PCB design and fabrication

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PCB

• Printed circuit board
• It’s the most likely platform upon which independent microchips are combined to form an overall system
• consists of an insulator substrate (plastic/epoxy)
  • provides thermal radiation
  • provides mechanical support
• On top of the pcb there are tracks of conductive material (usually copper)
  • provides wires that can communicate and connect the components of the system together

• On top of the pcb there are footprints that indicate locations where you expect that your chips will be installed
• copper tracks communicating between these footprints
• In the next step you perform installation of these chips in their locations
  • manually if the chips are DIP or surface mount packages
  • or using equipment in the case of BGAs

PCB Design

• There is an analogy between PCB design and microchip design
• The design tells where the copper tracks are gonna be
  • The PCB itself is an insulator substrate
  • All you create on top of it is the conductive copper tracks
• You have a certain amount of real estate which is the substrate
• Witin this substrate you have to distribute a number of resources your chips and passive components
• you also need to connect the input and output pins of these microchips in a specific way
  • To communicate with each other or with jacks that come off the pcb
• you have to determine ways inwhich these constraints (connections between chips) are satisfied
• These copper tracks provide an additional capacitive load increasing delay
• so you may find closure (a solution that functionaly connects the chips together but it creates a situation where the delay is too much)
  • In that case you try to place the chips in other locations and then do rerouting
• This reminds of placement and routing
  • The design flow and optimization process is very similar
  • with one difference, the pcb design is less computaionally intensive because the constrains alot more permissive

fabrication flow

• Once you have determined the routing tracks required, then go the fabrication stage
• the fabrication stage is very similar to photolithography
• we begin by placing the components and then routing between them
• then proceed to fabrication
• First step is to manufacture a mask created similary to the mask created for microchips
  • except in this case the mask is usually formed of solid material
  • the copper tracks are itched on the mask as openings
  • The mask and the substrate are gonna be roughly the same size
• Then you coat the entire pcb with copper
• align the mask with the pcb and use an itchent to each throguh the exposed parts and leave the unexposed parts
• then mount and solder the components

Multi layer

• when you fail to find a colsure for routing on a pcb, you will need another layer of metal wire to create crossovers
  • this is similar to the need for multiple metal layer in asics
• This is difficult in dip chips because they go through holes in pcb occupying both sides
• These two layer are gonna need to communicate
• this is created similar to vias in mircochips, you cut holes through the pcb, then when you deposite your copper these holes are filled simultaneously

• some pcbs require more than two layers
• you create the first layer of metal wires and cut the holes,
• then the second layer, and then you deposite insulator on top of the second layer, then you create via 2, then the third metal layer
• this is agian similar to the mircochip fabrication process