The adhesion strength between materials can directly impact the performance and durability of the interface. In the context of semiconductor packages that are composed of different layers with different material properties, ensuring that the bonded surfaces are kept together all throughout the manufacturing process, assembly, and operation is crucial for maintaining the integrity of the package, ensuring the device’s long-term reliability in challenging operating conditions. However, delamination, sometimes referred to as deadhesion, may occur wherein the encapsulating material becomes separated from an adjacent material at their interface.
This separation can occur at various critical points within the package, including the interfaces between the encapsulant and the semiconductor die (Type I), between the encapsulant and the leadframe (or substrate) (Type III), and between the encapsulant and the die paddle (Type IV). Among these delamination types, the loss of adhesion between the molding compound and the substrate is the most common mode of failure encountered during the processing and qualification of semiconductor packages.
Preventing Type III delamination has become significantly important over the recent years because of the more stringent reliability standards imposed to meet the requirements of the booming automotive industry.
Strategies to improve the adhesion strength of epoxy molding compounds on substrates
Improving the adhesion strength of epoxy molding compounds on substrates, which include leadframes, laminates, and even wafers, can be done using the recent advancements in materials engineering, manufacturing, and quality control and assurance. From a materials perspective, addressing Type III delaminations involves two primary strategies: (1) improving the epoxy molding compound formulation and (2) enhancing the substrate design. In this article, we will delve into these strategies, exploring the latest innovations and techniques that have emerged in epoxy molding compound formulation and substrate design to combat Type III delaminations.
Improving the EMC formulation
- Increasing the filler concentration to decrease the coefficient of thermal expansion (CTE)
Coefficient of thermal expansion (CTE) is a property which measures the changes in the dimensions of a material in response to changes in the temperature. The CTEs of the most commonly used substrates in semiconductor manufacturing range from 17 ppm/ °C (Cu) to as low as 2 ppm/ °C (Si). We would want the CTE of the EMC to be as low as possible to avoid high degrees of mismatch between those of the substrate.
When materials with significantly different CTE values are heated, they expand at different rates. This induces thermal stress at the interface of the materials, which increases the risk of thermal-stress induced delamination between the EMC and the substrate. To produce EMCs with low CTEs, epoxy resins with low CTEs can be used. Nevertheless, the most common strategy is to add fillers, such as silica and alumina, to reduce the overall CTE of the formulation.
- Modifying the EMC formulation to decrease its moisture sensitivity and absorption
Aside from thermal stress caused by CTE mismatch, volatility stress related to the moisture absorption properties of EMCs also affect their adhesion to substrates. This is particularly significant for semiconductor devices that need to undergo reflow at 260 °C. During reflow soldering, entrapped moisture within the semiconductor package evaporates, exerting stress and forming cracks within the package.
As crack propagation occurs, delamination between the package components may occur, causing the deadhesion between the EMC and the substrate. This failure mechanism is more commonly known as the “popcorn effect” and is considered to be one of the most common failure modes in semiconductor packages.
To avoid such from happening, EMCs may be formulated using epoxy resins that have less affinity to water, including but not limited to bi-phenyl, multi-aromatic, and dicyclopentadiene (DCPD) resins. Matching these epoxy resins to appropriate hardeners can help improve the hydrophobicity of the EMC. To illustrate this point, we can compare the moisture absorption of Hysol GR600-P1 with that of Hysol GR600-SL2. Although both molding compounds exhibits low moisture absorption, GR600-P1 (0.20%) absorbs lesser moisture than GR600-SL2. This is expected as GR600-P1 contains a multi-aromatic resin, whereas GR600-SL2 is based on a phenolic novolac resin. The aromatic compounds in GR600-P1 can contribute to hydrophobicity, reducing moisture absorption.
Here are some water absorption data for some of epoxy molding compounds available at CAPLINQ, along with their epoxy and hardener chemistry.
| Epoxy Molding Compound | Water Absorption | Epoxy/ Hardener Chemistry |
| Linqsol EMC-7560 | 0.4 | Multi-functional (MFN) resin + hydrophobic hardener |
| Linqsol EMC-9070 | 0.33 | Biphenyl (BP) resin + hydrophobic hardener |
| Linqsol EMC-7535 | 0.25 | Multi-aromatic resin (MAR) |
| Hysol GR720 | 0.4 | Epoxy O-cresol Novolac (EOCN) resin |
| Hysol GR60 | 0.34 | EOCN resin + phenalkamine (PN) |
| Hysol GR730HT | 0.24 | – |
| Hysol GR30 | 0.31 | EOCN + PN |
| Hysol GR300 | 0.33 | EOCN + PN |
| Hysol GR510 | 0.21 | MAR + EOCN resin + MAR hardener |
| Hysol GR510-HP | 0.24 | MAR + EOCN resin + MAR hardener |
| Hysol GR640HV | 0.52 | EOCN resin |
| Hysol GR640HV-L1 | 0.42 | EOCN resin |
| Hysol GR750 | 0.35 | MFN resin |
| Hysol GR700 C3D | 0.28 | MAR + low molecular weight (LMW) resin + MAR hardener |
| Hysol GR910-C | 0.3 | MAR + BP resin + MAR + MFN hardener |
| Hysol GR9810-1PF | 0.4 | BP + MFN resin |
| Hysol KL-G100S | 0.4 | – |
- Adding adhesion-promoting additives
Epoxy molding compounds are composite materials composed of organic and inorganic components. The interface between the organic resin and the inorganic fillers are often not load bearing and show poor mechanical properties. To address this issue and make sure that there is sufficient adhesion in the formulation, coupling agents, such as silanes, titanates, aluminum chelates, and zircoaluminates, are added.
