The document discusses the manufacturing process of low layer count PCBs, emphasizing the importance of material selection, design considerations, and technology advancements that influence cost and reliability. It highlights that not all fabricators are equally equipped to handle smaller features and tighter tolerances essential for modern designs. Additionally, it encourages utilizing fabricators' design for manufacturability (DFM) services during development to ensure quality.

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Low Layer Count PCBs
Why They Are Not All Created Equal
02.15.2024

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Why Talk About Low Layer Count Circuit Boards?
“…..anecdotal evidence tells us that most of the PCBs used
are double sided (two layer) and four layer in everyday
production….”
- Supporting American Printed Circuit Boards Act of 2022: An Appraisal

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Agenda
 The basics of 2- and 4-layer PCBs – what you had to look for …
 Technology changes that have made 2- and 4-layer PCBs more attractive –
then vs. now.
 What design considerations change the game in terms of cost and reliability –
you will be surprised.
 Material selection – what is the impact on performance?
 Everyone can make these, so everyone’s quality is the same – right?
 Cost vs. functionality – why 2- and 4-layer boards work for many applications.

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2/4 Layer PCB Basics
 Multilayer plated through-hole PCBs
invented in the 1960s.
 Double-sided plated through-hole PCBs
invented in the early 1940s.

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Old School DS & 4-Layer
 Look at how little copper is left

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Old School DS PCBs
 Big though holes and wide traces

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Old School – 2-Layer & 4-Layer Through Hole
 Quality of Plated Through Hole
– Very simple as the holes were large
– Plating a through hole is all about
getting fluid through the drilled hole
 Quality of the Etched Circuit
– Didn’t need to be very defined
– Plenty of space to meet IPC spec
– Impedance wasn’t a big issue
– Shorts and opens

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Old School 2- & 4-Layer PCBs
 Driven by cost or functionality

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What Changed?
 Component Sizes – 01005 Passives

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What Changed? Part Two
 BGAs and through holes with no components (VIAs)

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So Now …
 The rules change

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What Are The New Design Considerations?
 Fine lines and spaces -
0.004” and lower
 Very small holes – less than
10 mils
 Solder mask dams as small
as 4 or 5 mils

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How Do All Of These Design Considerations Affect Cost?
 They all:
– Require specialized equipment
• To be done right
– Have a high effect on yield
– Add time to process
 Why?
– Less room for error
– 0.004” trace - +/-0.0004
– 0.015” trace - +/-0.0015
Over-etched PCB

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Some ways to help …
 Different ways to treat vias
 Always use the largest features
possible – holes, traces and spaces
 Make sure you DFM your BGA
designs

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Material Selection on 2- and 4-Layer PCBs
 Old School Thinking:
– Use high Tg material on all
through hole PCBs
 Today:
– Unless you plan on doing
extensive re-work to the
PCB, do you really need it?
NOTE: Tg or Glass transition temperature, is the base material
parameter for the temperature (°C). It defines at what temperature the
base material becomes mechanically unstable. It is not the PCB
maximum operating temperature, but rather that which the PCBA can
endure for a short time before it deteriorates.

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Can You Really Re-Work These PCBs?
 Sure – SMD don’t require as much heat

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Here Is Where It Matters …
 It will take a ton of heat to get
the large through hole
components out.
 If you have a large PCB where
you will want to regularly replace
a component.
 Tg 130 vs Tg 180.

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Impedance Control and RF Circuits on 2/4 Layer
 Can it be done?
– Sure can
 Biggest challenges:
– Impedance features vs.
soldered features
– Important to keep the
soldered features the same
size and away from ground
planes
 Using trace design to create
the desired impedance
This rule aims to solve a manufacturing problem known as tombstone.
During reflow, a too large connection to an SMD pad acts like a heatsink,
creating a situation where one side of the component has a slightly lower
temperature. Because of this, one side of the component can lift up from
the PCB. This is especially true for small (0402 and smaller) components.

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Everyone Can Make 2- & 4-Layers, Right?
 Yes and no
 Through hole – YES
 Tight spaced, less than 10 mil
via and 5 mil dams – NO
 Equipment matters
 Registration and trace quality
 Solder mask dam quality

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Solder Mask Dam Challenges

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Everyone Can Make 2- & 4-Layers, Right?
 Quality Registration
 Old Way
– Panel tooling holes
 New Way
– Registration to features
on each panel or PCB to
improve quality and yield

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Even The Small Things Matter – Inkjet Printing
 Much faster and consistent process
 More functionality

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Cost vs. Functionality
 #1 rule of thumb – Always make it as big as you can.
– Improves long-term reliability
– Reduces cost
– More room for error – pure math
 If it needs small features and tight tolerances – make sure that your
fabricator has made the necessary investment to be successful.

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What’s Next For 2- & 4-Layer PCBs?
 Features will be getting smaller
 Semiconductors will be getting
more functionality
– Making 2/4 layers cost-effective
 Additional technology will be
developed and required
 Fabricators will need to invest to
be successful

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Summary
 Not all PCBs are created equal.
 Feature size is more important than layer
count.
 Not all fabricators have made the
investment in equipment needed to keep up
with the smallest features and tight
tolerances.
 Use fabricators’ DFM services to check your
work while in development.

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Our Products
Battery Packs Flex & Rigid-Flex PCBs Cable Assemblies Printed Circuit Boards
CNC Machining User Interfaces Flexible Heaters EC Fans & Motors

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 You can quote and order circuit boards up to 16-layers online with
InstantPCBQuote™.
– Standard and Expedited Lead Times
– Various Material and Surface Finish Options
– Controlled Impedance, Controlled Dielectric, Via in Pad
 Go to www.epectec.com/instantpcbquote/ to get started.
Did You Know?

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Q&A
 Questions?
– Enter any questions you may have
in the control panel
– If we don’t have time to get to it, we
will reply via email

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Thank You
Check out our website at www.epectec.com.
For more information email sales@epectec.com.
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