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Die Attach Materials

Die attach materials can broadly be broken down three ways by package type, application type and conductivity type. Package type will determine if you need an organic die attach material such as epoxy, or silicone or whether you require an inorganic die attach material such as silver glass, or solder. The application type will determine if you require a die attach paste, film or wafer back coating and conductivity type will determine whether you require an electrically conductive or non-electrically conductive die attach adhesive.

CAPLINQ offers industry-leading Die Attach Materials...

CAPLINQ is proud to offer this complete range of die attach materials as seen in the subcategories below. Please continue to our Learn More section to learn more about which die attach materials are available, the differences in their types and application methods, and how CAPLINQ can help you to select and order the die attach adhesive that is right for your application.

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Frequently Asked Questions

Frequently Asked Questions about Die Attach Materials & Adhesives

Why is Tg important when selecting the right die attach paste?

The glass transition temperature for organic adhesives is a temperature region where the polymers change from “glassy” to soft and rubbery. Increasing the temperature further continues the softening process as the viscosity drops too. Temperatures between the glass transition temperature and below the decomposition point of the adhesive are the best region for bonding. This window can span up to 100 degrees.

If Tg of the die attach is magnitudes lower than the wire bonding temperature, the die/chip could shift in the process, resulting in a weak bond. For thermoplastic adhesives, a too low Tg could result in the softening of the adhesive to the point that the die separates from the substrate at high enough temperatures.

Following these guidelines, we can avoid weak bonds and bleed out which typically happens at higher than necessary viscosities, caused by high temperatures.

Why is CTE important for die attach?

Using die attach pastes to bond substrates with different coefficients of thermal expansion (CTE) is a great challenge. Examples of difficult to bond combinations are ceramic to plastic, glass to plastic, and rubber to metal.

Here's what happens, temperature changes cause the different materials to expand according to their CTE leading to shear stresses and deformations after curing, or throughout the life of the device. The geometry of the design adds to the challenge.

The first point of approach is to select a die attach paste whose CTE is comparable to the CTE of both the die and substrate. Also, the die attach selected should contribute additional flexibility to the bond to compensate for the differences in material expansion. Thermoplastic adhesives are advantageous in this regard because of their characteristic low elastic modulus. They do not become brittle, unlike thermosetting adhesives.


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Conductive Die Attach Paste

Anyone involved with assembling microelectronics knows the significant role the die attach material plays. Whether it's connecting a semiconductor chip to a lead frame substrate, or connecting a semiconductor device to the designed circuit board, proper adhesion is critical.

There are numerous die attach materials available commercially with proprietary chemical recipes to meet the requirements of different applications or address one die attach challenge or the other.

What does it mean when you want to make a selection and are faced with product inventory sheets a mile long of die attach variants? To help, we'll go through the important die attach properties to aid your selection.

Very broadly, die attach material can be classified into polymeric adhesives, soft solders, metal-filled glass, and gold-silver eutectic alloys. This section will focus exclusively on conductive die attach pastes.

Conductive die attach pastes are adhesives that are typically composed of liquid resins responsible for its mechanical properties and fillers responsible for its electrical and thermal conductivity.

Conductive organic die attach pastes (thermoplastic and thermosets) are made of resins such as epoxy and silicone and fillers such as silver and gold. Silver sintering pastes are highly conductive (150~250W/mK) and are made up of powdered silver (nano/micron-sized) mixed with an organic resin.

Desirable properties of conductive die attach pastes

You'll need to compare your material properties, budget, manufacturing, and other constraints to the properties and ease of using the die attach paste options that most closely qualify.

Properties that depend on the chemical and physical properties of the die attach:

  • High thermal conductivity to dissipate heat generated in the device to heat sinks and other thermal management
  • High electrical conductivity to efficiently manage, distribute electrical energy
  • Good stability at the operating temperature
  • Low modulus to eliminate warpage and reduce stress
  • Free from ionic contamination/good corrosion resistance
  • Rapid bonding with good adhesion
  • No outgas interference
  • Reliability when exposed to mechanical vibration and shock

Properties relating to the ease of using the die attach for product manufacturing:

  • Curing cycle - decide between in-line curing (snap) or off-line batch cure(oven, UV). Quick curing is more profitable for high volume manufacturing.
  • Curing temperature that won’t affect parts.
  • Storage conditions and pot life - will the adhesive need to be stored frozen? How long will it have after mixing/reaction to use it?
  • A small difference in coefficient of thermal expansion between die, die attach, and substrate.
  • Reworkability that allows decoupling a die from it substrate which is useful for testing purposes when dealing with expensive parts.
  • Compatibility with different-sized constructions, e.g., large dies vs. thin, intricate geometries.
  • Die application, e.g., stacking.
  • Site preparation, e.g., cleaning and wetting, affect throughput.

