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I want to make my circuit board very small. I have seen many smaller boards with complex circuit. But I am not sure how do they do it? Is it all Custom circuit implemented on VHDL or some micro controller core printed on circuit board like the Black portion on the image below. I am new to PCB designing. Any suggestion would be appreciated.enter image description here

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  • \$\begingroup\$ If you need to ask what CoB is then you're too new to use CoB. Stick to BGA and/or WLCSP for now. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 6 '15 at 11:43
  • \$\begingroup\$ I have already printed two boards at fab house in china, all i need is reduced PCB and reduced cost as well. I am required to drop down the PCB size as low as possible. \$\endgroup\$ – Arjun Sep 6 '15 at 11:45
  • \$\begingroup\$ CoB requires specialized controlled-atmosphere attachment equipment; BGA and WLCSP use normal reflow. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 6 '15 at 11:47
  • \$\begingroup\$ Can I print AVR microcontroller like this in china at FAB house? or do I need to have my custom processor? \$\endgroup\$ – Arjun Sep 6 '15 at 11:51
  • \$\begingroup\$ You'll need to talk to your Atmel sales rep in order to find out which are available as a bare die. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 6 '15 at 11:56
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If you are using an AVR, you might be able to significantly reduce your footprint by using a Wafer Level Chip Scale package (WLCSP).

enter image description here

These can end up using even less board space than a Chip-on-board since you don't need room for the bonding wires. Instead, you fanout the connections from directly under the package, similar to a BGA.

enter image description here

The parts are much easier to work with than COB and you can order them from normal distributors like Digikey...

http://www.digikey.com/product-search/en?keywords=attiny%20wlcsp

You can read more about Atmel's WLCSP in this app note...

http://www.atmel.com/Images/doc42007.pdf

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