SMT COMPONENT ASSEMBLY

PCBs have conductive traces that allow electricity to flow through the board. Each SMT component on the board is placed in a specific location on the conductive pathway so the specific component can receive sufficient power to function. When considering the placement of components that use surface mount technology on a printed circuit boards (PCB), there are special considerations to be made.

CTE Considerations

There are a number of factors you must consider when establishing SMT component placement tolerance and spacing. One of the most important factors with regard to SMT component spacing and placement is CTE, or coefficient of thermal expansion. Many printed circuit boards are made of glass epoxy substrates with leadless ceramic chip carriers. When the CTE differential between the ceramic carriers and the epoxy substrate becomes too great, you may experience solder joint cracking, which happens after about 100 cycles.
The solution is to either make sure your substrate has an adequate CTE, to use a compliant top layer substrate or to use leaded ceramic chip carriers instead of leadless ones.

Placing Each SMT Component on the Board

Your SMT component placement will also be dependent on size and cost. Components that absorb more than 10 mW or conduct more than 10mA will require greater thermal and electrical considerations. Your power management components will need ground planes or power planes to control heat flow. High current connections will be determined by the acceptable voltage drop for the connection. When making layer transitions, high current paths will need two to four vias at each layer transition. When you place multiple vias at layer transitions, you will improve thermal conductivity, as well as increase reliability and reduce resistive and inductive losses.

When doing your SMT component placement, place connectors first, followed by power circuits, sensitive and precision circuits, critical circuit components and whichever additional components are necessary. You are choosing routing priority based on power levels, noise susceptibility and generation and routing capabilities. The number of layers you include will vary depending on power levels and the complexity of your design. Remember that since the copper cladding is produced in pairs, you should add layers in pairs as well.

After SMT Component Placement

After you have placed the components, if you are not the lead engineer or designer, you should make sure whoever is in the lead reviews the layout and makes any necessary adjustments to physical locations or routing paths, so you have laid out the circuit for optimal efficiency. Final considerations should include making sure there is a solder mask between pins and vias, that the silkscreen is concise and that sensitive circuits and nodes are protected from noise sources. Be open to correcting the PCB based on any feedback you get from the PCB designer during the review process.