Coupling agents work by creating chemical bonds between the inorganic and organic components, and they work even in harsh conditions. These additives have a special structure consisting of one part that can react with the “inclusion surface” or the inorganic fillers and another part that bonds with the polymer resin. In simpler terms, the structure of coupling agents can be simplified as A–B–C, Anchor–Bridge/Buffer–Couplant.
For example, silane coupling agents have a structure R–Si–(OR’)3. R reactive groups can form chemical bonds with organic resins, and each (OR’) group binds with the inorganic fillers. Aside from coupling the components in EMC formulations, coupling agents are also known to improve the hydrophobicity of the compound, reducing its moisture absorption that possibly improves the adhesion strength of the encapsulant to substrates. Therefore, the improvement in adhesion between the fillers and the organic matrix extends to the adhesion of the EMC to the device components, including the lead frame or substrate, thus decreasing the likelihood of delamination failure.
These are some of the most common EMC chemistry modification strategies employed to improve the adhesion strength of the plastic encapsulation on substrates. While these do improve adhesion and reduce the risk of Type III delamination, some property trade-offs exist. For example, EMCs with low moisture absorption and good adhesion strength often have medium to low glass transition temperatures. Also, for optically clear epoxy molding compounds, improving the adhesion strength may affect its anti-blue ray decay performance. As in all other things in life, striking the right balance between these properties is essential.
Substrate Preparation and Activation
- Surface preparation
As defined earlier, adhesion occurs either through mechanical interlocking or molecular and chemical interactions between the two surfaces. In large part, mechanical adhesion depends on the area available for bonding. That is to say that surfaces with higher surface areas would yield more sites at which mechanical interlocking may occur. Materials may also bond chemically, which requires the surfaces to be clean and contamination-free and to have high surface energies for easier bonding. Therefore, improving the adhesion between the EMC and the substrate is a two-way street. It does not only depend on having the right EMC formulation but also on having good substrate (surface) preparation.
Prior to molding, metal lead frames can be cleaned through degreasing and sanding (abrasion). Degreasing is performed to remove loosely attached dirt and contaminants on the surface often using volatile solvents, such as acetone, toluene, alcohols, and some ketones. In this technique, the surface is wiped or rinsed repeatedly using the solvent and then dried to remove residual volatiles. Meanwhile, abrasion is done to remove heavy contaminants in the surface, which may include dirt and oxide layers. Aside from that, abrasion roughens the surface and therefore, increases the surface area available for bonding.
Chemical treatments can also be performed not only to improve the chemical properties (i.e., surface energy) of the surface but also physical properties. A well-known technique is chemical etching, where the substrate is immersed in an etchant. The etching solution will vary on the type of substrate. Appropriate cleaning procedures are done after etching to ensure that the surface is free from residual chemicals. In cases where degreasing, abrasion, and/or chemical treatments are not enough, physical methods are also performed. Similar to chemical methods, physical techniques are performed to modify surface chemistry. There are a lot of physical methods, including flame treatment, corona discharge, and plasma treatment.
For chemical and physical activation, the effective time period of sufficient adhesion highly depends on the substrate and the treatment employed. Determining this “window for bonding” is important to make sure that the treatment does not lose its effectiveness.
- Use of adhesion promoters
Similar to coupling agents added in the EMC formulations, adhesion promoters may also be added on the substrate to facilitate its bonding with the EMC. Adhesion promoters work in a similar fashion to coupling agents. One “tail” (end of the structure) forms a bond with the substrate, and the other tail links to the EMC. Different adhesion promoters have different base chemistries but all of them work on a molecular level. For example, Aculon’s primers have self-assembled monolayers of phosphonates (SAMPs) that can improve the adhesion of metals to polymers. For EMCs, hydroxyl functional adhesion promoters can be used, especially if the EMC is based on phenolic epoxy or if it requires a thermal cure above 60–70 °C. Hydroxyl primers also work for iso-cyanates and melamine formaldehydes. Aculon’s adhesion promoters have exhibited superior performance in improving adhesion of die attach adhesives and EMCs to different lead frames, including Au, Pd, Cu, Ag, and Ni.
Thiol-, acrylate-, and hydroxyl-based adhesion promoters work best with different types of materials.
| Thiols | Acrylates | Hydroxyls | |
| Adhesion Examples | Epoxy (Meth)acrylics | (Meth)acrylates, amines, thiol based | Polyurethanes and iso-cyanates, melamine formaldehydes, phenolic epoxy based |
| Application | Spray / Dip / Wipe | Spray / Dip / Wipe | Spray / Dip / Wipe |
| Visible to Naked Eye | No | No | No |
| Thickness (nm) | 2+ | 2+ | 2+ |
| Dry Time (sec) | < 20 | < 30 | < 30 |
| Cure Required | For <5 min cure, only Ti, Al and epoxy need a cure | For <5 min cure, only Ti, Al and epoxy need a cure | For <5 min cure, only Ti, Al and epoxy need a cure |
Aside from that, adhesion promoters are easy to apply. Once the intended part or surface is sufficiently clean, adhesion promoters or primers can be applied through dipping, wiping, or spraying. For each application method, post-application curing is recommended to achieve the best performance.
Strong adhesion is the cornerstone of semiconductor packaging reliability. CAPLINQ offers epoxy molding compounds with proven excellent adhesion to substrates, and we also offer effective adhesion promoters. Reach out to us to explore our comprehensive solutions for your semiconductor packaging needs.
