Applications for Bonding Technologies

9.1 Wire Bonding

Wire bonding is the manufacturing process for creating metallurgical bond connections between a thin wire (12.5 – 50 µm for gold fine-wires and 150 – 500 µm for aluminum thick-wires) and a suitably coated circuit carrier through friction welding. In principle this process consists of pressure welding with the aid of ultrasound. The metallurgical bond is mainly caused by frictional heat created through the relative movement between the two bonding partner materials. To achieve high reliability over longer time and under difficult environmental conditions high quality requirements regarding material and mechanical strength properties must be met by the surfaces of the bonding partners.

Multiple silicon chips are combined to a functional unit on a circuit carrier (for example PCB board, DCB substrate, thick film ceramic) which is mostly encased in a hybrid housing for environmental protection. The metallic conductors mounted inside the housing then serve as connections to the outside. If higher current carrying capacity is needed, as for example in power electronics, Al thick-wire bonding is employed (Fig. 9.1).

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Fig. 9.1: Bond connection: Al thick-wire on clad AlSi; (a) Macro photograph, b) micro structure with ruptured wire.

9.2 AlSi Clad Strip for Bond Connections

Besides electroplated or chemically deposited gold coatings AlSi clad semi- finished materials are used for layered systems on circuit carriers and in hybrid housings (or for lead frames). These strip materials are manufactured by cold roll cladding of an AlSi1 alloy material onto Cu or Cu alloy strip (see chapter 3.2.1).

To achieve a strong metallurgical bond between these two components a suitable surface preparation of the carrier strip and a high degree of deformation are required, followed by diffusion annealing of the clad strip.

Depending on the requirements of the assembly technology for the final product, i.e. using connectors or soldering attachment, the clad strips are coated in the terminal area with a suitable surface layer with either an electroplated hard gold (AuCo0.3) layer or selectively by electroplating or hot dip tinning with pure tin or a tin alloy. To ensure the best bond properties during the coating process the AlSi surface is protected against corrosion by masking during this processing step.

• Examples of AlSi clad strips for bond connections

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• Materials

Inlay cladding	AISi1
Substrate materials	Cu, CuFe2P, CuSn6 others

• Dimensions (typical values)

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B : max. 150 mm d: 0.2 - 1.5 mm s : 10 - 70 μm b : max. 60 mm

• Quality Criteria and Tolerances

Mechanical strength and dimensional tolerances for AlSi clad strips mostly follow the standards EN 1652, EN 1654, and EN 1758 for Cu and Cu alloys. Depending on the end application the following parameters for the coating layer are tested for and documented during manufacturing:

• Layer thickness *Adhesion strength
• Bond properties *Solderability

9.3 Electroless Metal Deposition on Printed Circuit Boards

Printed circuit boards are also used as circuit carriers. Since many of the conductive structures are not electrically connected on the circuit board electroless processes are applied to create the final surface coatings (see chapter 7.1.2). Besides bonding to Nickel/Gold surfaces soldering is the most widely used process to create conductive connections. This can also be performed on electroless deposited tin surface coatings.

Fig. 9.2: Electroless deposition on a printed circuit board

References

Kaspar, F.: Drahtbonden zur Kontaktierung auf elektronischen Baugruppen. VDE-Fachbericht 55 (1999) 97-103

Freudenberger, R.; Ganz, J.; Kaspar, F.; Marka, E.: REDOR reduktives Goldbad. Metalloberfläche 49 (1995) H.11, 859-862

Falk, J.; Lutz, K.; Berchthold, L.; Ritz, K.: Oberflächen zum Drahtbonden. DVS-Bericht 141: Verbindungstechnik in der Elektronik. (1992) 178

Sheaffer, M.: Drahtbonden für neue Bauelementeumhüllungen. DVS-Bericht 141: Verbindungstechnik in der Elektronik. (1992) 48 DVS-Merkblatt 2810: Drahtbonden, Düsseldorf: DVS-Verlag 1992

Ganz, J.; Kaspar, F.: AlSi-plattierte Bänder für Bondverbindungen mit hoher Zuverlässigkeit in der Gehäusetechnik. PLUS 11 (2005) 2057 -2058