Aqueous Washable Thermal Resistant Coatings and Adhesives

Aqueous solutions are mixtures where the solvent is water. Non-aqueous mixtures are comprised of organic media that is reactive or simply a solvent. Aqueous washable coatings refer to products that are dissolved and removed by pure water (e.g. deionized water, DIW) or a media that primarily comprises water (e.g. detergent). Thermal resistance is a relative term, varying between markets. In automotive, testing on electronic components are carried out at 150°C [2], while semiconductor-packaging processes require resistance above soldering temperatures. For purposes of this publication, thermal resistance is defined as a minimum of 200°C. Washable coatings may be cast from various media, both aqueous and organic, yet their removal is conducted in aqueous liquids. Temporary washable coatings are applied, processed, and washed away without harm to the substrate.

These products are able to perform planarizing, bonding, sawing, laser, and plasma processing.

A washable coating is applied to sustain a process and removed later (e.g. temporary adhesive, Fig. 1). Examples of water washable and water resistant products are shown in tables 1 & 2.

Thermal resistance in polymers improves with the presence of aromatic rings (phenyl groups) within its chemical structure. Phenyl groups are more stable due to their inherent lower reactivity, higher energy of oxidation, and aromaticity, a phenomenon based upon electron delocalization with varying resonation states (Fig. 2).

Unlike permanent compounds that are cross-linked, washable products are non-reactive within the parameters of their intended use. With some exceptions, cross-linking is discouraged when the polymer must be washed away.

Heat resistant phenyl groups are common in impermeable, dense, and inert properties of well-known polyimide (PI) and benzocyclobutene (BCB). The same properties are found in DaeCoat™ products, but unlike PI and BCB, they are ideal for temporary applications as they are removable by dissolving or emulsifying in water. The occurrence of ionizable functional units mixed with the phenyl rings along the molecular chain cause a shift towards aqueous solubility. What is formed between the products is a continuum from water resistance to high solubility. Molecular ionized groups provide high water dispersion and appear transparent. Without ionization, organic moieties are preserved and dispersed by emulsifiers, appearing translucent (Fig. 3).

The washable products exist in film form (~50 μm). For uniformity, polyester (PET) release layers bind the adhesive film that is peeled away prior to bonding. Washable bonding films include DaeCoat™ 532 and 537 (Fig. 4).

Applications of washable coatings include temporary protection and bonding processes. The products may be spray coated over irregular features, spin-coated for uniformity, or use as a film form as bonding agents with porous carriers (Tables 3 & 4).

Successful coating applications depend upon an intimate knowledge of the substrate. Affixing materials on mirror finish or highly textured (large topography) substrates require coupling mechanisms that include optimization of chemical-physical phenomena. Condensation of phenyl containing molecules, as described in this paper (Figs. 2&3), may require a primer with smooth surfaces, while spray coating conforms to textures and irregular features. These applications have been shown to improve adhesion.

Spray coating is a simple and cost effective way to mask high topography features during EMI-shielding processes. This approach can be used to mask low profile LGAs and high standoff BGAs [3]. In one process (Fig. 5), the features are coated with the water washable mask (Fig. 6), transferred to carriers for sputter coating, and washed to remove the mask. In some cases, water resistant masks are used in EMI processes that involve saw dicing. These sacrificial coatings help reduce chipping due to saw vibration (Fig. 7).

A simple water washable product for laser processing is DaeCoat™ 525. This product has >300 °C resistance and has been tuned with ingredients to provide smooth coatings, excellent laser interaction, and water washing upon contact, product is applied, cured, and laser processed. During rinsing, the heat activation zone (HAZ) is rinsed with water to produce clean surfaces (Fig. 8).

Washable coatings aid plasma etch processes due to their substrate compatibility and simplification. DaeCoat™ 534 exhibits properties of high laser interaction, surface patterning, and plasma resistance for dry etch processing. For a given etch chemistry and conditions, performance is consistent or better than common plasma resistant photoresists. Processed coatings exhibit a textured appearance with minimal loss (Si etch ≥90 μm, selectivity ~1000:1) while maintaining high solubility for subsequent water washing in common process tooling (Fig. 9).

