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62

Chip Scale Review May • June • 2019

[ChipScaleReview.com

]

The dense aerosol is transported by a

conditioned gas to the print head where

a focusing sheath gas is introduced to

collimate and accelerate the aerosol down

onto the substrate. This collimation process

takes place by forcing the aerosol and

sheath gas through a converging nozzle that

compresses the aerosol stream. The jet of

droplets exits the print head at high velocity

(>50m/s) and impinges upon the substrate,

adhering to its surface and producing a fine

line of conductive ink. The high velocity of

the print stream enables an amazingly high

stand-off distance of up to 5mm, greatly

relaxing the fixturing requirements in the

Z-dimension. This feature also allows for

conformal printing on non-planar surfaces,

such as stacked ICs or chip-on-board

interconnects. There is a shutter mechanism

built into the print head to pause printing as

directed by the controller unit.

Continuing with the discussion above,

electrical interconnects or other circuit

patterns are then printed onto the substrate

by means of precision multi-axis motion

controllers and drive equipment in up

to five axes of motion, depending on the

application requirements. Any arbitrary

digitized form can be printed on planar or non-planar surfaces with print resolutions as

fine as 10µm. The printing takes place in normal production atmospheres—no special

heating or pressurization of the printing volume is required.

Finally, a post-printing process is applied to sinter, dry or cure the ink to the desired

electrical and mechanical properties. This usually consists of

applying heat, UV light or laser light, depending on the ink and

substrate materials.

Capabilities needed

The non-contact direct printing process needs to be capable of

printing very fine features with lines as small as 10µm at 20µm

pitch. Wider features, up to 2.5mm, should also be able to be

printed. Also, the process must be able to be applied to many

types of substrates including FR4, ceramics, ceramic/PTFE

composites, metals, glass, plastics, Si and GaAs (

Figure 5

). The

post-printing process is dependent upon the ink and substrate

material, but can take place at room temperature using in situ

UV or laser light, or may require heating.

Summary

Wire bonds will not suddenly be replaced by printed

interconnects. However, given the limitations of wire bonds in

terms of both space requirements and inherent reliability, it is not

surprising to see more production customers looking to printed

solutions. While this article focused on space requirements and

reliability concerns, the other critically important issue that needs

to be addressed is the issue of electrical performance. There have

been a number of studies comparing the RF performance of printed

interconnects to that of wire bonding. The results show no reduction

in performance in signals up through the microwave spectrum.

References

1.

MIL-STD-883F Method 2011 [htt ps://nepp.nasa.gov/ DocUploads/31ECBD46-FFA0-43AE-82C09A3B2B6FE26B/ std883.pdf]

Figure 5:

Optomec Aerosol Jet printing on a stacked die with 60µm bond pads. The figure shows 35µm

printed wires deposited via Aerosol Jet from 3+ mm off the substrate.

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