Engineering Design And Manufacture Engineering Essay

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Key technology relevant to the design and the status of development

Information on MEMS industry:

Narvey and Nathanson at Westinghouse developed the first transistor in 1967 with moving mechanical elements. During 1980s and early 1990s, most of the MEMS industry went to develop different processes. Today, most of the efforts are going into designing structures using the available MEMS processes.

MEMS (Micro-Electro-Mechanical systems) are usually a single or a number of microm-achined devices with an associated ControVsignal-processing-circuit providing a defined function. Pressure sensors and accelerometers are some of the examples. While the electronics are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using micromachining processes that selectively engrave away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

Information on types of die attach:

Die attach is also known as Die Mount or Die Bond. It is a process of attaching silicon chips onto the die substrate or die cavity of the support structure of the semiconductor package.

There are two common die attach processes:

Adhesive die attach

Eutectic die attach

In this design project, only adhesive die attach process is considered. Some adhesives like silver, polyimide, epoxy and silver-filled glass are mainly used as die attach material to bond the die on the die substrate. The adhesive is first dispensed in controlled volume and pattern on the die substrate. Then the die for mounting is ejected from the wafer by one or more ejector needles.

While being ejected, computer controller vacuum pick-up tools retrieve the die from the wafer tape and positions it on the adhesive. These complicated processes are done by a special machine called "Die bonder" (Figure 1).

As the pick-up tools press the die onto the substrate with certain pressure, some epoxy will be squeezed out of the edges of the die. The squeezed epoxy is called die attach fillet (Figure 2). The amount of epoxy should be calculated accurately because too much die attach fillet may lead to contamination of the die surface but too little of it may lead to die lifting or die cracking.

Figure 1 - Palomar's Model 3500-III Die Bonder can perform high accuracy epoxy die attach with dispensing

Figure 2 - the die attach fillet between the die and die pad (substrate)

Information of die attaching tools:

Die bonder requires various tools to operate.

Ejector needles/pins (Figure 3) - before attaching a die, the die must be first picked-up and ejected from the wafer tape. By using ejector needles or pins, the machine is able to push the backside of the die upwards to separate it from the wafer tape. The number of the ejector needles depends on the size of die, specification and aspect ratio. Usually most of the tapers of the pins are smooth, highly-polished and with small angle. It can prevent large tape disturbance when the pins penetrate the wafer tape.

Figure 3 - Different sizes of ejector pins

Figure 4 - Four-sided die Attach Collets

Die attach collet/pick-up tool (Figure 4) -The die on the wafer tape is picked up by this tool. Some of the working principles are contacting surface or pushing the die up off the surface of the adhesive film by ejector pin. Conical vacuum pick-up tools are typically used for die pick-and-place into epoxy. The plastic materials are commonly specified as opposed to metal because they are less likely to damage the die and sensitive devices. Some of the tips of pick-up tools are made of tungsten carbide, which can cause damage to the die surface. To prevent die scratching, antistatic plastic tools must be used instead. Replaceable rubber tips (Figure 5) may also be used on fragile dice.

Figure 5 - Rubber Tips for Die Pick-up Tools

Figure 6 - Epoxy Dispensing Tools

Epoxy dispensing tools (Figure 6) - Put die attach material such as silver-filled epoxy or silver-filled paste on the die substrate. Most of the tools are customized to specific requirements. For convenience, the bodies of tools are standardized to fit in most of the die bonder and die bonders are also usually equipped with adaptor heads allowing quick replacement of dispensing tools.

Figure 7 - Rubber epoxy Stamping Tools

Epoxy stamping tools - Depending on different types of MEMS or application, epoxy stamping tools may also have different shape and pattern. To ensure there is correct amount of epoxy deposited on the die pad every time, a matrix or regular dots pattern is needed. Different dot matrix patterns are designed to satisfy the needs of different die sizes. It can be made of steel or rubber. The steel stamping tool is usually for small to medium-sized dice, giving a rectangular tip in medium-sized dice and a round tip in small dice. Rubber epoxy stamping tool (Figure 7) is usually for larger dice. When the die is pressed, the epoxy is spread.

Material of die packing:

Plastic packages cost less than ceramic and metal packages so it is more attractive for MEMS packaging. Plastics packages also weigh less than ceramic and metal packages. However, plastic packages may absorb moisture and cause reliability.

