BY MICHELLE VELDERRAIN, NuSil Technology LLC
Silicone adhesives are becoming more widely used in microelectronic hybrid assemblies due to their inherently low elastic modulus, which relieves stress between substrates with different coefficients of thermal expansion (CTE) during thermal cycling. They can be formulated with varied mechanical and chemical properties that influence the choice of chemicals and processes for rework. The chemical compatibility of the package substrate must also be considered, as well as any other associated hazards.
To better understand how various chemicals affect silicones, it is important to understand their general composition. Silicone adhesives are defined by the silicone polymer, which is characterized by the siloxane bond (-Si-O-Si) where the silicon atom will have a least one bond to an organic molecule commonly referred to as polyorganosiloxanes (-R2SiO-)n. The most common organic group found on the silicon atom for adhesives is methyl (CH3). Other organic groups can be reacted onto the silicon atom, giving the silicone different chemical and physical properties, such as solvent resistance and increased thermal stability. Other functional groups will be present based on the specific cure chemistry for the particular formulation. Hydrosilation (a.k.a. platinum cure or addition cure) is the most commonly used for microelectronic applications since it has no reaction by-products, minimal shrinkage, and can be heat accelerated.
By themselves, silicone polymers have weak mechanical properties when cross-linked into a cured matrix, so they are reinforced with fillers such as fumed silica and/or silicone resins, increasing the elastic strength of the cured silicone rubber. These methods of reinforcing the silicone also affect how the silicone adhesive is applied and the application.
Silicone adhesives containing silicone resins have a rheology similar to honey, and can coat a substrate by flowing around intricate design patterns. However, adding silica gives adhesive shear thinning properties – thixotropic – like ketchup; these will not flow without shear. These are used as glob tops and in other applications where the adhesive is not desired to flow. Silicones also have intrinsically high dielectric strength that can be optimized to have electrical or thermally conductive properties by adding ceramic or metallic fillers. This allows use in many applications from a dam-and-fill adhesive to coatings, to a thermal interface material (TIM) where the silicone adhesive matrix can be > 80 % filler (w/w).
Figure 1. Applications for silicone adhesives in microelectronic packaging
There are generally two methods to remove the silicone for rework, each with its own set of challenges. Soaking in solvents, such as xylene, has been used to swell and soften the silicone to allow removal by mechanical tools. This method risks damaging the substrates and assembled package, due to the intricate and compact nature of microelectronic assemblies. It also may fail to remove any remaining silicone residues on substrate surfaces. The use of silicone digesters or emulsifiers is another method gaining in popularity. They comprise weak acids or bases that cleave the siloxane bonds, reverting the cured silicone matrix into discrete polyorganosiloxane molecules. Using silicone digesters can greatly reduce the need for mechanical methods, thus decreasing the potential for damaging the part as well as leaving minimal silicone residues on the substrate.
Individual combinations of silicones, substrates, and cleaning solutions were evaluated, and a removal-rate rating system was developed to be used as a reference guide and assist in selection of the optimal silicone-removal solutions based on assembly configuration, material substrates, and silicone. The substrates and silicones chosen are commonly used in microelectronic assemblies (Figure 1). The silicones were cured between two test panels and then immersed in the selected cleaning solutions. Mechanical motion, which may influence silicone delamination from the substrate, was minimized.
Table 1. Results ranked 1 – 4 based on time (hours).
The silicones evaluated were a silica-filled adhesive (SFA), alumina-filled TIM, silver-filled die attach (DA), and a resin-filled adhesive (RFA). Material substrates evaluated were copper, aluminum, and epoxy (FR4). Cleaning solutions evaluated were two common solvents and three silicone digesters. The solvents used were isopropylalcohol (IPA) and xylene. Two commercially available silicone digesting solutions were chosen; each designed specifically to remove cured silicone by breaking the siloxane bonds and dissolving back into silicone digesting solution. These are referred to as D1 and D2, where each contains a proprietary active ingredient. D2, recommended for use below 49°C, is not compatible with certain plastics, and may react slightly with aluminum. Both these solutions are not water-soluble, but are compatible with many composites and do not contain halogenated solvents. The final silicone digesting solution, referred to as D3, is a 1% tetra-n-butylammonium fluoride trihydrate (TBAF) in Dowanol propylene glycol methyl ether acetate (PMA) solution.
