Lead frames and moulding for electronic packages
Stamping, selective electroplating, joining and injection moulding are key contributors to the integration of lead frames for smart cards and other miniature semiconductor packaging applications.
Electronic packaging for semiconductor and other microsystems demand utmost precision and reliability.
Post-moulding assembly of housing shells with lead frames is one accepted method of assembly, but for premium performance, overmoulding of metallic inserts is a must.
Overmoulding, whereby inserts are placed into tightly matched sections of a mould cavity can provide support for long lead frame conductor path lengths, and provide compact dimensions for high strength and tight feedthroughs.
The classic way to package components is to first attach semiconductors on top of a metallic lead frame and to overmould a thermosetting resin.
But rigid packages are not useful for many sensor or optical chip functions.
In such situations premoulded packages are used; these are lead frames with a thermoplastic semi-shell casing.
Chip protection is achieved by potting with a mechanically soft (eg silicon gel) or optical quality resin.
Reel-to-reel moulding of delicate package structures has proven its suitability for mass production: Tyco Electronics has already produced more than one billion such smart card substrates.
The design of a specific package incorporating semiconductors is of interest in many applications where additional functions can be integrated into one module, as an alternative to assembling a standard package with additional separate components.
In such "mechatronic" packages the customer deals only with a single module.
One example of such a package is an automotive housing which incorporates the bare chip driving the exciter current of a generator, together with passive components (resistors and capacitors) and sliding brushes.
The lead wires of the resistors and capacitors are resistance welded to the lead frame which has been stamped, plated and pre-moulded.
This subassembly is finally overmoulded with thermoplastic using a short-temperature-rise process.
The completed mechatronic assembly is then mounted in a generator housing.
Insert-moulded housings are of special interest when wire bonding is the chosen interconnection method.
The advantages of ceramic substrate technology to create micro-hybrid circuits are miniaturisation and robustness.
Selective electroplating of stamped lead frames with high purity gold is a key technology to allow use of both gold thin wire or aluminium wire loops (corresponding with ball-wedge or wedge-wedge operation).
Surface cleanliness has to be maintained throughout the complete process chain - and this is a challenge for the moulding as well as the connected process steps.
Besides cleanliness, resistance to resonance phenomena is vital for the bondability of lead frame tips.
This is ensured by highly compressed, nearly gap-free embedding of the metal pad into the polymer material.
For extremely flat applications, an adhesive joint between a conductor grid and a protective dielectric film offers advantages overmoulded 3D designs.
Hermeticity can be achieved with a small overlap of metal and polymer.
Smart cards are the largest and most challenging application for this lamination technology.
Thermal activation of adhesive in a reel-to-reel lamination yields high productivity and attractive cost, and the matching of both metallic and polymer contours without an accumulative shift can be achieved with special production knowhow.
The conductor grids used in the examples above are manufactured by precision stamping, a process of shear-cutting of contours, which can be supplemented by bending or local coining.
Stamping typically involves the use of progressive die tools, which incorporate multiple stamping and punching stations to progressively form the grids from the base metal strip material.
The resulting comb structures are either separated or continuously connected with a remaining carrier strip.
Typical strip material thickness for high speed tools is from 0.07mm to greater than 1mm.
Base materials are most commonly copper alloys, occasionally steel or aluminium alloys.
The design of injection moulding tools for insert moulding depends on the manoeuvrability of the insert components.
In optimal situations the conductive grid can be presented to the tool as a plane structure and be transported attached to the carrier strip and guided in a strip path through the tool.
A feeding system automates the sequence.
Additional work steps may be incorporated via a carrier strip loop before or after the injection mould step.
Typical additional work step examples are bending of a previously flat part in a press with a bending tool before transfer into a mould or disconnecting metallic connecting paths using a cutting tool, after mechanical integrity is achieved by moulding.
One product example is a stamped, electroplated and bent strip that is reel-to-reel moulded.
Practical limits to further integration are offset by the added risk of down time.
Insertion parts of three dimensional nature are handled as separate parts and require considerable effort to properly position them in the mould.
The time required to equip up to several cavities becomes a considerable part of the production cycle.
Down time is often minimised by having double or even triple versions of the lower mould, for use on a moulding press with rotating table.
In this approach, moulding, positioning of the inserts and eventually gathering up the freshly moulded parts takes place simultaneously.
Besides manual operations, there is the option of robotic handling, mostly applied for high volume runs in high labour cost areas.
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