Next-generation Material Sets
BY NIGEL HACKETT AND DAN LOSKOT
n the 1960s, device manufacturers began to investigate the possibility of encapsulating their products with some of the new plastic materials available, rather than using the traditional glass, ceramic, or metal packaging techniques. The motivation behind this decision was a direct result of the drive to implement cost-effective manufacturing processes that were compatible with mass production techniques. Today, the same motivation is once again driving change in the industry.
Lead-free directives such as RoHS and the push for manufacturers to become environmentally safe or “green” are calling for changes to the manufacturing process. As a highly significant part of that process, encapsulation – in its popular manifestation of transfer molding – is striving to meet the demand for new formulations. However, legislative requirements do not remove the need to reduce manufacturing costs, maintain fast product development cycles, and deliver reliable products. In general, it is becoming increasingly important for these efforts to align in the form of compatible material sets.
Step-by-step Look at Molding
The process of modern molding is used to encapsulate components in a plastic material to protect ICs or passive devices. Transfer molding is perhaps the most widely used molding process in the semiconductor industry, a trend which is related to the technique’s ability to mold small and complex components effectively. Mold compounds can be used to encapsulate a range of electronic packages, including capacitors, transistors, central processing units, and memory devices. In basic terms, the process can be considered in two stages. First, the components to be encapsulated are transferred into mold cavities. Following this stage, a mold compound, having been liquefied by either heat or pressure, is forced into the cavity where it solidifies into a plastic encapsulated device (Figure 1).
 Figure 1. Close-up view of an encapsulated chip. |
In basic terms, mold compounds are composite materials comprising organic resins, such as epoxy resin, which can be melted. Given the natural adhesive properties of epoxy resins, compounds also require a mold release agent to enable the extraction of the component from the mold. Also featured in modern mold compounds are phenolic hardeners, silicas, pigments, and catalysts to accelerate cure reactions.
When selecting a molding compound, factors to be considered include glass transition temperature, moisture absorption rate, strength, co-efficiency of thermal expansion, thermal conductivity, and adhesive ability. The correct selection of a mold compound has numerous manufacturing benefits, and can prevent problems such as package stress and delamination.
Given that mold compounds are specifically formulated to meet a range of defined performance requirements, a change in formulation properties will inevitably affect the whole manufacturing process.
Lead-free Developments
As the name suggests, recent directives such as RoHS have legislated for the removal of hazardous substances from manufacturing processes. The most significant implication of this directive regards the use of lead which, from 2006, will be eliminated from all solders used in electronic equipment. The most immediate consequence for electronics manufacturers is that all tin/lead replacement solder alloys under consideration have significantly higher melting temperatures. This means that board soldering temperatures have the potential to reach between 245° and 260°C, with certain devices, such as stacked package configurations, requiring over 300°C solder resistance. Naturally, higher temperatures mean that package stress is increased, raising the possibility that package failures could occur unless precise alterations are made to current processes.
In terms of molding, failure has been identified as occurring at the back end of the process when the encapsulant is required to absorb a small amount of moisture to reach equilibrium with the surrounding environment. If the package is exposed to excessively high temperatures and undergoes thermal shock, any moisture will turn to steam and generate a level of internal pressure that cannot be contained. The difference in pressure will result in either the formation of a thin, hard coating which will peel or in the reduction in the adhesive strength – either of which will result in delamination.
The transition to lead-free requires that mold compounds are formulated to withstand the increasing temperatures required by lead-free solder alloys. A traditional reaction to this within the industry has been to increase the adhesive properties of the compounds, creating hardier encapsulants to reduce gross package cracking. However, less visible, internal damage caused by temperature shocks may still occur. These alternative compounds have been found to contain significant moldability compromises. At lower temperatures, the techniques have failed when exposed to forces that are too high for a rigid material. A different and more comprehensive approach has been to use flexible epoxy resins at higher temperatures. As the devices heat up, the components are then able to expand at different rates, meaning that they do not get pulled apart. In these processes, the epoxy resin in the mold compound is able to absorb the shock of the higher temperatures, thus eliminating the potential for delamination.