A summary of the washing conditions from Tables 3&4 is provided (Fig. 10). DaeCoat™ and DaeClean™ products exhibit good substrate compatibility when used at suggested processing conditions. Water resistant materials require special cleaners (detergents and emulsifiers) diluted into water with best results using a 2-tank counter-current flow. Surface tension reduction is achieved by maintaining concentration of the cleaning fluid at ≥10% (Fig. 11). Rinsing follows complete dissolving & removal with final substrate cleanliness defined by the purity of the DI water supply in use. Substrates that are sensitive to water or the drying of water may use a post wash in isopropanol (IPA) to scavenge residual moisture.

Data and observations are presented on the use of washable coatings for A) temporary bonding with carrier technologies, B) thermal resistance, and C) planarizing coatings to affix items or temporarily protect high topography.

For subsequent analytical testing, quartz substrates as are chosen and prepared at Daetec along with 100 mm (4–8″) silicon wafers (1-0-0, ~525 μm) re-manufactured from Wollemi Technical, Inc. (Taiwan, www.wollemi.com.tw). Materials used include commercially available spin-coated adhesives and other developmental products produced at Daetec. UV-cure applications are conducted with free-radical resins available from BASF.

Coatings are produced on a Brewer Science, Inc. CB-100 spin-coater, while thick coatings with slit tooling and spray washing uses custom tooling designed at Daetec. Metrology data is generated by a XP-1 stylus profiler, AFP-200 atomic force profiler, and a Xi-100 optical profiler. Where applicable, equipment settings include a 5 mg stylus load, minimum 4 mm distance, and speed of 0.5 mm/sec. Modified thermogravimetric test methodology for outgas supported by laboratory electronic gauges (+/− O.1mg). UV cure equipment is Intelli-Ray 400 microprocessor system (Uvitron International, www.uvitron.com).

Washable coatings for adhesive applications are commonly used with porous carriers. Such carriers provide benefits for both bonding and debonding. Carrier construction may be from stainless, ceramic, glass, or carbon fiber (CF), in all cases, there is a perforated base with topside hydrophobic porous spreader. High porosity allows a pathway for the removal liquid to interact with the adhesive (Fig. 12).

Films of DaeCoat™ 532 were adhered between ceramic LTCC substrates and glass porous carriers using best conditions of 180°C and l–5psi for ≤10min (Fig. 13), at confirmed substrate temperatures using a hot plate or oven with attention to thermal conductivity (Fig. 14).

While thermalgravimetric analysis (TGA) data for DaeCoat™ 615 and 655 suggests minimum thermal resistance at >200°C (Fig. 15), the resistance of all other washable coatings approach 300°C (Tables 1 & 2). Improved resistance is possible with conditions of inert atmosphere (N₂) and limited duration. Furthermore, thermal resistance is time dependent as most data reported here is for periods of 30min or more. Dwell times of a few minutes at high temperatures >250°C (solder bonding) are accepted by most or all of DaeCoat™ products.

Planarizing over large topographies is achieved by using a 100% solids system and curing by UV, DaeCoat™ 555 or 655. Common slit-coating equipment as used in panel processing is used to apply thick coatings with little or no waste. Once material is dispensed and spread, the substrate continues on a conveyer through a knife-controlled orifice to create a uniform thickness. Thickness is maintained by material rheology until the panel enters a UV cabinet where it is flood exposed and cured in seconds to a rigid and hard matrix. The planarizing coating is designed to hold items in place during slicing, grinding, lithography, or thermal processing. Demonstration of the planarizing coating on glass is presented (Fig. 16).

This paper introduces thermal resistant washable coatings and adhesives demonstrated for temporary bonding with or without carriers, their details of thermal resistance, and unique materials that produce thick coatings for planarization. These materials are built upon phenyl containing molecules that exhibit a range of water resistance to allow customers to use them for a wide range of processing while being removed by simple washing practices.

Water resistant coatings and adhesives exhibit phobic properties which are compatible with common manufacturing processes (i.e. water, acid plating baths, acid etchants), yet are able to be removed in water mixtures (DaeClean™ detergents and emulsifiers). These emulsifiers exhibit high hydrophile and lipophile balance values (HLB>10). High HLB molecules exhibit a long chain lipophile that makes contact and interacts with water resistant materials, while the hydrophile maintains water solubility. Phobic substance removal efficiency is aided by high HLB nonionic emulsifiers to allow these species to be rinsed away with water (Fig. 17).

Daetec thanks the staff at DPSS Lasers [4] and SPTS Technologies [5] for their support in making this work possible.