TWI Knowledge Summary
Die attachment
by Norman Stockham
The process
The process of mounting a semiconductor die/chip to a substrate or package is known as die attach. Connections to the die are then usually made using wire bonding technology ( Fig.1). Die can vary in size from less than 0.5 x 0.5mm to greater than 50 x 50mm, the choice of attachment material being dictated by the size, substrate material (e.g. ceramic, polymer, glass metal) device requirements and operating environment.
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Fig.1 Solder die attach/wire bonded connections |
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
Current status
Die attach is an established process which has been evolving over the last 40 years. Initial applications usually employed eutectic bonding or soldering on ceramics or metal substrates. Due to the development of non-hermetic packaging, high volume production and larger die, adhesives have become the predominant attachment medium. These materials are still evolving to meet the requirements for faster manufacturing systems and more severe component requirements.
The current main die attach materials are listed below:
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:
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Limitations:
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Typical adhesive die materials:
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Soldering die attach
Soldering is mainly used on high power devices because of its good thermal/electrical conductivity and ability to absorb stresses due to expansion mismatch.
Benefits:
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Limitations:
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Typical solder die attach materials
| 63Pb-37Sn
95Pb-5Sn Pb-In-Ag 80Au-20Sn 65Sn-25Ag-10Sb |
(183°C)
*
(310°C) (310-314°C) (280°C) (233°C) |
* Indicates melting temperature
Eutectic bonding
A eutectic bond is formed by heating two (or more) materials (e.g. Au and Si) in a joint such that they diffuse together to form an alloy composition (e.g. a 97Au-3Si eutectic) that melts at a lower temperature than the base materials (e.g. a 97Au-3Si eutectic melts at 363°C). The eutectic bond can be produced by heating the die then scrubbing it against a gold foil/metallisation or by introducing a eutectic foil (e.g. Au-Si) into the joint.
Benefits:
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Limitations:
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Typical eutectic die attach materials
| Au97-Si3
Au88-Sn12 Au80-Sn20 Pb63-35Sn-1.8Sb |
(363°C)
* - most common
(350°C) (280°C) (230°C) |
* Eutectic melt temperature
Glass die attach
The glass is introduced to the joint as a paste/frit, then heated (to 350-450°C, for instance) until it softens to form a low viscosity liquid that will wet the die and substrate. Silver particles can be added to the glass (~80%Ag) to enhance the thermal and electrical conductivity of the material.
Benefits:
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Limitations:
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Typical glass die attach materials
Lead borate based glass - 80%Ag
Important current issues
The main challenges for the die attachment process are:
- Faster, cheaper manufacturing techniques (e.g. placement, cure)
- Higher power (hotter) devices
- Cleaner processes
- Environmentally friendly materials/processes
- High ambient temperature/harsh environment service requirements
- Dimensionally stable materials
- High accuracy miniature placement systems
- Heat/mechanically sensitive assemblies
- Larger device sizes.
Benefits
Current die attach techniques, particularly those based on adhesives, offer a rapid, low cost, technically sound method of attaching standard electronic devices.
Risks and special care requirements
The risks and special care requirements associated with die attach are based on making the correct choices of materials and processes for a specific application. As with most applications, the choice is often a compromise between conflicting requirements, e.g. device performance, cost, service life requirements. Die attach cannot be considered in isolation as its performance is influenced by subsequent processing/packaging/service conditions and it can affect other processes/materials (e.g. wire bond site contamination).
Storage and handling of materials is also an important factor in obtaining high quality joints.
You can also use the Weldasearch literature database to supplement what you find in JoinIT.
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