*  CAD  (Computer  Aided  Design)  FOR    PRINTED  WIRING  BOARDS  

*  DESIGN  FOR  MANUFACTURABILITY  

ConDnued..  

Layout will automatically insert footprints into a board based on the information in the Capture "PCB Footprint" field.  

Design rules must be entered into the layout system before the traces can be placed. Design Rules are required in terms of the minimum spacing allowed between electrical items on the design and for assigning minimum values to certain parameters which may be requ i red by the manufac tur ing processes.  

To  set  a  board  layout  finally  before  rou1ng  begins:  o   Define  resource  files  and  the  target  directory  for  the  design  o   Select  the  units  of  measurement  o   Create  the  board  outline  o   Define  the  layer  stack  o   Set  system  grids  o   Specify  the  component  types  used  in  the  design  o   Define  the  manufacturing  technology  and  complexity  o   Set  spacing  rules  o   Define  padstacks  o   Define  vias  o   Select  colour  se?ngs  for  the  graphical  display  of  your  design  

Board  Outline  and  Grid  Se?ngs  

Spacing  rules  and  Netlist  selec1on  for  rou1ng  

Fan  out  Fan-out is a method for routing surface mount pads where densely packed boards that prohibit routing on surface layers on your board (like BGA array fine pitch with high pin count IC). Fan-out routes surface mount pads to a via so the pad can be routed to other layers like POWER and GND plane.  Fan-­‐out  applica1ons:  o   Mul1layer  boards  that  include  power  and  ground  planes  o   Densely  packed  boards  that  prohibit  rou1ng  on  the  surface  layers  o   Boards  that  include  fine-­‐pitch  components  that  impede  surface  rou1ng  o   Boards  that  need  minimum  route  exposure  

Placing components in Layout Module- a short demo by video

Auto (Routing)

•  Rou1ng  follows  placement  •  Netlist  –key  to  rou1ng  successfully  •  Set  design  rules  before  rou1ng  •  Different  packages  have  different  rou1ng  algorithms  •  Speed  is  not  the  key  factor,  successful  comple1on  with  shorter  interconnect  lengths  is  the  issue  

•  Try  manual,  assisted  and  fully  auto  •  High-­‐density  board  rou1ng  requires  1me,  modifica1on  of  design  rules  if  required  and  complete  understanding  of  layout  

Auto  rou1ng:  single-­‐sided  board:  a  demo  

Auto  rou1ng:  double-­‐sided  board:  a  demo  

Auto  rou1ng:  four-­‐layer  board:  a  demo  

Gerber  file  genera1on  (Tech  files)  

Example of a Drill Report file

Signal Checks Conductor Width Spacing Annular Ring Drill to Copper Hole Registration Text Features Missing Copper Features Connection Missing Holes Unconnected Lines Rout to Copper

Plane Checks Drill to Copper Annular Ring Spacing Conductor Width Thermal Air gap / Spoke Width Missing Copper Rout to Copper Drill Registration Clearance smaller than hole

Solder Mask Checks Solder Mask Clearance Coverage Rout to Mask Spacing Missing Solder Mask Clearance Exposed Lines Partial Clearances

Silk Screen Checks Silk Screen to Mask Spacing Silk Screen to Copper Spacing Silk Screen to Hole Spacing Silk Screen to Rout Spacing Line Width Text Height Silk Screen Over Copper Text

Drill Checks Hole Size Duplicate Holes Hole Spacing Touching Holes Plane Shorts Holes to Rout Missing Holes

Source: Sierra Circuits

DFM  Checklist  

(anti-pad)

Thermal relief pads are connected to the plane and isolation pads are not connected to the plane.

A special pattern etched around a via or a plated through-hole to connect it into a power or ground plane. A thermal relief pad is necessary to prevent too much heat being absorbed into the power or ground plane when the board is being soldered.

Via/Pin  with  thermal                            Via/Pin  without  thermal  

Thermal reliefs on a PCB are to make it easier to solder and desolder through-hole components when you have a PCB with internal planes, large Cu poured planes or wide conducting traces. Without the relief pattern the plane will act as a heat sink drawing much of the heat away from the lead you are trying to solder or desolder. This could be a cold solder joint or the need to apply excess heat while soldering, resulting in a damage to the board.

Board Design for BGAs

q  Solder mask should cover the vias (tenting process) q Solder mask is a material which prevents ‘bridging’ of tracks q Minimizes cross talk between tracks q SMD and NSMD defined vias are both common for BGA pads; reliability is an issue; non solder mask defined pads are better

q  High routing density achieved with this design process q  HDI interconnect technology is a requirement for BGA/CSPs

q  Designers need to be aware of !!

Textbook for this course: ‘Fundamentals of Microsystems Packaging’ by Rao Tummala

m  

END of CHAPTER