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IPC Class 2 VS Class 3: The Different Design Rules | Sierra Circuits
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IPC Class 2 VS Class 3: The
Different Design Rules
by Lucy Iantosca | Jul 21, 2020 | 6 comments
As a circuit board manufacturer, designers often ask us about the
difference between IPC Class 2 and Class 3. Class 1 does exist although
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IPC Class 2 VS Class 3: The Different Design Rules | Sierra Circuits
we rarely produce boards that fall into this classification. Most of the
times, even if the end-use of the product only requires Class 1, we will
make it Class 2 just to ensure a better performance. This article will
help you understand the different design rules for IPC Class 2 and Class
3 circuit boards.
There are four IPC classifications. Class 1 is assigned to general
electric boards with a limited life and a “simple” function, such as the
ones you can find in remote controls. Class 2 is for dedicated service
electronic products. This means that you expect the board to have an
extended life so you can place it in a television, a computer, or an air
conditioner. Class 3 PCBs are tighter in tolerances as opposed to Class
1 and Class 2 boards. John Perry, Director of Printed Board Standards
and Technology at IPC, explained:
“Class 3 includes products where continued high performance or
performance-on-demand is critical, product downtime cannot be
tolerated, end-use environment may be uncommonly harsh, and the
product must function when required.”
These circuit boards are highly reliable. They are used to achieve high
performances in the military or in medical, for instance. IPC-6012DS
Class 3A includes space and military avionics. It is the highest class for
printed circuit boards.
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Contents [ hide ]
1 Class 1 – General electronic products
2 Class 2 – Dedicated service electronic products
3 Class 3 – High-performance electronic products
4 IPC 6012 Class 3/A
5 What are the differences between Class 2 and Class 3 for assembly?
6 What are the differences between Class 2 and Class 3 for PCB
manufacturing?
6.1 Annular ring and drill breakout
6.1.1 IPC annular ring acceptance criteria
6.2 What are the design rules for annular rings?
6.2.1 IPC Class 2 Drill & Pad Diameter for 1/2 oz Copper
6.2.2 IPC Class 3 Drill & Pad Diameter for 1/2 oz Copper
6.3 PCB dielectric requirement
6.4 PCB through-hole plating requirement
7 PCB cross-section to verify the spec requirement
8 Inspection and acceptance criteria
8.1 Documentation
Class 1 – General electronic products
Class 1 boards are assigned to general electronic boards with a limited
life and a simple function. This class includes most typical everyday
products. The class 1 boards allow various cosmetic defects as long as
it doesn’t affect the functioning of the board. The reliability of the
product isn’t a critical factor in these types of boards. For instance, they
can be found in TV remote controls, LED lights, kids’ toys, etc. They are
the most inexpensive boards to manufacture in the industry but they
come with a limited life expectancy.
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Class 2 – Dedicated service electronic
products
Class 2 boards have higher reliability and extended life. They follow more
stringent standards than class 1 but allow some cosmetic
imperfections.
Here, uninterrupted service is preferable, but not critical. The class 2
products aren’t exposed to extreme environmental conditions. The board
is expected to run continuously but its operation is not extremely critical.
These kinds of boards are implemented in your laptops, smartphones,
tablets, communication equipment, etc.
Class 3 – High-performance electronic
products
Class 3 boards must provide a continued performance or performance on
demand. There can be no equipment downtime, and the end-use
environment may be exceptionally harsh. High levels of inspection and
testing are performed on these boards with stringent standards. This
makes the class 3 boards highly reliable. This category includes critical
systems such as life support systems, military equipment, electronic
monitoring systems, automotive PCB, etc.
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IPC 6012 Class 3/A
The IPC-6012 class 3/A is relatively a new class that includes space and
military avionics. This is the highest class for printed circuits. The class
3/A boards call for very stringent manufacturing criteria since the boards
should remain operational in critical conditions such as Outerspace, etc.
These boards are expensive to manufacture compared to the other
classes since they need to be close to perfection. They are found in
aerospace, military airborne systems, and missile systems.
