Bare Copper Pad Reflow Soldering

I received a request from customer to assemble ceramic substrate (alumina oxide)that printed with a low temperature Copper thick film conductor (the overglaze layer is polymer). The Copper pads are exposed without any coating. This ceramic substrate went through double sided reflow in N2 environment(around 100units of SMT components on each side), followed by the third time hot-bar reflow for lead-frame pins assembly at the 2-edges of the ceramic. The Proto-run result showed that the exposed Copper pads were partially oxidized after 2nd time reflow. Hence, the 3rd time reflow couldn't form acceptable joints. I'm thinking of using OSP technology to prevent/minimize the oxidation from happening especially during 1st & 2nd reflow. Would appreciate any suggestion/comment to prevent/minimize oxidation on bare copper pads.

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| I received a request from customer to assemble ceramic substrate (alumina oxide)that printed with a low temperature Copper thick film conductor (the overglaze layer is polymer). The Copper pads are exposed without any coating. | This ceramic substrate went through double sided reflow in N2 environment(around 100units of SMT components on each side), followed by the third time hot-bar reflow for lead-frame pins assembly at the 2-edges of the ceramic. | The Proto-run result showed that the exposed Copper pads were partially oxidized after 2nd time reflow. Hence, the 3rd time reflow couldn't form acceptable joints. | I'm thinking of using OSP technology to prevent/minimize the oxidation from happening especially during 1st & 2nd reflow. | Would appreciate any suggestion/comment to prevent/minimize oxidation on bare copper pads. | OSP should help, but I wonder why your hot bar process can't burn through any oxidation from the earlier reflow processes? Maybe you can screen print over these pads during the 1st reflow process and then hot bar the solder from the printer.

$0.02

-Kevin |

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| | I received a request from customer to assemble ceramic substrate (alumina oxide)that printed with a low temperature Copper thick film conductor (the overglaze layer is polymer). The Copper pads are exposed without any coating. | | This ceramic substrate went through double sided reflow in N2 environment(around 100units of SMT components on each side), followed by the third time hot-bar reflow for lead-frame pins assembly at the 2-edges of the ceramic. | | The Proto-run result showed that the exposed Copper pads were partially oxidized after 2nd time reflow. Hence, the 3rd time reflow couldn't form acceptable joints. | | I'm thinking of using OSP technology to prevent/minimize the oxidation from happening especially during 1st & 2nd reflow. | | Would appreciate any suggestion/comment to prevent/minimize oxidation on bare copper pads. | | | OSP should help, but I wonder why your hot bar process can't burn through any oxidation from the earlier reflow processes? Maybe you can screen print over these pads during the 1st reflow process and then hot bar the solder from the printer. | | $0.02 | | -Kevin | | | Never thought of screening over bare copper pads and reflowing before additional screening and reflow. Looks like tin/lead plating and reflow leaving a big hump. No matter, use an OSP but recognize every time you heat and clean, the stuff goes away leaving you the same, closer to bare copper, oxidation problems.

By the way, not trying to be a dog (it's a genetic thing or?) as Justin described (and my apologies to all the women professionals of the world (most of you know far more than me, but who doesn't - I've lead too sheltered a life and cannot control myself), the substrate you describe is aluminum oxide. Aluminum oxide is called alumina - but I know you knew that.

It's just very difficult, if not impossible to expect bare copper not to oxidize beyond wettability. Scratch a penny and watch, in front of you eyes, how easily it oxidizes.

Earl Moon

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