Engineering Specification Development Adhesive Engineering Essay

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The project is aimed at designing a small adhesive die attach machine for attaching MEMS dies to PCB substrates printed with MEMS (Micro-Electro-Mechanical Systems) of different designs. The system should be able to:

PCB substrates are position on the machine table manually and the machine should be able to detect the locate the positioning of the substrate within an acceptable tolerance;

pick up diced MEMS dies from a 4 inch wafer;

dispense die attach epoxy to the die pad on the substrate;

attach the die to the die pad with sufficient pressure;

maintain the bonding operation at a stable temperature;

easy to use with computer control;

low noise;

high repeatability, and

safe operation.

Section 2

Key Technology Issues and Status of Development

Research on key technology issues relevant to the design of "Adhesive Die Attach Machine for MEMS Applications" and the status of development

Introduction to MEMS technology

Micro Electro Mechanical Systems is popularly known as MEMS. MEMS is a diverse technology which is an amalgamation of all the faculties of engineering and science. MEMS consist of sensors, actuators, passive components, power management circuits, analog and digital integrated circuits. MEMS devices can be used as miniature sensors, controllers or actuators. They have become increasingly dominant in every aspect of commercial marketplace as the technologies for micro fabrication continue to be developed.

The following are the manufacturing process steps to attach a die to the substrate.

The die attach adhesive is dispensed on the die pad in a pattern to optimize attachment material coverage between the backside of the die and the substrate.

The die is placed on the substrate with a pick and place machine. As the die is placed, the adhesive spreads to cover the die attach pad.

The adhesive is cured, typically with heat.

In wire bonded applications, encapsulation and sealing takes place to complete the assembly process.

The requirements of a die attach material will depend on the application, but may include:

Good mechanical strength (e.g. up to 150°C)

Process temperature that will not affect the die function

Absorption of stress from thermal expansion mismatch between the die and substrate

Joint fatigue resistance - mechanical and thermal

Electrical/thermal conduction or isolation

Chemical inertness with low outgassing

Reworkable

Ability to automate process

Technology issues

1. Die Picking

2. Die Transfer

3. Adhesive Dispensing

4. Die Bonding

Die Picking

- Die can be directly picked from the wafer after the wafer sawing process.

- Die bonder are often equipped with ejector needle system to push against the backside of the wafer to eject the die.

- Computer controller vacuum pick-up tools are used to lift the dies

Die Transfer:

- The die pick-up arm of the die attach machine will send the die to the appropriate position and load the die on the substrate.

- Die bonding pressure has to be controlled.

- The substrate may be heated to facilitate partial curing of the epoxy die attach.

Adhesive Dispensing

Jet dispensing of die bonding paste is much faster than conventional needle dispensing. Patterns of the past can be dispensed that cannot be achieved with a needle dispenser.

Speedline/Camalot Dispenser

Die Bonding

- The die pick-up arm of the die attach machine will send the die to the appropriate position and load the die on the substrate.

- Die bonding pressure has to be controlled.

- The substrate may be heated to facilitate partial curing of the epoxy die attach.

Ball Bonding Process

1. Place the ball on the die and the crescent bond on the circuit to minimize the force applied to the die.

2. Heat substrate with die to 130 - 150°C during wire bonding.

3. Select appropriate size wire to allow for ball size and wire bond placement accuracy on bond pads. Bonds should not overhang the outside of bond pads to avoid contact with exposed metallization from other areas of the die.

Wedge Bonding Process

1. Bonding may be done in any order with first bond, stitch bond, or last bond placed on die.

2. Heat substrate with die to 130 - 150°C during wire bonding.

3. Select appropriate size wire to allow for bond, tail, and wire bond placement accuracy on bond pads. Bonds should not overhang the outside of bond pads to avoid contact with exposed metallization from other areas of the die.

Section 3

Functional decomposition of the "Adhesive Die Attach Machine for MEMS Applications" in sub-systems

This Adhesive Die Attach Machine for MEMS Applications can be functionally decomposed into 5 sub- systems: pick-and-place system, component presentation system, substrate presentation system, vision system, and epoxy dispensing system.

Pick-and-place system:

There are two kinds of pick-and-place system which is ball-screw and belt systems.

Component presentation system:

A die attach machine should support all standard formats of component presentation on one machine in a single pass, including all sizes of wafers, waffle packs and grip rings. Waffle vision which provides searching for die in large waffle pack cavities, is also a critical feature.

Substrate presentation system:

There are several types of substrate presentation systems, including belt, gripper, walking beam and manual work holder. 