The major difference between all these classes is the degree of
inspection. The classes define the permissible defects while
manufacturing the boards.
What are the differences between Class 2 and
Class 3 for assembly?
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Umut Tosun, Application Technology Manager at Zestron America,
explained, “The major differences between Class 2 and Class 3 are
found in component placement for surface-mount components,
cleanliness requirements based on residual contaminants on the
assemblies, plating thicknesses as defined in plating through-hole and
on the surface of PCBs.”
During assembly, surface-mount components might be slightly placed
off pad. This is what we call a visual defect since it does not usually
affect the electrical and mechanical performance. It, therefore, does not
matter for Class 2 circuit boards. However, Class 3 does not accept any
imperfection and this type of assembly misstep will cause the circuit
board to fail the inspection.
The amount of barrel fill required for through-hole leads is 50% for Class
2 and 75% for Class 3. As it can be delicate to get the paste into small
plated through-holes (PTH), Sierra’s advice is to design your PTH 15 mils
over the diameter of the lead. This way, you will have 7.5 mils on each
side, which will make it easier for the paste to fill the barrel.
Factors
Class 2
Class 3
Surface-
Can be slightly placed off pad.
Imperfections are
mount
(Considered as a visual defect,
not acceptable
components
doesn’t affect the electrical and
including visual
mechanical performance)
flaws.
This kind of
imperfection will
cause the circuit
board to fail the
inspection
Amount of
barrel fill
Through-hole leads 50%
Through-hole leads
75%
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What are the differences between Class 2 and
Class 3 for PCB manufacturing?
Annular ring and drill breakout
90 degrees annular breakout
Another topic Class 2 and Class 3 differ on is drill breakouts. Class 2
allows breakouts from the annular ring whereas Class 3 does not accept
any lifted or fractured annular rings. Class 3 boards need to be highly
reliable and when there is a breakout, it is too difficult to find out how
much is really broken out and how much it really affects the connection
with the pad. For Class 2, 90 degrees breakout of the hole from land is
allowed provided minimum lateral spacing is maintained.
IPC Class 3 acceptable annular ring
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Conductor to land junction area
The conductor junction cannot be reduced by more than 20% of the
minimum conductor width specified on the engineering drawing. The
conductor junction should never be less than 2 mils or the minimum line
width, whichever is smaller. For Class 3, the minimum internal annular
ring cannot be less than 1 mil. The external annular ring cannot be less
than 2 mils. It is measured from the inside of the PTH barrel to the edge
of the land pad and may have a 20% reduction of the minimum annular
ring in isolated areas due to defects, like pits, nicks, pinholes, or dents.
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IPC Class 2 VS Class 3: The Different Design Rules | Sierra Circuits
There will be a difference between the designed annular ring and the
manufactured / actual annular ring. This is due to shifting in materials
during the circuit board manufacturing process. To meet the Class 3
requirements, Sierra uses Pluritec machines to discover the shift in
material, software to re-scale the drill locations, and vision drilling to
accurately place the drills.
IPC annular ring acceptance criteria
Feature
Class 1
Class 3
Class 3
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Feature
Class 1
Class 3
Class 3
Plated-
180⁰ annular ring
90⁰ annular ring
The minimum
through
breakout from the
breakout from the
annular ring
hole
land is acceptable
land is acceptable
should not be
provided the
provided the
less than
minimum lateral
minimum lateral
0.05mm.
spacing is
spacing is
maintained.
maintained.
The minimum
external annular
The land/conductor
The land/conductor
ring may have a
junction should not
junction should not
20% reduction of
be reduced by more
be reduced by more
the minimum
than 30% of the
than 20% of the
annular ring.
minimum conductor
minimum conductor
width.
width.
The conductor
junction should not
be less than
0.05mm or the
minimum line
width, whichever is
smaller.
What are the design rules for annular rings?