‘Green’ Mold Compounds
Industrial and legislative environmental concerns have culminated in a list of materials used in the electronics industry gaining banned status. “Green” mold compounds are materials that do not include bromine (Br) or antimony (Sb), both of which have been identified as being environmentally hazardous. Some halogen materials are likely to produce dioxin and other hazardous gases during the incineration process. The additives contained in mold compounds to prevent burning have historically been brominated or chlorinated substances in combination with antimony based materials. With these new halogen-based elements banned, manufacturers have been forced to find an alternative flame retardant.
Different approaches have been employed across the industry to find a replacement. In recent years, mold compound suppliers responded to these environmental concerns by using inorganic red phosphorus as an alternative flame retardant ingredient. However, this replacement has been associated with high-profile failures and has drawn concern throughout the industry. Although the phosphorus initially appeared to function efficiently as a flame retardant, it was discovered that it would fail when exposed to moist conditions and broke down, and has since been ruled out as an alternative flame retardant by most manufacturers.
Most recently, a patented flame retardant system has been developed, incorporating transition metal oxides. This new system meets all environmental requirements and is resistant to cracking when processed at higher temperatures. The system fulfils a multitude of other process demands, possessing high molding capability, and is a cost-effective option.
The Future: Material Sets
The reduction in package size and imposition of environmental concerns that necessitate process change does not remove the need to continually reduce manufacturing costs and maintain fast development cycles that deliver consistent product reliability and compatibility. More simply, the need to be environmentally friendly does not mean that processes can be any less competitive. It is for this reason that comprehensive material sets are becoming increasingly important to modern manufacturers.
As mold compounds react to current industrial changes, so do other elements of the overall manufacturing process. It is the responsibility of the mold compound supplier to ensure that the compound is compatible with other materials. With lower temperatures, mold components could withstand diverse production requirements. However, the compounds are now being pushed to their limits and it is no longer enough to think of the molding process in isolation. This need calls for a more comprehensive, “A-Z” approach in which all elements are considered.
There are two key areas in which molding compounds need to be optimized for interaction. First, die attach materials that tie the device to the cavity and provide the thermal/electrical conductivity between the device and package must be selected carefully. As such, without a compatible die attach, problems associated with expansion, cracking, and adhesion can occur. Equally, depending on the package design, there could potentially be a reaction with the underfill protecting the device if it is not compatible with the mold compound, causing similar problems.
The current situation will only be exacerbated in the future, as packages continue to diminish in size. For example, thinner sections of molding compound and layers of die attach will be required to generate the same high performance levels. As components get smaller, the increase in heat will require efficient thermal management techniques that are compatible with new processes. As a consequence, it is now necessary to ensure that the materials used in the package are compatible, even before it reaches the molding stage.
 Figure 2. New material sets coming to market. |
Suppliers that can deliver on every element of the manufacturing process are now more in demand than ever. Packaging companies are turning to suppliers who can provide the technical expertise, facilities, and time to qualify to be able to test and guarantee compatible material sets for use in next-generation packages (Figure 2). Most recently, major improvements have been seen where different materials (such as die attach and mold compounds) are co-developed – ensuring compatibility is designed into the materials from the outset. Increasingly, it is becoming less effective for manufacturers to conduct the evaluation and testing processes internally, as comprehensive materials suppliers eliminate the need for multiple vendor and product evaluations.
Conclusion
Legislative and manufacturing changes have not altered molding’s status as a highly useful and effective encapsulant technique. Indeed, as more and more stress is placed on components by the manufacturing process, molding has never been more necessary. However, precise formulation changes are required to ensure that compounds are able to cope with changing conditions. Ten years ago, material sets were not necessary. It was acceptable for materials companies to specialize in one core technology. However, it is now difficult to consider any part of the assembly process – including molding – in isolation. As the industry transitions to more challenging technologies, it also seems to be moving towards material sets, delivered by suppliers that are able to optimize product development cycles cost effectively.
NIGEL HACKETT, global director of business development, and DAN LOSKOT, global product manager of semiconductor liquids, may be contacted through Douglass Dixon, marketing manager, at Henkel Corp., 15350 Barranca Parkway, Irvine, CA 92618; 949/789-2517; e-mail: [email protected].