Moisture absorption may causes MEMS sensors to sense incorrect data because moisture can cause adhesion between the poly-silicon and silicon substrate which does not allow sensors to react quickly to sensed signal appropriately. Delamination may also exist if moisture stays in the package.

Failure of die attach:

Die Lifting - detachment of the die from the die pad or cavity. Common Causes: contamination on the die pad or cavity, die backside contamination, excessive die attach voids, incomplete die attach coverage, inadequate die attach curing

Die Cracking - occurrence of fracture anywhere in the die. Common Causes in the context of Die Attach: excessive die attach voids, die overhang or insufficient die attach coverage, insufficient bond line thickness, excessive die ejection force on the wafer tape, absence of die attach voids. Another critical aspect of adhesive die attach is the ejection of the die from the wafer tape during the pick-and-place system's retrieval operation. The use of inappropriate or worn-out ejector needle and improper ejection parameter settings can cause die backside tool marks or microcracks that can eventually lead to die cracking (Figure 8).

Figure 8 - Wrong ejector pins/placement will cause die cracking

Adhesive Shorting - electrical shorting between exposed metal lines, bond pads, bonds, or wires as a result of adhesive accidentally dripping on the surface of the die (sometimes called 'epoxy on die'). Common Causes: incorrect die attach material viscosity, incorrect adhesive dispensation

Bond Lifting - lifting of the first or second bond from the die or leadfinger, respectively. From the die attach process point of view, this is often due to resin bleeding of the die attach material into the bond pads or leadfingers, inhibiting good intermetallic formation. See also Wirebonding.

Die Scratching - inducement of any mechanical damage on the die, as when an operator scratches a die with tweezers due to mishandling. Common Causes: insufficient operator training, worn-out or contaminated pick-and-place tool, disorderly workplace, use of improper tools

Die Metallization Smearing - depression or deformation of any metal line on the die surface. Common Causes: dirty or worn-out die attach pick-and-place tool, wafer mishandling

Section 3

3.1 Functional decomposition of the machine into sub-systems

Figure 9 - Simple decomposition of an Epoxy die attach machine

The Epoxy Die attach machine can be decomposed into following sections:

Power System - Supply electricity for all components inside the machine.

Vision System - Detect the PCB substrates on the machine table, wafer in the wafer holder.

Control System - Control all the moving components inside the machine with acceptable tolerance.

Material Handling (Warpped with noise reducing material):

Machine Table - The place where the PCB substrates are placed, the die are attached and epoxy are dispensed.

Wafer (up to 4 in.) - Die can be directly picked from the wafer after the wafer sawing process, computer controlled vacuum pick-up tools and ejector needles are equipped in the wafer holder.

Epoxy Curing - After dice are attached onto the substrates, they are sent to oven for stabilizing the polymer. For using UV cured epoxy, they are exposed under UV lights.

Cooling System - There is a water jacket with temperature sensors inside the machine to maintain constant temperature.

Safety - The machine is equipped with a transplant plastic cover and emergency stop switch.

3.2 Functional specification of the machine, breaking down into sub-systems

Power System:

The machine is equipped with transformer to suit different voltage supply. Rate operating voltage: 300 - 500V.

Vision System:

Optical Bond Centering

Parallel Pre-Bond IQC (Integrated Quality Control and Rejected Pad Detection)

Optical Dispense Centering (Integrated Quality Control and Rejected Pad Detection)

Post-Bond IQCThere Detect the PCB substrates on the machine table, wafer in the wafer holder.

Control System - Control all the moving components inside the machine with acceptable tolerance.

Material Handling (Warpped with noise reducing material):

Machine Table - The place where the PCB substrates are placed, the die are attached and epoxy are dispensed.

Wafer (up to 4 in.) - Die can be directly picked from the wafer after the wafer sawing process, computer controlled vacuum pick-up tools and ejector needles are equipped in the wafer holder.

Epoxy Curing - After dice are attached onto the substrates, they are sent to oven for stabilizing the polymer. For using UV cured epoxy, they are exposed under UV lights.

Cooling System - There is a water jacket inside the machine to prevent overheating.

Safety - The machine is equipped with a transplant plastic cover and emergency stop switch.

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