Substitution of metallic housings

Electronic assemblies are often operated under harsh environmental conditions in industrial technology or automotive engineering and must therefore be protected from environmental influences. Thermoset encapsulation of electronic assemblies offers an alternative to metallic housings.

The environmental influences to which the assemblies are exposed include extreme ambient temperatures and aggressive media. The ingress of media due to ageing or corrosion of the housing can lead to the failure of electronic assemblies. Hermetic encapsulation can prevent the effects of media and the associated damage.

Although metal enclosures enable hermetic encapsulation, this is associated with high costs. Plastic enclosures are a cost-effective alternative, but also require a precise assessment of reliability aspects in relation to a given application.

State-of-the-art thermosets are used to encapsulate electronic assemblies. As a rule, moulding compounds based on epoxy resins are used in the production of standardized chip housings, which are processed by transfer moulding. As the packaging service providers are mainly based in Asia, the availability of the transfer molding process in Europe is limited. This applies in particular to applications with small and medium quantities.

In the area of second-level packaging, thermoset encapsulation can also be used to encase complete assemblies. In addition to the encapsulation of individual chips, transfer molding also represents a solution for second level packaging. The process is well established in electronic packaging, but involves the use of high-purity and therefore expensive molding compounds and is not widely used in Europe.

In addition to transfer molding, injection molding of thermosets can also potentially offer a solution for second level packaging. Injection molding is a widespread process for processing thermoplastics in Europe. The use of injection molding in thermoset processing is also on the rise again.

Injection molding of thermosets is less established in electronics than transfer molding, but more cost-effective materials are available for thermoset injection molding, so testing the suitability of these materials for second level packaging is of interest. A number of challenges have to be overcome when encapsulating complete assemblies.

On the one hand, the structure of complete assemblies is more complex than that of housings for individual chips and can hardly be standardized. Reliability under the relevant environmental conditions must therefore be ensured individually for each application. On the other hand, the peripheral contacting of media-tight encapsulated assemblies also poses a challenge.

Second level packaging through thermoset injection molding

The suitability of thermoset injection molding for board-level packaging was validated at Hahn-Schickard using a demonstrator(Fig. 1) [1]. Various component types (including CR, CC, MLF, TSSOP, BGA and ElKo) were mounted on a PCB and encapsulated using thermoset injection molding. Encapsulated and non-encapsulated assemblies were subjected to temperature shock tests at temperature levels of +150 °C and -40 °C with a holding time of 15 min each [2]. Figure 2 shows an example of the result of the end-of-life tests for two component types (CR1206 and MLF28).

Fig. 2: Behavior of different component types in the temperature shock test, a) ceramic resistors of type CR1206, b) chip package of type MLF28, E: encapsulated, O: not encapsulated [2]

The probability of failure for encapsulated and non-encapsulated assemblies was determined for both component types. It is noticeable that the thermoset encapsulation leads to an increase in the service life of ceramic resistors compared to non-encapsulated assemblies, but to a reduction in the service life of MLF housings. This result proves that the component types used in a specific application must be considered individually. In addition to experimental reliability tests, simulation methods can also be used to validate the service life [3].

Functionalization of packages

Fig. 3: Demonstrator for seal overmoulding of connectors [5]As a rule, encapsulation with thermosets is only used to realize the housing function. However, it is also possible to use the surface of packages for functionalization and thus increase the integration density [4]. For this purpose, a process chain can be used in which the moulding process is followed by the steps of laser structuring, activation, rinsing and electroless chemical metallization. Due to the roughening, laser structuring enables selective metallization without the thermosets used having to contain special additives. Commercial palladium-based systems are available for activation. To demonstrate the potential of this technology, Hahn-Schickard, in collaboration with project partner FAPS, built a demonstrator equipped with conductive tracks on the thermoset housing and vias for contacting the embedded chip(Fig. 3) [4]. The same technology can also be used to implement other functions such as antennas.

Overmolding connectors for peripheral contacting

Reliable solutions are required for the electrical contacting of systems with external sources. One possible approach is the overmolding of connectors. The suitability of this approach was evaluated at Hahn-Schickard using a demonstrator in which a printed circuit board was fitted with contacting elements and encapsulated by thermoset injection molding in a single process step(Fig. 4) [5].

The encapsulated components were subjected to a temperature shock test and stored in a humidity-heat cabinet. The holding temperatures in the temperature shock test were -40 °C and +150 °C, the change time was 10 s and the holding time per half cycle was 15 min. A total of 1000 thermal shock cycles were carried out. All components passed the subsequent electrical function test. It was thus demonstrated that the combination of injection molding technology with electronic encapsulation using thermosetting molding compounds enables the complete encapsulation of printed circuit boards and connectors in a single process step [5].

Conclusions & Acknowledgements

Fig. 4: Demonstrator for seal encapsulation of connectors [5]The basic suitability of thermoset injection molding for encapsulating complete electronic assemblies (second level packaging) has been demonstrated. For specific applications, the effects of encapsulation on service life must be considered in relation to the component types used. Simulation methods can make a contribution to reliability assurance.

The surface of packages can be functionalized by applying conductive tracks or antennas. Through-plating to the underlying package is possible.

The use of thermoset injection molding enables the encapsulation of printed circuit boards and the overmolding of connectors in a single process step.

The IGF projects 18253 N, 19758 N and 20288 N of the research association Hahn-Schickard-Gesellschaft für angewandte Forschung e. V. were funded via the AiF as part of the program for the promotion of joint industrial research and development (IGF) by the Federal Ministry of Economics and Technology on the basis of a resolution of the German Bundestag.

References:

[1] R. Kulkarni; P. Wappler; M. Soltani; M. Haybat; T. Günther; T. Grözinger; A. Zimmermann: An Assessment of Thermoset Injection Molding for Thin-Walled Conformal Encapsulation of Board-Level Electronic Packages, Journal of Manufacturing and Materials Processing, 3, 1, 18 (2019)
[2] R. Kulkarni; M. Soltani; P. Wappler; T. Günther; K.-P. Fritz; T. Grözinger; A. Zimmermann: Reliability Study of Electronic Components on Board-Level Packages Encapsulated by Thermoset Injection Molding, Journal of Manufacturing and Materials Processing, 4, 1, 26 (2020)
[3] R. Kulkarni; M. Soltani; S. Krafft; T. Groezinger; A. Zimmermann: Coupled simulations for lifetime prediction of board level packages encapsulated by thermoset injection molding based on the Coffin-Manson relation, Microelectronics Reliability, 114, 113813 (2020)
[4] S. Petillon; F. Häußler; S. Weser; M. Haybat; W. Eberhardt; J. Franke; A. Zimmermann: 3D Conductive Tracks on Thermoset Packages for Advanced System Integration, International Congress Molded Interconnect Devices 14, 570-594 (2021)
[5] Hahn-Schickard-Gesellschaft für angewandte Forschung e. V., Integration of electrical contacts and connectors in the thermoset encapsulation housing as a 1-shot process (DuroConnect), IGF project 20288 N

Hahn-Schickard Society for Applied Research e. V.

Allmandring 9b, 70569 Stuttgart

Romit Kulkarni, Phone +49 711 685-84780, This email address is being protected from spambots. You need JavaScript enabled to view it. (Second Level Packaging)
Simon Petillon, Phone +49 711 685-83772, This email address is being protected from spambots. You need JavaScript enabled to view it. (Functionalization of packages)
Mehmet Haybat, tel. +49 711 685-84809, This email address is being protected from spambots. You need JavaScript enabled to view it. (seal overmolding)