Results and Discussion
The results of the study demonstrated removal effectiveness of solvent or silicone emulsifier (Table 1), effects of silicone type on removal rate, and change in substrates. As hypothesized, the solvents did not dissolve or break down the siloxane bonds. IPA did not produce delamination between the silicone and substrate within 24 hours in these test conditions, where as xylene showed slightly better performance on aluminum.
The silicone emulsifiers evaluated ranked from most to least effective in dissolving silicone within the shortest time as follows: D3 > D2 > D1 (Figure 2). D3 dissolved most silicones within 8 hours, and in some cases, within 3 hours. D2 completely dissolved most silicones within 24 hours. The silicone formulation may have a significant effect on how easily the silicone can be removed, with less influence from the substrates on removal. For example, the DA and RFA silicones were not dissolved within 24 hours in all but one test condition. D2 was the only silicone emulsifier able to dissolve RFA when adhered to copper and epoxy. On the other hand, the TIMs were dissolved within 8 hours in all but two test conditions. The TIM and DA formulations each contain ~80% (w/w) of dense fillers, and low concentrations of silicone overall. Each reacted with the silicone digesters quite differently, where the DA was difficult to dissolve on all substrates, and the TIM was digested relatively quickly; the rate varied slightly based on substrate.
The effects on substrates from the solvents and silicone emulsifiers were inconclusive based on weight loss. Final weights of only the samples, where the silicone was completely removed through delamination and/or digestion within 24 hours, were measured. Any weight loss or weight gain was less than 0.05% and considered insignificant. All the silicone digesters changed the color/appearance of the aluminum to some degree. There were also visual changes seen in the color of copper substrates exposed to D1 after 1 hour at 40°C. The cause of the appearance change and what effect it may have on substrate is unclear. Further substrate exposure experiments will be conducted to determine if there are any significant surface effects from the silicone emulsifiers. In practice, depending on the substrate, observed surface effects may or may not affect the device performance.
Aspects that influence the removal/emulsification rates of the cured silicone include removal conditions (time/temperature), solubility of silicone in cleaning solution, and adhesion.
Figure 2. Ranking comparison of Silicone Digesters
The difficulty of removing silicone from within a microelectronic device itself is much more complex since the device is composed of various substrates, the type of silicone/s is unknown, and there is minimal exposed surface area. There are techniques that can accelerate the silicone emulsification process when these challenges exist.
Sonication is commonly used to clean with minimal mechanical agitation in many applications and is recommended for silicone emulsification. Combinations of solvent and silicone emulsifier may also be used where the silicone matrix swells from absorbing solvent and allows easier access of silicone emulsifying solutions into silicone. When heat is used to accelerate the chemical breakdown of the silicone, properties such as boiling point, flash point, and heat sensitivity of active ingredients must be considered.
Solvents were not as efficient as silicone digesters in removing cured silicone. Each silicone digesting solution dissolved the silicone within 24 hours, with the exception of the DA. Commercially available silicone emulsifiers evaluated may not have dissolved the cured silicone as quickly as the 1% TBAF solution, but they demonstrated a reasonable effectiveness in removing various types of silicone and ease-of-use. Using additional methods such as sonication and/or slightly elevated temperatures can help increase the rate of dissolving cured silicone since microelectronic packages are composed of several materials and the silicone may have limited exposed surface area. A general-purpose silicone emulsifier can be used when taking into account the substrate material compatibility and the cleaning solution’s ideal performance conditions. Research is recommended to determine material compatibility, and ensure that elevated temperatures will not degrade temperature-sensitive chemicals in silicone emulsifier solution.
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MICHELLE VELDERRAIN, technical specialist, may be contacted at NuSil Technology LLC, 1050 Cindy Lane, Carpinteria, CA 93013; 805/684-8780; E-mail: firstname.lastname@example.org