To achieve acceptance for Class 2 and Class 3, follow the tables below
published by Altium. The first one gives the annular ring requirements
for mechanically drilled blind, buried, and through holes on ½ oz copper:
IPC Class 2 Drill & Pad Diameter for 1/2 oz Copper
Drill
Pad
Anti-Pad
PCB Thickness
Aspect Ratio
0.006''
0.016''
0.026''
Up to 0.039''
6.5:1
0.008''
0.018''
0.028''
Up to 0.062''
7.75:1
0.010''
0.020''
0.030''
Up to 0.100''
10:01
0.012''
0.022''
0.032''
Up to 0.120''
10:01
0.0135''
0.024''
0.034''
Up to 0.135''
10:01
IPC Class 3 Drill & Pad Diameter for 1/2 oz Copper
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Drill
Pad
Anti-Pad
PCB Thickness
Aspect Ratio
0.008''
0.023''
0.033''
Up to 0.062''
7.75:1
0.010''
0.025''
0.035''
Up to 0.100''
10:01
0.012''
0.027''
0.037''
Up to 0.120''
10:01
0.0135''
0.028''
0.038''
Up to 0.135''
10:01
And these tables are for various copper thicknesses:
Drill & Pad Diameter
8 Layers or Less
>8 Layers
IPC Class 2
Pad Diameter Over
Pad Diameter Over
Drill
Drill
1/4 oz Copper
0.010''
0.010''
3/8 oz Copper
0.010''
0.010''
1/2 oz Copper
0.010''
0.010''
1 oz Copper
0.012''
0.012''
2 oz Copper
0.014''
0.014''
3 oz Copper
0.016''
0.016''
4 oz Copper
0.018''
0.018''
Drill &
Pad
Diameter
8 Layers
or Less
Drill &
>8 Layers
Pad
Diameter
8 Layers
or Less
>8 Layers
IPC Class
Pad
Pad
IPC Class
Pad
Pad
2
Diameter
Diameter
3A
Diameter
Diameter
Over Drill
Over Drill
Over Drill
Over Drill
0.013''
0.015''
0.013''
0.015''
0.013''
0.015''
0.013''
0.015''
1/4 oz
Copper
3/8 oz
Copper
1/2 oz
Copper
1/2 oz
Copper
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Drill &
Pad
Diameter
1 oz
8 Layers
or Less
0.015''
Drill &
>8 Layers
Diameter
0.017''
Copper
2 oz
1 oz
8 Layers
or Less
>8 Layers
0.015''
0.017''
0.016''
0.018''
Copper
0.016''
0.018''
Copper
3 oz
Pad
2 oz
Copper
0.019''
0.021''
0.022''
0.024''
Copper
4 oz
Copper
PCB dielectric requirement
The minimum dielectric for Class 2 and Class 3 is 3.5 mils.
PCB through-hole plating requirement
Class 3 requirements are as well more astringent for voids in copper. On
Circuitnet, Paul Reid, Program Coordinator at PWB Interconnect
Solutions, said, “A copper void is where the copper plating in the barrel
of the hole is missing exposing the dielectric material of the drilled hole.
Class 2 allows one void in 5% of the holes. Class 3 and 3/A allows no
voids.” The plating thickness requirement for Class 2 is 0.8 mil as
opposed to 1 mil for Class 3.
These are just a few requirements that differ between Class 2 and Class
3. As usual, the best advice we can give you is to communicate with your
PCB manufacturer. They will guide you and help you get it right the first
time. You should also request a cross-section of your board to make
sure that your shop met your Class 2 or Class 3 requirements. For a
more detailed explanation read our post on 6 DFM issues.
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PCB cross-section to verify the spec
requirement
Visual and X-ray inspections are not always enough to ensure the
integrity of a board. To make sure that your PCB manufacturer met your
requirements, ask for a cross-section analysis. This destructive
technique is the best way to verify your PCB internal structure, mostly
using a microscope. The test can check for various aspects, such as
cracks, voids in solder joints, through-hole filling, etc.
