INTRODUCTION

The green mother board inside of every electronics is what’s holding all the electronics and wiring together coherently. This board has multiple layers and intricate materials for different layers to achieve the density we needed for today's miniaturized electronics. In this article, we will cover 3 common types of printed circuit boards, PCB fundamentals, what engineers can do during design and PCB manufacturability basics.

BASICS

There are 3 types of PCB as shown below

Printed Circuit Board (PCB)

• via structure:
• stacked or staggered when using laser/uVias
• Through-hole with plated through hole, blind, and buried.
• Surface Copper thickness=copper + plating
• Internal layer copper thickness=1/2 oz [17.5um]
• Dielectric
  • FR4 - fire retardant material 4 - it's a type of fiberglass woven together
  • Dielectric constant DK= 4
• It is generally used for the main logic/mother board of a computing system where many high speed interfaces are needed to transfer data from memory to CPU and other I/O hardwares such as USB.
• Example Multi-layer 4 layer stackup

Flexible Printed Circuit Board (FPCB)

• via structure:
  • staggered when using laser/uVias
  • Note: stacked via on FPCB is not common
• Surface Copper thickness=copper + plating
• Internal layer copper thickness=1/3 oz [17.5um]
• Copper type: Rolled Annealed (RA)
  • Its grained structure and smooth surface is optimal for applications requiring bending.
• Dielectric
  • Polyimid (a Kapton like film)
  • Dielectric constant DK= 3.4

Application: It is generally used for circuits that do not have heavy or large components such as an Antenna bracket boards where few passive RF matching network are placed.

Example 2 layer stackup

Rigid PCB

• via structure:
  • staggered when using laser/uVias
  • Note: stacked via on FPCB is not common
• Surface Copper thickness=copper + plating
• Internal layer copper thickness=1/3 oz [17.5um]
• Copper type: Rolled Annealed (RA)
  • Its grained structure and smooth surface is optimal for applications requiring bending.
• Dielectric
  • Polyimide (a Kapton like film)
  • Dielectric constant DK= 3.4

Application: used for modules like display or camera that needs to connect with the main PCB boards within a tight product such as a phone.

Example 2 layer stackup

PCB Fundamentals

• Solder Resist
• Solder Mask (commonly referred in PCB)
  • it's the green material made of lacquer like polymer used to protect the circuit from the elements and to prevent solder from bridging and causing a short circuit.
• Coverlay (Term generally used in FPCB and Rigid Boards)
  • Ink based
    • flexible liquid photoimageable solder mask (LPI)
      • Use as solder mask for component footprint in the stiffened flex areas
  • Poyimide based
    • Polyimide
      • Use in the flexible alreas w/o components
• Prepreg
  • it's the adhesive material like a double sided tape that is used to bond adjacent PCB layers together.
• Copper Foil Types
  • ED Copper (Electrodeposited Copper Foil)
    • widely used in rigid boards
  • RA copper (Rolled Annealed Copper Foil)
    • widely used in FPCB boards
    • Note: Rolling/grain direction matters in FPCB design. You want the rolling/rain direction of the copper foil to be perpendicular to the direction of bending or else the flexible property of RA copper is negated and could result in trace cracking.
• Substrate/Dielectric
  • Fiberglass (FR4)
    • Generally used in rigid PCBs
  • Polyamide (PI)
    • Generally used in FPCB
• Stiffener
  • Typically used in FPCB or Rigid PCB to strengthen the specific area of the board such as components, connectors and keep these areas flat and rigid.
  • Materials
    • Aluminum
    • PI
    • FR4
    • SUS (stainless steel)
• EMI Film
  • EMI film is generally used on FPCB for EMI shielding
    • e.g. Tasuta EMI film is widely used for EMI shielding on flexes with ~ 80 dB of isolation
• Vias
  • Laser drilled microVia [uVia]
    • uVias has a smaller drill size that is used with fine BGA pitch (i.e small pad size) where a mechanical drill is too large for it.
  • Mechanical drilled throughole via
• Drill
  • Drill size: it's the final hole size
  • pad size: the total diameter of the PAD (drill size+annular ring)
  • annular ring is the concentric ring extending from the drill to the pad perimeter.
    • Annular ring is required for a plated through hole.
• Surface finish
• It is used to protect copper pads before soldering
  • ENIG
    • One of the most popular ones and highly resistance to oxidation but at higher cost
  • OSP
    • it's made of organic material but has a short shelf life and can't easily rubbed off.
• Via Filling
  • Conductive - Good for thermal performance but costly
  • Non-conductive
• Silk Screen
  • It's used to provide visual information on the PCB surface such as component package orientation, reference designator, etc.

