Honeywell

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Inkjet Printing Enables New Semiconductor Packaging Technologies
Keywords: Inkjet printing, Soldermask, QFN
The need for ever smaller and cost effective semiconductors is driven by wearable electronics, automotive and IoT applications and is not slowing down. New manufacturing approaches are needed. Additive manufacturing and printed electronics technologies like inkjet printing of functional materials have a number of unique benefits over traditional semiconductor technologies that enable further miniaturization and process cost reduction. Unlike with traditional dispense technologies, inkjet deposits a large number of very small droplets (picoliter volume) of functional liquid materials on a substrate. Thousands of individually addressable nozzles are utilized to dispense these materials in a non-contact way. The technology enables digitally patterned layers as well as homogeneous layers, even on 3D surfaces and 3D structures. Besides in advanced semiconductor packaging, these benefits are recognized in several other industries such as (flexible) printed circuit boards, photo voltaic (PV), display, 3D printing, and even pharmaceutics where inkjet technology finds its way into industrial manufacturing. The feature size of inkjet printing is not compatible with semiconductor front-end processes because sub-micron patterning is required. However, many wafer based back-end-of-line and strip based packaging process steps need features of tens of microns up to millimeters, and can benefit from inkjet. Therefore inkjet printing is being adopted by some of the largest semiconductor producers in the world. This paper will review the advantages of inkjet in comparison to traditional technologies, and highlights a few specific applications, one of which (routed QFN) is summarized below. A typical example of inkjet enabled miniaturization is routed QFN packaging. It offers higher I/O pin count wrt traditional QFN, combined with excellent thermal and electrical properties by applying a copper lead frame. Because the more complicated fan-out structure would lead to free standing lead fingers, the manufacturing process for routed QFN lead frames is a two-step procedure. First the lead fingers are etched halfway through the lead frame. After die attach, wire bonding and molding, the lead frame is etched back from bottom side yielding the final structure of free standing lead fingers (now supported by molding compound) and protruding solder contacts. The challenge however is that after back etching of the lead frame, the exposed copper is to be encapsulated without contaminating the contacts and leaving enough stand-off for reliable soldering. Encapsulating the exposed copper is a difficult task for molding, dispense or screen printing technology. The required layer thickness is in the tens of micron range, too thin for molding. For dispense and screen printing technology the feature sizes are a challenge and they easily contaminate the contact points due to overfilling by dispense or contact with a screen. Inkjet however precisely prints small droplets of dielectric material in a thin layer around the contacts and has no problems with the 3D topology of the lead frames. The ink jetted material creates a closed layer, contamination free contacts and nice fillets around the contact point for reliable soldering. Inkjet printable solder mask offers the right specifications in terms of layer thickness, resistivity and adequate reliability in terms of adhesion and solder resistance.
Wouter Brok,
Meyer Burger (Netherlands) BV
EINDHOVEN, Brabant
Netherlands


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