Challenges associated with die attach paste applications that should guide your selection

  • Void formation - causes poor adhesion, poor heat transfer, and die tilting for large die sizes. It also interferes with electrical performance
  • Bond line control - inconsistent bond thickness could lead to cracks and fractures as a result of different rates of thermal expansion and contraction of the die and adhesive. It can also affect the quality of wire bonding in semiconductor packages
  • Bleed out - When adhesive seeps out of the bond, it can contaminate the die surface.

A chemical formulation might satisfy one or more of these parameters. CAPLINQ can help you decide on the best die attach paste for your unique application, contact us today or learn more below.

Difference between die attach film and die attach paste

Die attach pastes can be deposited into any shape making them ideal where the components to be bonded come in many geometries. They are suitable for high mass manufacturing but require additional equipment and efficient process control to avoid common defects such as voiding and bond thickness variation. There's also the curing process to consider.

Die attach film, on the other hand, is easier to control in comparison. The supplier (pre-cut) or user (sheets or rolls) cuts the film into the desired shape to fit the die. Automation is straightforward as the cut pieces of film are picked and positioned. Die films are the go-to method for die stacking.

Difference between thermoplastic and thermoset die attach

Thermosets

The original conductive die attach polymers were thermoset polymers and are still widely used today. After polymerization, these adhesives exhibit a rigid three-dimensional structure, a consequence of chemical bonds between adjacent chains. When heat is applied, these interlinked chains cannot move freely, hence are not characterized by reversible phase change qualities.

The amount of interlinking between chains varies among different chemistries of thermoset polymers. High cross-linking means a hard, rigid and often brittle material, while low cross-linking means the material is more pliable and can be softened (they do not lose their original shape) by heating to high temperatures. They are commonly used in circuit board and semiconductor/device packaging for low to medium power applications.

A disadvantage of thermosets is that the end user is responsible for the polymerization step which makes precise repeatability difficult due to many hard to control factors.

Thermoplastics

These were formulated to overcome the negative qualities of thermoset polymers. After polymerization, they are linear in structure. In fact, the manufacturer polymerizes the adhesive fully under controlled temperatures, reaction rates, times, and pressures. This ensures repeatable die attach properties.

Their linear structure allows them to be re-melted because their long polymer chains simply slide past one another on heating. Cooling the adhesive returns it to the solid state. As a die attach, a thermoplastic need not be fluid for effective bonding, the viscosity that allows the plastic to flow into the microstructure when pressure is applied should serve as a guide. The need for pressure means additional equipment is required compared to thermosets. They are also instrumental in circuit board device and packaging.

Difference between polymeric resin with silver filler and sintered silver pastes

An advanced type of conductive die attach paste is the sintered silver die attach. Unlike thermoset and thermoplastic polymers which fail at temperatures near 200 °C, sintered silver adhesives are suitable for high-temperature applications above 300 °C.

Sintered silver pastes consist of nano and micro-sized silver particles. Some formulations combine the silver powder with organic components that function as thinners and binders which burn out at high temperatures. Bonding between the die and substrate is not a result of phase transition of the paste but atomic diffusion assisted by temperature. The bonding occurs below the melting point of the connecting materials and the silver too.

Sintered silver pastes with nano-sized silver are a new invention, and research is ongoing in this area to refine the bonding process without the need for applied pressure.

Dispensing the die attach paste

The rheological properties of a paste are a function of its metal content, the viscosity and surface tension of the carrier resin, the shape and size of the particles, and other physical properties.

Die attach pastes can be dispensed using a capillary tool by printing or writing. Dispensing is scalable to fast operating cycles with automation and multiple orifices.

Printing/shower dispensing

This is the choice method for small-sized dies where a shot of material is dispensed through the needle/orifice. For large dies, the encoded pattern is dispensed completely in one shot.

Writing

This is a slower process that caters to large-sized dies and offers flexibility plus high throughput rates with the use of dedicated dispensing tools. Here, a needle dispenses continuous lines on a moving substrate to create the desired pattern.

Selecting the best conductive die attach paste for your unique application

Numerous die attach formulations are available, up to the thousands, and they differ widely in properties. Let us know the unique requirements of your product or process, and we can help pinpoint the best adhesives for your needs. CAPLINQ offers a variety of conductive die attach pastes for the following applications:

  • Laminate and lead frame packaging,
  • Semiconductors: QFN/QFP, SOP/DIP, LGA, BGA,
  • LED packages, and
  • Discrete electronic components: capacitors, inductors, transistors