Design For Manufacturability (DFM)

A product must select suitable technology and specification based on the product end use, dimension, high speed interface, and component density.

• Routing
  • no accidental shorts (i.e wrongly named two nets the same, accidentally short power to ground via a short pad)
  • Minimum Trace width and spacing
  • Controlled impedance
    • Note: In typical layout CAD tool, constraint manager forces line width and spacing as well as applying impedance geometry for each designated net.
• Drill size
  • Correct drill size are used for vias
    • Notes: In a typical layout CAD tool, a padstack is used to define the pad size and technology for each PCB layer.
• Spacing Clearance
  • Component to component clearance specification must be met
  • Component to line or Pads clearance must be met
  • Trace or component to edge clearance of the board must be met

Please see DFM for more details.

Reliability

To prevent components from falling out and BGA cracking during mechanical stresses, the following techniques should be applied.

• Potting
  • Helps stabilize and protect electronic components during bending or mechanical shocks (i.e drop test) by curing an encapsulating material over the components. Generally used in FPCBS where the structural integrity is flimsy.
• Underfill
  • Helps stabilize and protect electronic components during bending or mechanical shocks (i.e drop test) by underfilling the BGAs for main ICs with black sticky materials.

Stackup

• Key Items
  • Solder Mask
  • Prepreg
  • Copper
  • Substrate
• Drill Technology
• Anylayer (HDI)
  • Via Type
    • Laser drilled- uVia
  • Via Structure
    • Stacked vias
    • Staggered vias
  • Layers 4-layer
    • Copper/Substrate[fiberglass-FR4]/Copper/substrate/Copper
    • signal/placement->GND->PWR->signal/placement
• Via Size
  • PAD
    • e.g 12 mil
  • Drill
    • E.g 18 would be more common for a PTH and 12/4 for u-via

Panelization

• An individual PCB is generally made from a large panel of PCBs where the custom defined PCBs are arranged in a certain pattern to maximize the utilization of PCB panel.
• A full panel of smaller PCBs is typically further divided into assembly sub-panels with extra material around the edges for machine handling.
  • Note: It's the reason that ordering 10 vs 20 small PCBs causing the same price.

Testing

• Short and open test for all traces and vias must be tested for each board or a sample of boards.
• impedance for all controlled impedance interfaces on all layers must be tested for each board
  • Note: a coupon trace is embedded on the outer edges of the Panel of for each impedance controlled layer that one can verify whether or not the impedance is manufactured correctly.

PCBA

PCBA stands for assembled PCB (i.e components are placed and soldered on on the bare PCB boards). In order for proper assembly on components. Design for Assembly must be reviewed with the contract manufacturers before PCB design release.

• Please see DFA for PCB design for common assembly checklist and overview.

SUMMARY & CONCLUSION

• 3 types of PCBs, regular one, flexible one, and hybrid one are commonly used on a PCB design.
• DFM is important for design of a reliable circuit that accounts for tolerance and manufacturing limits.
• Short circuit and open circuit tests are done to verify all routing is correct.
• DFA is important for correct assembly and reflow of electronic components onto the bare PCB boards

PCB manufacturing and design are critical parameters for PCB design. As a design engineer, we need to have a good knowledge of limits and tolerance with each PCB technology so that we can have confidence that our design and products are manufacturable.