Specialty Chemicals, Adhesives & Plastics
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Our broad selection of die attach paste formulations provide the reliability and performance today’s leadframe and high-density laminate packages demand. Each package type – from BGAs to LGAs to Smart Cards – has different requirements, which is why we offer a suite of products that cater to the unique needs of those devices. Die attach pastes offer a low modulus for stress reduction and warpage elimination, as well as Bismaleimide (BMI) formulations for low moisture absorption to avoid package cracking during high temperature processing. Most of our semiconductor grade materials will pass the NASA outgassing standards if cured properly.
Process flexibility and superior performance underscore Henkel’s complete portfolio of die attach pastes for leadframe and laminate devices. Incorporated into applications such as automotive electronics, where temperature control and unfailing function are critical, LOCTITE ABLESTIK die attach pastes deliver high thermal conductivity and high reliability. Robust adhesion to various metal surfaces including palladium, copper, silver, gold and PPF along with proven low-bleed formulas make our die attach materials the products of choice for semiconductor packaging specialists.
Guidelines on Die Size
Small = 2mm and below
Med = 2mm x 2mm to 5mm x 5mm
Large = >5mm x 5mm
The original die attach polymers were thermoset polymers and are still the most widely used today. After polymerization, these adhesives exhibit a rigid three-dimensional structure, a consequence of chemical bonds between adjacent chains (cross-linking). When heat is applied, these interlinked chains cannot move freely, hence their phase changes are not reversible.
The amount of interlinking between chains depends on the chemistry 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. Thermosets are commonly used in circuit board and semiconductor/device packaging for low to medium power applications. A disadvantage of thermosets is that sometimes, precise repeatability of the polymerization step can be difficult.
Thermoplastics were formulated to overcome the negative qualities of thermoset polymers. After polymerization, they are linear in structure. 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 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 also means additional equipment is required compared to thermosets. They are instrumental in circuit board device and packaging.
Many die-attach materials are kept at -40C in order to keep the chemistry from advancing at high(er) temperatures. Even so, these materials (no matter whether they are cyanate esters or epoxies) do need to see higher temperatures (often > 150C) before the cure is ever initiated. So though the material may "age", the cure will not progress if it is kept at lower temperatures.
The first and most dramatic effect is the viscosity. Especially important in high volume production, if the viscosity changes over time, then the equipment settings may need to be continuously altered in order the continually dispense the same amount of material part after part. Furthermore, the CoA often states that the viscosity will remain within a specified range - so the shelf-life is implemented to protect the manufacturer beyond a known period that they have tested.
This said, the biggest risk is that it will not dispense as expected. If it does dispense fine, and you are able to dispense the material in such a way that you can mount a die, then (though - and I can't stress this enough - we do NOT guarantee it), you can use it with a high level of confidence that the material will act as expected once cured.
Silver migration and dendritic silver formation underneath components between the two terminals is a known phenomenon in microelectronics. It can occur when metal structures (tracks, terminals, …) are subjected to a wet environment, with current passing through them and some acids to speed up the dissolution of the metals (or even quicker, their oxides which may have formed on the surface) in question.
In practice, however, this is something that does not happen too much in high reliability applications as most critical electronics are almost always shielded or protected by some form of encapsulation which eliminates most of the conditions mentioned before. In cheaper or unprotected setups though, this is something to keep in mind. Please also note that this is not only happening with silver adhesive, but actually can happen with all types of metal (Cu tracks, Sn terminations on components, etc.) including Ag containing solder. Even gold substrates in the right circumstances will be subject to this form of electrochemical migration and dendrite formation.
We do not have data on extensive storage at lower or greater temperatures than the ones indicated in the Technical data sheet. This is at the customer`s own risk. Therefore the properties can just be guaranteed for the particular temperature and time range indicated on the TDS. Those are the ones tested to be the best for the product.
Generally speaking – Gold is “more difficult” to bond than Silver. But nearly all our die attach style products are compatible with both Gold and Silver
If stored below the recommended temperature, the product could crystallize, especially epoxies. Low temperatures could cause the formation of “Freeze-Thaw Voids” in the syringes. Any void formation can affect the dispensing characteristics of the adhesive, causing non-uniform dispensed volume.