Vision system:

A vision system is composed of four major elements: vision engine, wafer/component camera, substrate camera and upward-looking camera.

Epoxy dispensing system:

High-performance volumetric screw-pump dispensers yield a highly repeatable epoxy dispensing from large patterns to ultrasmall dots, with the same needle.

Pick-and-place system

Component presentation system

Adhesive Die Attach Machine for MEMS Applications

Substrate presentation system

Vision system

Epoxy dispensing system

Functional specification of the "Adhesive Die Attach Machine for MEMS Applications".

Pick-and-place system:

The system must include bondforce calibration to maintain bond line thickness and prevent die damage due to excessive force. Automatic calibration via load cell provides a consistent bond process. A load cell is also used to automatically calibrate the needle for epoxy writing, which is critical for maintaining the required gap between the tip of the needle and the substrate. The operating range should cover 0 to 1000 grams of bond force. Lower force is important for bonding very thin die, or die with sensitive features such as air bridges and vias found on gallium arsenide (GaAs). High bond force (1000grams or more) is critical for very large die or applications requiring high viscosity epoxy.

Component presentation system:

Automatic wafer presentation systems should efficiently present large quantities and multiple types of components to the pick and place system, up to 200 of 50mm waffle packs or Gel-Paks, up to 25 different wafers or grip rings, or a combination of these, in a single pass. Easy loading and unloading is advantageous, as is detection of ink dots, missing corners and partial die rejects. Wafer mapping, which directs the pick and place system, will precisely locate good die on non-inked wafers. To maximize yield, and minimize cost, especially in higher-volume applications, the die should be picked directly from the wafer.

Substrate presentation system:

Flexible tooling that is easy to use and can be changed over quickly is important, as is a touch probe that provides quantitative feedback so that adjustments can be made for planarity. Recommended is a touch probe with 1μm repeatability. Gripper and walking-beam indexers are less flexible, and are used for the more dedicated IC die attach.

In addition to flexible tooling, a substrate presentation system should accommodate a wide range of substrate sizes, from under 50mm to 200mm, on a standard edge belt, on Auer Boats or on custom carriers. Generally, presenting different sizes and types of substrates includes substrates under 50mm and flex over 50mm to 200mm as well as TO-Packages. Substrate presentation includes Auer Boats and custom carriers, standard edge belt and edge belt with custom carrier. 

Vision system:

A die attach machine should be supported by a comprehensive library of vision algorithms, and an engineering group willing to develop new algorithms for unique requirements, including pattern/template matching, circle matching, edge search, center-point search (blob analysis), symmetrical, and multi-search. Pattern or template matching enables the programming of unique features or dedicated fiducials. This does not require templates of any specific shape or construction to recognize features as small as 150μm, depending on camera magnification and the selected algorithm.

Epoxy dispensing system:

Preferred dispensing techniques include volumetric (auger screw) dispensing, time/pressure dispensing with multi-needle shower head, stamping/gang stamping and epoxy writing with programmable dispensing patterns. Process control and automatic programming should ensure a consistent epoxy pattern and volume, X-Y speed and auger speed. An epoxy low-level detector is also important for continuous operation.

High-performance volumetric screw-pump dispensers yield a highly repeatable epoxy dispensing from large patterns to ultrasmall dots, with the same needle. This eliminates multiple custom stamping tools, which can be costly and difficult to maintain. Additional features include solder-paste capability, easy removal and cleaning and automatic needle calibration. 

Section 4

Adhesive die attach

Adhesive bonding has been in widespread use for over 30 years. Adhesives can be made electrically/thermally conducting (e.g. silver loaded epoxy) or electrically isolating.

Benefits:

Ease of automation 

Low curing temperatures 

Reduced die stresses 

Low cost 

Wide range of die sizes 

Special plated surfaces are not required 

Rework is possible

Limitations:

Outgassing 

Contamination/bleed 

Voiding (in some cases) 

Inferior thermal/electrical conductivity 

Dimensional changes during processing and service life 

Harsh environment sensitivity

Typical adhesive die materials:

Epoxy thermoset resins 

Acrylic thermoplastic resins 

Silicone resins

The requirements of a die attach material will depend on the application, but may include:

Good mechanical strength (e.g. up to 150°C)

Process temperature that will not affect the die function

Absorption of stress from thermal expansion mismatch between the die and substrate

Joint fatigue resistance - mechanical and thermal

Electrical/thermal conduction or isolation

Chemical inertness with low outgassing

Reworkable

Ability to automate process

Dispensing adhesives

The most common systems used in dispensing adhesives use a time-pressure dispensing valve, auger pump, positive displacement pump, or a jetting valve. Each technique has its unique advantages and disadvantages.

