Saturday, September 25, 2021

Underwater laser cutting and silver sintering to make ceramic circuit boards

 

Thermal stress cracking can be completely avoided by CO2 laser cutting thin alumina sheets underwater.  I also show how to formulate and apply silver paste, then sinter in a kiln to produce double-sided ceramic printed circuit boards with conductive vias.

60W CW CO2 laser at 80% power.  10mm/sec.  Standard lens focal length (50mm).  2mm water above ceramic.  180 passes to cut through 0.75mm thick alumina.  

Silver paste: 97% silver powder, 3% glass powder by mass.  Particle size 1 micron or less. Add poly vinyl alcohol mold release until desired consistency reached.

Paste applied with 4 mil thick vinyl stencil.  Dried in air 10 minutes, then rapidly brought up to 900*C, held for 10 minutes, then rapidly brought back down to room temperature.  Total cycle about 45 minutes.

I measured electrical conductivity of the finished traces from my process with vinyl stencils: 4 milliohms per square at 10 micron final thickness.  This is pretty close to the Dupont published spec ( less than 2 milliohm/sq at 16 micron thick)

Underwater CO2 laser cutting reference: https://sci-hub.se/10.1016/J.JEURCERAMSOC.2011.06.015
Raspberry Pi picoReflow oven controller: https://apollo.open-resource.org/mission:resources:picoreflow

Applied Science on Patreon: https://www.patreon.com/AppliedScience

3 comments:

  1. It might be reasonable to add a bit of gallium o the paste. Gallium wets glass and would form an alloy with silver.
    Most likely would also wet the glass particles from the glaze.
    On the other hand, if wetting is sufficient, it might turn out that glass in the paste isn't needed any longer. Plus, the resulting alloy would have way lower melting temperature.
    It's all a matter of experimentation. Some interesting reference to gallium alloys in dentistry: https://nvlpubs.nist.gov/nistpubs/Legacy/RPT/nbsreport4313.pdf

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  2. While looking at the Coefficient of Thermal Expansion (CTE) for alumina, higher temperatures have greater CTE, that seems to explain why heating it didn't work. It might be worth a try to cut the alumina sheet on dry ice. The CTE at dry ice temperatures is about half of what it is at room temperature. A possible draw back is that the thermal conductivity increases as you drop the temperature. I suspect that could be dealt with by running at a faster cutting speed.
    http://www.matweb.com/search/DataSheet.aspx?MatGUID=c8c56ad547ae4cfabad15977bfb537f1

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  3. Hey Ben! Would love to get in contact with you about an exciting opportunity. Please email me at mary@madeinnetwork.com for more details! Thanks so much.

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