Product design for manufacturing (DFM) checklist

By Mark Zetter

Product design for manufacturing considerations can culminate into an exhausting list of items depending on the electronics product and product requirements.

Taking measures to consider as many aspects of a product’s design requirements as early as possible during the product development phase can help companies save time and money.

Contract manufacturers with experience in product design can be an OEM’s best source for helping to put together a checklist of product design considerations.

Below are some items broken out by activity or process companies should consider.

 

Design, component selection

  • Verify components are available in production quantities
  • Minimize number, and number of, different components
  • Confirm no end-of-life (EOL) components designed in
  • Minimize sole-source devices, components
  • Multiple, alternative manufacturers specified for all components
  • Second sources checked for mechanical and electrical fit
  • Component tolerance issues identified and evaluated
  • Component temperature ranges considered
  • BGA vs fine-pitch QFP considerations
  • Trim pots eliminated where possible
  • Design for environment: for recycle, re-use, repair

 

General board layout considerations

  • Visible orientation marks on silkscreen layer for all polarized components
  • Polarized components in proper directions
  • Layer numbers visible on PCB or break-offs
  • Names on artwork

 

Board assembly manufacturing processes

  • Workmanship requirements established (e.g., IPC, military, aerospace)
  • Number of processes identified and minimized
  • Hand soldering minimized
  • Hand insertion and assembly minimized

 

Through-hole components

  • Non-plated tooling holes for through-hole automation
  • Drill sizes: lead diameter + X for auto-inserted through-hole (may suggest size of ‘finished hole’ instead so design house can determine which size drill to use to allow for plate up)
  • Dual in-line package (DIP), axial and radial inserted components (note some exceptions on non-plated, single-sided boards)
  • Customary for DIP socket drill holes to be 0.040″
  • Orientation of auto-insert components at 0°, 90°, 180°, and 270°
  • DIP components oriented same way on volume boards
  • Axial components oriented same way on volume boards
  • Transistor lead layouts spaced properly, in-line, pad-to-pad
  • No stand-up axial components
  • Minimum annular rings on through-hole pads

 

SMT land patterns

  • Fine-pitch land patterns at 60% pad width/pitch
  • Land patterns match to actual physical components
  • Use of land patterns fall in with DFM optimization
  • No natural bridges on fine-pitch SMT
  • Non-solder mask defined pads for BGAs

 

Wave-soldered bottom side SMT components

  • Confirm lower-yielding process required
  • Bottom-side SMT wave land patterns optimized
  • Component orientation optimized for waved SMT components
  • 4-sided SMT and fine-pitch SMT not soldered
  • Minimum component-to-component spacing equal to height of tallest of two components
  • Other wave solder shadowing considerations to prevent ‘skips’ (solder surface tension does not break when waving, therefore solder cannot properly wet the interface to form a solder joint)
  • No SMT components on bottom within clinch head footprint on mixed-technology PCBs

 

Hardware

  • PCB includes stainless steel to prevent rusting, repel solder
  • Tinned, plastic encapsulated microcircuits, etc…will solder to board
  • Phillips or allen-head screws for ease of assembly
  • Silpad washers on power devices where required
  • Identify rivscrew use, where possible
  • Non-plated mounting holes, without pads, where/if possible

 

Board mechanicals

  • Three (3) fiducial marks 0.060″ on each side with SMT via-holes covered with soldermask (normal strategy except for certain ICT-enabled designs)
  • Layer stack-up identification on board or break-off tabs
  • Component-to-edge-of-board considerations identified
  • Panelization or break-offs discussed with ODM or contract manufacturing partners

 

Printed circuit board specifications

  • Optimum board finish specified (e.g., gold flash for boards with fine ball grid array (FBGA) or QFPs 0.020″ pitch and smaller)
  • Boards specified for Bare-Board Test (BBT)
  • Gerber files generated in proper format format (with embedded aperture lists) (some formats include various high-level commands/controls that allow the Gerber data creator to precisely specify the photoplot)
  • No soldermask between 0.020″, or smaller, fine-pitch
  • Controlled impedance specified where required
  • Special board materials, construction specified for high-frequency boards
  • Liquid photo-imageable soldermask, standard
  • SMT paste layer(s) with no fiducial or other pads

 

Test strategy

  • Test plan established before design is started
  • Failure modes considered
  • Identify self-test, manual functional test, all boundary-scan board test, automated functional test, in-circuit/combinational test (ICT)
  • Avoid test turrets and hooks (use SMT loops if required)

 

In-circuit test (ICT)partial checklist

  • Enablers for integrated circuits (IC) and oscillators – with resistor pull-downs
  • Test pads for unused IC pins for complex ICs
  • Boundary scan enabled devices (note chaining, layout requirements)
  • One test pad per net – accessible from bottom side
  • Additional pads for power nets
  • Minimum test pad is 0.030″ (check with ODM or contract manufacturer – sometimes 0.040″ square preferred)
  • Okay to use through-hole leads as test points
  • Two (2) tooling holes required: diagonally opposite, minimum 0.093″, 0.125″ (non-plated preferred)
  • Use of 0.050″ and 0.075″ center-center test points minimized or eliminated
  • Pads with via-holes used as test points have no soldermask covering them, other vias masked
  • ICT tooling hole to test pad clearance 0.125″
  • Board edge to test pad clearance 0.100″
  • On-board batteries have disconnect jumper

 


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