Storage above the recommended temperature will decrease the shelf life but most likely will not change the chemistry or “destroy” the product (which could happen at lower temp). The same applies when you want to put a syringe back into the freezer, after using it. It is not recommended at all but the worst case scenario is to increase viscosity, decrease shelf life and maybe acquire some moisture causing voids.
It also depends on how long you plan to store it in an alternate temperature. But, to get the most out of the product we suggest to get a suitable freezer and to use the products as instructed in the TDS.
In free-radically cured adhesives such as acrylates (Bismaleimides as well but not to the same extent), oxygen is known to inhibit polymerisation. In the bulk of the material, once the finite level of oxygen is depleted, the polymerisation process will initiate. This is more valid for certain UV and anaerobic cure materials, but typically not applicable for epoxy based materials (including semi-sintering) with “embedded hardener”.
At the adhesive fillet's surface, oxygen is continuously replenished by oxygen molecules in the atmosphere. The end result can lead to variations in cure and cured properties between the center of the die and the outer perimeter resulting in a tacky surface. Under Nitrogen, adhesives formulated with the secondary thermal cure initiator canbe thermally cured without prior exposure to light.
The whole idea with these mixers is both to maintain homogeneity of the die attach material and also to avoid air bubbles after a freeze-thaw process. Mixers such as THINKY and UNIX can help in streamlining the production process.
The air bubbles are caused by the thawing of the syringes and in extreme cases can actually pull in air from behind the plunger into the syringe. Air is generally not a disaster unless it is concentrated in "air bubbles" which would cause "voids" and "spitting" during a die attach dispense.
The "vacuum option" is designed to also remove the air and prevent voids. In our experience, simply using a planetary mix without the vacuum option is plenty sufficient. The small air bubbles when mixed back in a homogenous paste are not seen again during die attach dispense and the vacuum option is an expensive and unnecessary luxury.
Conductive die attach pastes find the most significant application, about 80% of the die attach market, in attaching chips to leadframes where their electrical conductivity is an important parameter. These pastes are made of organic resins mixed with inorganic fillers. The resins could be thermosetting such as epoxy, phenolics, cyanate esters, and silicone or thermoplastics such as polyimides, and polyurethanes. Metal fillers such as silver, copper, gold, nickel, palladium have been used. The resin is responsible for the mechanical properties of the adhesive, and the fillers are responsible for its electrical and thermal conductivity. Conductive die attach pastes form electrical connections between leadframes, I/O leads, active and passive devices, and other circuit components. They also act as channels that facilitate electrostatic dissipation and electrical grounding.
Based on their electrical conductivity, conductive die attach pastes can be classified as:
A consequence of chips and devices becoming smaller is that they are susceptible to high temperatures experienced during electrical transmission. Without sufficient heat dissipation from a die attach material to appropriate heat sinks in a circuit, destruction and failure of parts become inevitable. Thermal conductivity is, therefore, a critical parameter affecting performance. For a paste to qualify as electrically conductive, its minimum thermal conductivity as specified by Method 5011 of the MILD-STD-883 standard is 1.5 W/m·K.
Numerous conductive die attach pastes are available in the market, each with its unique differentiating property. The essential properties to consider that have far-reaching effects on the performance of any given product are: filler composition, morphology, manufacturing, and processing requirements.
The majority of polymer resin used in formulating conductive die attach are insulators by nature, so fillers are incorporated to increase their electrical and thermal conductivity. Fillers that have been used commercially are silver (429 W/m·K), copper (401 W/m·K), gold (318 W/m·K), and nickel (90-92 W/m·K). Their thermal conductivity values indicate why silver is so popular.
Understandably, silver-filled die attach pastes are expensive. However, silver exhibits other superior qualities including the fact that silver oxide is stable and conductive. On the other hand, copper-filled variants experience a decrease in conductivity with oxide growth after exposure to high temperatures. Nickel has a low oxidation rate, and given its lower thermal conductivity value, is a preferred filler in low-cost anisotropic adhesives.