Below is a cross-section of a Class 2 circuit board:
And this is a cross-section of a Class 3A board:
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At Sierra, we do in-process cross-sections for every circuit board we
manufacture at each step of the building process. We check for
dielectric, plasma etch, solder mask thickness, copper, plating, etc. And
if we don’t meet the customer’s requirements, we reject the board and
build another one. If you need a report, you can ask for a final crosssection. We will send you a document with everything we tested and the
results. The micro section analysis report looks like this:
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Inspection and acceptance criteria
After the end-use of the product, the degree of inspection is what you
should take into account when choosing which classification your PCB
should fall into. Keep in mind that inspection is one of the factors that
drive up the cost when an assembly goes from Class 2 to Class 3.
When you think about it, PCBA (Printed Circuit Board Assembly) is not a
simple task. The board needs to properly function once assembled with
all its components, materials, and the solder to hold them together.
Depending on which class your board falls into, the requirements you
will have to meet for inspection will differ. This is when some IPC
documents come in handy to set the level of acceptance criteria for each
class of products.
Documentation
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On Circuitnet, Leo Lambert, Vice President of EPTAC, made a list of the
most significant documents, which are “the IPC 2220 series for circuit
board design and fabrication, the IPC 6010 series documents for board
performance and quality, IPC-A 600 for board Acceptability
requirements, J-STD-001 for soldering requirements and IPC-A-610 for
Acceptability requirements.”
There is a misconception that links Class 3 boards only to the aerospace
field. It is often true but Class 3 is not exclusive to aerospace or any
other industry. The criteria for the four IPC classes are based upon the
application of the product. Therefore, Class 3 can also be the criteria for
avionics, military, industrial, and medical applications.
It makes sense that a lot of Class 3 boards are for aerospace. The
products launched into space have to be highly reliable to prevent any
failure that could be critical. And the additional inspection is just too
pricey for the commercial and consumer market.
When you require a Class 3 circuit board, it implies that the product has
to be built according to the complete IPC criteria. This means that the
design and manufacture teams must take into account laminate
selection, plating thickness, annular ring requirements, manufacturing
processes, material qualifications, facilities arrangements, inspection
criteria, etc. in order to produce the board that meets all the Class 3
requirements.
For more design information, check with our DESIGN SERVICE team.
In another article, we will discuss Class 3 for flex according to IPC-6013,
the document about the qualification and performance specification for
flexible PCBs.
Meanwhile, you can read our article about IPC-2581 set to retire Gerber
files and IPC-6012 or IPC-A-600, which standard should you use?
DOWNLOAD OUR IPC CLASS 3 DESIGN GUIDE:
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6 Comments
BEST Inc on January 15, 2019 at 6:01 AM
Very Informative blog! Thanks for sharing the differences between Class
2 and Class 3 for PCB assembly. However, here my eagerness increases
to know about the IPC-6013 standard. As I have already aware of other
courses like IPC 610, IPC 620, IPC J STD-001 etc but not about IPC 6013
and its design rules.
Reply
Adrian Adamson on July 2, 2019 at 11:34 AM
It looks like the cross-section figures for both your Class-2 example and
your class-3A example meet the Class-3 critiera. Does your class-2
example have any class-3 violations?
Reply
Steve Selby on December 3, 2019 at 6:23 AM
I have a question concerning the requirements for Class 3 when it
comes to connector mechanical holding assists which could be plastic
or metal snap designs intended for holding connector in place for
soldering process. My question is if you utilize a metal snapping
component which does not have any electrical connections and the
interface holes have copper through hole and annular ring top and
bottom do they have to be soldered? If yes what is the criteria for
acceptance to meet class 3?
Reply
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Lucy Iantosca on December 9, 2019 at 12:55 AM
At this location, soldering is not required. You can simply attach
the component bodies with screws and nuts.
Reply
HealthyBooklet.com on October 7, 2020 at 11:15 AM
December 11, 2019 Class 2 vs. Class 3 Where are the mayor differences
in class 2 and class 3 other than in the IPC-A-610 standard? In which
standards is the difference most significant and notable? A.M
Reply
Ronald Smith on October 21, 2020 at 5:22 AM
This is an important post which takes me to a quick look at the
difference between IPC-A-610 Class 2 and Class 3 and also explains the
various classes of boards as well and their working.
Reply
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