The True Cost of Hermeticity in Microelectronic Packaging Open Access

In both the Department of Defense (DoD) and commercial technologies, hermeticity has been traditionally used as a means of increasing the reliability and integrity of the assembly [1]. Hermeticity is achieved by creating an assembly with a gas-tight seal in order to protect the internal components from the environment.

Within a given environment, moisture is considered one of the main contributors to corrosion of semiconductors. The permeability of the package components, encapsulation materials and sealing methods must be considered due to the risk of moisture exposure. The permeability of some typical packaging materials is shown in Fig.1 [2]. Other contaminants, such as acids and greases, can also cause corrosion within microelectronic packages.

Creating a reliable, hermetically sealed package can prove difficult to achieve depending upon the design configuration and application. Alternative concepts to achieve reliability without hermeticity have been considered. This paper will review current trends in the materials and methods used in creating reliable microelectronic packages.

A basic metal package is comprised of a chassis, a substrate, a lid and a connector used to mate with the next higher assembly (NHA). Chassis materials typical of this package style include aluminum and titanium. The chassis housings are designed in such a way that they can enclose the substrate within a cavity using a sealed lid. The substrate contains all of the electrical components and routing for this microelectronic assembly. The substrate is attached to the chassis via either a mechanical hardware attachment, such as a clamping, or an adhesive bonding method. In order to achieve hermeticity, the lid is attached to the chassis typically via a laser welding process. The material of the lid chosen is dependent upon the material of the chassis itself. In order to get a good weld joint which is hermetically sealed, a compatible lid and chassis material set must be used. Additionally, proper precautions must be taken to ensure that no moisture is being trapped in the package during the laser welding process. This can be achieved via various means such using vacuum bake operations or laser welding in a controlled glove box environment. An exploded view of an example metal package is shown in Fig.2 [3].

Hermetic metal packages are producible, although obtaining an optimal hermetic seal can prove challenging. Since electrical performance typically drives the shape factor and design of the chassis cavity and lid, the package does not always end up being the ideal configuration for the laser welding process. Furthermore, the use of glass sealed interconnects can also prove troublesome for the laser weld process. Stresses in the connectors, as well as misfiring of the laser near the glass, can lead to cracks in the seal and loss of hermeticity.

If required, rework can also be challenging due to the thickness and fit of the lid within the chassis. The lid must be machined out of the assembly in such a way that the chassis is not damaged and can maintain the same fit, form and function as the original chassis design. Imperfections in the chassis after the machining process can lead to an inability to re-weld the assembly and ultimately could make the assembly unusable.

The robustness of a metal structure is one of the advantages of this style. Once manufactured to be hermetic, metal packages tend to be more durable in comparison to other packaging design styles. Having a complete metal structure surrounding the microelectronics lends itself to protecting any type of damage from the hardware. That being said, proper adhesion of the substrate and the components to the chassis is required in order to withstand vibration and temperature cycling requirements. Coefficient of thermal expansion (CTE) needs to be considered in the assembly material selection process to prevent undesired stresses within the assembly.

Electrical performance of a metal package is dependent on the design and NHA restrictions. Electrical isolation within the package can be challenging due to the difficulty of the laser welding process. Internal welds can be used along cavity walls that separate components susceptible to “cross talk”, however increasing the risk of loss of hermeticity. If the assembly allows, various materials can also be included into the assembly which act as radio frequency (RF) absorbers to improve RF isolation.

A ceramic package is comprised of a substrate, base plate, ring frame, connector and lid. The substrate being used in this style of packaging is typically either high temperature co-fired ceramic (HTCC) or low temperature co-fired ceramic (LTCC). Each ceramic type has its own respective advantages and disadvantages with respect to electrical performance and mechanical shrinkage tolerances but is not the subject of this paper. For the purposes of this study, ceramic will be considered as a general term covering all types of ceramic substrates. The base plate and ring frame are both metal components, typically selected based upon the CTE of the ceramic being used. An example of a ceramic package design is shown in Fig.3 [4].

These metal components, as well as the RF connectors, are soldered or brazed to the ceramic substrate using high temperature solder types such as AuSn or CuSil. Once the package components are soldered, and verified as being hermetic, the microelectronic components are placed into the package. Establishing hermeticity in this package style requires very clean processing in order to prevent the likelihood of voiding within the solder joints of the package.

