PCB Manufacturing
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.