are three main methods of wire bonding: Thermocompression, Ultrsonic, and Thermosonic.
• Thermocompression bonding was extensively used until Ultrasonic bonding technology became more prevalent. Thermocompression bonding requires a high-force on a surface with a high temperature of around 300°C. It provides excellent, reliable Al-Au bonds with flexibility in the bonding direction allowed. The wire material is Au but the pad can be Au or Aluminum.
• Ultrasonic bonding is the most common bonding technology used for Al bond wires and is performed at ambient temperature. Bonding is formed as a wedge bond by pressure and vibrational energy.
• Thermosonic bonding is used for Au wires and currently comprises about 90% of all wire bonding. It is done at temperatures of around 100°C to 240°C. Bonding is formed when the ultrasonic energy combines with the capillary technique of thermocompression bonding. Occasionally, it is used for Au wedge-wedge bonding, but mostly it is best suited for a ball-wedge bond.
The exact material and method of wire bonding will be specified by our customers. Additionally, the wire specifications are determined by the amount of current to be carried through the device as per the customer's requirements.
Die Attach
Die Attach (also known as Die Mount or Die Bond) is the process of attaching the silicon chip to the die pad or die cavity of the support structure (e.g., the leadframe) of the semiconductor package. There are two common die attach processes, i.e., adhesive die attach and eutectic die attach. For high-powered applications, the die is usually eutectic bonded onto the package for good heat conduction. For low-cost, low-powered applications, the die is often glued directly onto a substrate using an epoxy adhesive.
Both of these processes use special die attach equipment and die attach tools to mount the die.
Adhesive die attach uses adhesives such as polyimide, epoxy and silver-filled glass as die attach material to mount the die on the die pad or cavity. The adhesive is first dispensed in controlled amounts on the die pad or cavity. The die for mounting is then ejected from the wafer by one or more ejector needles.
While being ejected, a pick-and-place tool commonly known as a 'collet' then retrieves the die from the wafer tape and positions it on the adhesive. All of the above steps are done by special die attach equipment or 'die bonders.
The mass of epoxy climbing the edges of the die is known as the die attach fillet. Excessive die attach fillet may lead to die attach contamination of the die surface. Too little of it may lead to die lifting or die cracking.
The attach materials and process will be specified by the customers.
Encapsulation
In the encapsulation stage, the die is encapsulated with ceramic, plastic, or epoxy to prevent physical damage or corrosion. As in other packaging stages, the customer will specify encapsulation requirement such as hermetic sealing, moisture contamination, physical package options, sensitivity of the dies to stress from packaging materials, height requirement of the capped devices, multi-die capability, etc.