The ring frame creates a cavity to which a lid is attached via a seam seal or laser weld operation. Depending upon the electrical requirements, the ringframe may be used to create cavities around each of the microelectronic components. As required, internal welds may be used around each cavity wall to meet electrical isolation requirements. Each additional weld increases the risk of hermeticity loss during manufacturing and alters the residual stresses of the assembly. However, if done properly, internal welds can be used to improve the structural reliability of large lid configurations by reducing the allowable deflection of the lid during changes in temperature and pressure.

Due to the fragility of ceramics, these substrate types are prone to damage and chip-outs. While there are military standard requirements which allow a certain level of defects to be present on a ceramic substrate, the dense electrical routing and components present within the substrate often lead to packages being unusable in such cases. The fragility of the material should be considered in the design of the package such that assembly is robust enough to withstand manufacturing and handling.

Rework of a ceramic package may be achieved in a similar fashion to the metal package assemblies. The de-lid process is typically more successful in a ceramic package style design due to the lid being placed on top of the ring frame instead of being dropped into a metal chassis. There is still however risk associated with damaging the assembly and internal electrical components during the rework process.

Package robustness is one of the main disadvantages to this style of packaging. Thermal cycling imparts high stress on ceramic packages and must be taken into account when selecting the material set of the assembly. Similarly, mating to the NHA can also impose stress on the package and must be considered in this type of design.

Multi-layer ceramic packages allow for a high density of components and internal routing to be used within the design. Additionally, the dielectric and loss properties of available ceramic materials are also advantageous for many design applications.

Reliability without hermeticity can be achieved by using mechanical structures and environmental coatings to protect electrical components. Often this method is used on traditional printed wiring board (PWB) designs that are required to be more reliable for a given application. The PWB may contain a mixture of surface mount and chip and wire technologies. Depending upon the NHA design and electrical requirements, ring frames or huts may be placed around components. This can provide both electrical isolation benefits and protection from mechanical damage during manufacturing and handling. For a given application, a base plate or heat sink may be also attached to the PWB via either epoxy or solder. An example of epoxy bonded microelectronic components on a PWB can be seen in Fig.4 [5].

Manufacturing of this type of assembly is typically easier compared to the other assembly types due to the matured nature of surface mount technologies. However, rework of an assembly is a major concern depending on the timing within the manufacturing process flow. Once an assembly is environmentally coated, it requires a burdensome chemical and mechanical process to remove the coating and allow for rework of the electrical components. Often the labor required for such a rework will be high and may require a trade study to determine if it is worth the cost of the rework. Improvement on chip level and pre-coating test processes will continue to reduce the need for rework, although electrical performance is not the only driver for rework processes. Visual defects and changes in requirements can also initiate the need for rework of an assembly.

The structural integrity of this assembly type is largely driven by the application and what additional hardware is being added to the assembly. How the PWB interface to the NHA, as well as the general stresses of the NHA, need to be considered when designing the assembly. Additionally, the environmental conditions and mating style of the NHA application must be considered when selecting the type of coating to be used.

Reliability in microelectronic packaging has been, and will continue to be, a major concern that must be taken into account early in the design of a package. Although reliability can be achieved both with and without hermeticity, the packaging style is going to largely be driven by electrical, environmental and cost requirements.

Although metal packages are typically more robust due to the structure of a chassis style design, they also tend to be larger in size and weight. Additionally, metal style packages are difficult to delid and can lead to scrapped packages if rework is required after the assembly is hermetically sealed.

Ceramic packages offer the benefit of increased component and routing density, but in turn are more fragile and prone to damage during manufacturing. Special care must be taken in design to ensure the package assembly is robust enough to withstand the environmental requirements of a given application.

Printed wiring boards designed to be reliable without hermeticity can be a cost effective solution for many applications but does not lend itself to be easily reworked due to the environmental coatings being used. This option may not be feasible depending upon the reliability of the electrical components being used, as well as the requirements of the next higher assembly.

Many aspects of the design must be reviewed when determining what package type should be used for a given application. Electrical requirements will often drive which package type should be used. However, the mechanical and environmental requirements may not allow for such a package design to be feasible and must be considered. The earlier in the design phase that these aspects can be reviewed and analyzed will reduce program risk by providing more time in the case that a major design change is required. Characteristics such as electrical performance, producibility, structural integrity and cost must be considered when choosing a packaging solution for a given application.

The author wishes to thank A. Bailey and S. Smalley, Northrop Grumman Mission Systems, for their kind support and for providing technical guidance used to make this publication possible.