Even though the thermal conductivity values of the filler metals are substantial, those of the formulated pastes are considerably lower. A value such as 1.9 W/m·K is not unusual for commercially available silver-filled pastes, while others as high as 7-11 W/m·K are also available.
The filler percentage of a composite mixture is important because of the underlying trade-offs. A large amount of metal is necessary for greater conduction, but this affects physical properties like elasticity and mechanical strength.
According to percolation theory, a critical filler concentration exists where the 3-D network structure is established, and the increase in conductivity becomes significant by several orders of magnitude. Beyond this concentration, conductivity does not increase as noticeably with an increase in filler composition. Therefore, only a minimum quantity of filler is required to achieve a degree of electrical performance that is inherent to the composite. Typical values for ICAs lie between 25-30 vol % and 5-10 vol % for ACAs.
The characteristics of the filler material such as size, shape, orientation, and distribution in the mixture affect the viscosity/rheology of the formulation. These properties come into play in the bond line thickness. Since thin bond lines enhance thermal conductivity, fine particle sizes are preferable and it’s why metal powders with diameter sizes between 5-20 µm are typical.
More than connecting materials together, adhesives act as load distributors. Assembled packages having different CTEs need a low modulus adhesive to compensate for the mismatch. Increasing the filler loading for the sake of high conductivity increases the tensile modulus of a die attach paste but decreases the ultimate strength. This could result in cracking during thermal cycling or vibration. Manufacturers use plasticizers to lower the modulus and add flexibility.
Parameters like temperature, cure time, pressure, storage conditions, and shelf life, impact not only the strength and reliability of a die bond, but also design and manufacturing decisions.
In addition to the resin and filler, other components like hardeners, thinners and curing agents are added to modify cure rates, viscosity, and other mixtures properties. The die attach paste can be supplied as a single component where the catalyst is latent (usually by storing below -40°C ) until exposed to the given cure temperature. Supplied as two components, you get a separate resin, called part A, and the catalyst, part B. Either or both part A and B may contain the metal filler. The parts are weighed, mixed, degassed and cured. Cost-cutting objectives favor formulations that are solvent-free and have shorter cure cycles.
Curing involves the polymerization of the resins to solidify them into strong adhesive joints. Curing agents and sufficient heat (oven, UV) are often required for the reaction to occur and the type of catalyst will influence the cure time and temperature.
Typical pastes cure anywhere between 80°C to 180°C and some two-component die attach can cure at room temperature. Snap cure (often one-component) pastes derived from modified cyanate esters can cure in under 1-2 minutes making them an excellent choice for high volume manufacturing.
One-component formulations often need to be stored in a freezer, and this reduces the ease of using it. If stored incorrectly, adhesives can polymerize prematurely, absorb moisture, or crystallize. Also, if left too long before dispensing at room temperature, an adhesive mixture can separate leading to die bond inconsistencies where some parts are resin-rich and will flow excessively, while other parts are filler-rich and don’t flow easily. Die attach formulations based on bismaleimide resins were developed to counteract the tendency to absorb moisture.
Conductive (and non-conductive) die attach pastes may be syringe dispensed or stencil printed depending on the viscosity of the mixture.
Except for the difference in their composition and application, non-conductive die attach pastes are not far off from their conductive cousins. They are made of organic resins like epoxy, polyimides, silicones, acrylates and non-conductive fillers that could be metal, metal oxides, metal nitrides and more. Examples of ceramic and inorganic fillers are silica (SiO2), alumina (Al2O3), and beryllium oxide (BeO). An organic filler like PTFE has also been used successfully.
These pastes find the most use in die attach for semiconductors and surface mount devices, where a strong adhesion that can withstand physical and thermal stresses combined with strong electrical insulation is the priority. Example applications are automotives, sensors, consumer electronics, memory cards, RFID cards, USBs and LEDs.