Time-Pressure Dispensing Valve: The time-pressure dispensing valve consists of a syringe containing adhesive which is directly attached to the dispensing tip. Adhesive is fed from the syringe using pressure in a time-controlled manner. Pressure is removed to stop material flow. Fluid flow is proportional to the amount and duration of the applied pressure. Since the air pressure is kept constant over time, as the syringe is emptied, dot sizes decrease because the plunger does not advance as far with each air shot. This variability can be adjusted by increasing the air shot size, but is often operator-dependent and can lower throughput. Time-pressure systems are the most economical dispensing solutions, but have a lot of variation in their results, and are limited in the minimum dot size they can produce.

Auger Pump: Rotary auger pumps use an Archimedean screw turning in a cartridge to push the material through the pump. In some cases, the pump uses an electromagnetic clutch to engage and disengage the constant-speed DC screw motor. Low-pressure air maintains a steady flow of the material into the pump. A precision-controlled auger pump is programmable, and uses a DC servo motor with an encoder to precisely control rotation. A programmed dispense signal provides a direct and specific point-to-point indexed rotation of the auger while regulating speed, thus precisely controlling the quantity dispensed. When auger pumps are used to dispense adhesives with fillers, filler size and properties should be taken into consideration. If the filler is abrasive, suitable auger screw material like tungsten carbide should be selected. Needle size and auger screw clearance should be twice the filler size for easy flow of the adhesive. If this size is too small, filler material will clog the needle or the auger resulting in inconsistent dispensing.

Positive Displacement Pump: Positive displacement pumps (Figure 4-3) use a piston to force material through a needle. The piston motion inside the dispensing system is controlled by a DC servo motor with precision encoder. The displacement of the piston in the chamber results in an equivalent positive displacement of fluid through the pump. The deposition time is extremely fast and is solely dependent on the piston size and the length of piston stroke inside the chamber. A change in viscosity does not affect the amount of material dispensed through the pump. If the piston is not seated well, adhesive leaks through the sides of the chamber. Constant pressure is critical for filling the chamber; a drop in pressure results in an insufficient amount of adhesive in the chamber resulting in a smaller amount of material being dispensed. Higher pressures may lead to adhesive leaking.

Jetting Valve: Jet dispensing also called non-contact dispensing - provides the highest speed, delivering adhesive dots from a height between 1mm and 3mm above the board. In addition, it minimizes problems with adhesive tailing. Different sized dots can be applied by simply programming the valve to jet multiple shots into the same location. This allows tight process control, better repeatability, and better dot consistency.

Eliminates Z-axis motion during dispensing.

Positive shutoff prevents tailing.

Dispensing speeds as high as 1000 dots/min.

Fluid stream can be placed in areas where needle will not fit allowing tighter spacing.

Reduced chances of damaging die.

Positive Displacement Pump

Jetting Pump

Used for filled and unfilled adhesives.

Used for filled and unfilled adhesives.

Adhesive flow rates of 5000mg/sec with 1% accuracy can be dispensed.

Very repeatable dots/line. Can dispense wide viscosity range without any process issues.

Easy to clean.

Relatively easy to clean. Does not use needle tips.

0.008 inch dot or line size can be dispensed accurately.

Useful to dispense very small dots 0.006 inch very accurately.

Die Bonding

Ball Bonding Process

1. Place the ball on the die and the crescent bond on the circuit to minimize the force applied to the die.

2. Heat substrate with die to 130 - 150°C during wire bonding.

3. Select appropriate size wire to allow for ball size and wire bond placement accuracy on bond pads. Bonds should not overhang the outside of bond pads to avoid contact with exposed metallization from other areas of the die.

Wedge Bonding Process

1. Bonding may be done in any order with first bond, stitch bond, or last bond placed on die.

2. Heat substrate with die to 130 - 150°C during wire bonding.

3. Select appropriate size wire to allow for bond, tail, and wire bond placement accuracy on bond pads. Bonds should not overhang the outside of bond pads to avoid contact with exposed metallization from other areas of the die.

Section 5

Considerations for manufacturing and assembly

According to the specification of previous specification, there are several sub-systems need to be manufactured by ourselves, while others can be purchased from the suppliers. In a buyer's perspective, these sub-systems are from different suppliers, so making them to be a whole entirely will be a very important issue.

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