There are applications where pastes with low thermal and electrical conductivity are sufficient. There are other situations, however, where pastes need high thermal conductivity because temperature control is critical. This second category are thermally conductive and electrically insulating die attach pastes. Unfilled polymer resins are natural thermal insulators (0.1–0.5 W/m·K), and the added fillers increase thermal conductivity tenfold or better. Method 5011 of MIL-STD-883 standard specifies the minimum thermal conductivity for electrically insulative die attach as 0.15 W/m·K.
The thermal conductivity of silver-filled die attach pastes makes it excellent for heat dissipation, but in electronic applications where a high thermal conductivity must be combined with low electrical conductivity, ceramic fillers such as alumina and silica are good alternatives. Examples are packaging materials for power semiconductor devices where the heat generated needs to be dissipated to ensure its long life and reliability.
Thermal dissipation through the die attach occurs mainly through conduction and convection. Therefore, the morphology of the filler materials, their shapes, sizes and their distribution in the resin influence the process. Smooth ceramic powders less than 20 µm in diameter are typical.
The thermal conductivity of diamond is impressive, as high as 1500-2000 W/m·K depending on its purity, and that of diamond-filled pastes can exceed 12 W/m·K. However, the price of diamond is prohibitive for everyday applications. Fillers that have found commercial use are alumina (99%, 40 W/m·K), aluminum nitride (170-260 W/m·K), boron nitride (130-260 W/m·K), and silicon carbide (270 W/m·K).
Boron nitride at a loading level of 40 wt% can produce a mixture with a bulk thermal conductivity of 8-10 W/m·K, although the thixotropic end result can be difficult to dispense. On the other hand, alumina is low cost and a popular choice due to its more manageable flow properties. When mixed with organic resin, the resulting paste has a thermal conductivity of 1.5 W/m·K at a loading level of 75 wt%.
Therefore, although the thermal conductivity of a paste may be increased by adding more filler as far as the density of the resin will allow (typically, 85%–90% by weight and 40%–50% by volume), there is a point where doing so is no longer beneficial, negatively impacting other properties of the paste, e.g., viscosity. To counter this, thixotropic additives are introduced to improve flow properties. You will find that many commercially available die attach pastes are formulated using similar core resins, fillers, and catalysts but differ in the quantities of these ingredients.
Another factor affecting the thermal conductivity of a paste other than type and percentage of filler is the curing requirement of the paste.
The resin part of the paste is responsible for providing shear and tensile strength to the adhesion throughout all the possible conditions an assembly will encounter. Apart from thermal conductivity and flow properties, the type of filler used influences the CTE of the paste which is critical when bonding dissimilar materials together. Unfortunately, while these fillers may improve the CTE, their addition negatively impacts the bond strength. It becomes a trade-off decision about what the percentage of filler loading should be for an application.
Non-conductive pastes may be based on thermoset or thermoplastic resins. The paste can be supplied as a single component with a latent catalyst/ hardener or a two-component mixture, requiring expert weighing, mixing and degassing of the resulting composite.
Die attach pastes (conductive and non-conductive) can be deposited into any shape through syringe dispensing or stencil printing, making them ideal, where the components to be bonded, have interesting geometries. They can also be scaled for mass manufacturing and automation using high-end equipment that is optimized for all the critical parameters to avoid common defects such as voiding and bond thickness variation.
A dimensionless ratio that measures the ability of a material to store electrical energy when exposed to an electric field. The dielectric constant for non-conductive die attach pastes used for electrical insulation will typically lie between 2-5. A higher value in the range 6-12 is acceptable where some electrical conductivity can be overlooked.
It measures the power loss in a material experiencing an alternating electric field, a ratio of the power dissipated to the power applied. A low value implies the die attach paste, the substrates to which it adheres, and the surrounding circuit components will experience only low levels of heating. At ambient temperatures, typical DF values lie between 0.003 to 0.030 at 1 KHz, and can go up to 0.050 at 1 MHz.
It measures the maximum voltage that, when applied across a material, won’t cause dielectric breakdown. Beyond this point (breakdown voltage), a non-conductive die attach paste becomes electrically conductive. Also, dielectric strength decreases as temperature or frequency increases.