Standards of printed boards: Class 2 vs. Class 3
Where are the major 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?
The IPC standards that most clearly define the differences between classes would be IPC-
2222/IPC-6012 Design/Performance for rigid PCB) and IPC-2223/IPC6013 (Design/Performance
for flexible PCB).
There are three Classes of printed boards called out in IPC-6011 that include Class 1 General
Electronic Products, Class 2 Dedicated Service Electronic Products and Class 3 High Reliability
Electronic Products. Class 1 has the lower requirements and Class 3 with the toughest
requirements. There is a new Class called out in IPC-6012, Class 3/A, which includes Space and
Military Avionics which is the highest Class for printed circuits. The Class 1, 2 3 and 3/A
specifications are called out in IPC-6012 Rigid. The requirements are also found in IPC 6013 Flex,
IPC-6014 PCMCIA, IPC-6015 MCM-L, IPC-HF-318 High Frequency, IPC-A-600 and other IPC
documents. Since there are four standards Classes 1, 2, 3, and 3/A the most requirements
appear to be in the IPC-6012-2010 document as outlined in appendices A and B.
it is important to note the major difference in the four Classes is in the degree of inspection and
what level of acceptance that one inspects too. The fabricator generally tries to make the best
product that they can. The implementation of the Classes is done mostly by sorting product to
the Class that is specified.
For example let's take the requirement for voids in copper. 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 1 allows three voids per hole in 10% of the holes (6012) or 5% of the holes (A-610). Class
2 allows 1 void in 5% of the holes. Class 3 and 3/A allows no voids. The goal of the fabricator is
to produce product with no voids. If there are voids what the fabricator would do is inspect and
then scrap the boards that have voids based on the Class that is specified. What I find is that the
Class most often is level of inspection rather than a planned goal of the fabricator. On occasion
the fabricator will plan to produce the printed board to a given Class level but mostly it is the
same processes that produce the four different classes and inspection is used to sort to the
specified class. The Classes are, for the most part, a sorting for requirements based on
inspection and measurement.
The first difference between the two classes of products is based upon the definition itself. Class
2 products are defined as products where continued performance and extended life is required,
and for which uninterrupted service is desired but not critical.Whereas, Class 3 products demand
continued high performance and equipment downtime cannot be tolerated such as life support
systems and other critical systems e.g. pacemaker, satellite, radar signals, etc.
The IPC-CH-65B handbook provides list of IPC performance specifications that provide
information on how materials are evaluated on electronic assemblies and define how materials
must perform especially for Class 3, high performance electronics.
IPC-A-600 - Acceptance of Printed Circuit Boards
ANSI/J-STD-001 - Requirements for Soldered Electrical and Electronic Assemblies
IPC-A-610 - Acceptability of Electronic Assemblies
IEC-61189-5 - Test Methods for Electronic Materials, Interconnection Structures and Assemblies
- Part 5: Test Methods for Printed Board Assemblies
The major differences between Class 2 & Class 3 is 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.
Although, there are many other differences, these are the ones that would primarily impact the
performance requirements of Class 3 product.
This is a big answer and there are many parts to the answer. Let me start by stating there are
two issues to address, the product and the process and both are necessary for building a class 3
product and recommended for a class 2 product.
For a product to be built to any class level it has to be designed to that class level from the
board fabrication material selection to the final assembly. Additionally it has to be built in a
facility that has the proper environment, quality management system, and continuous
improvement plan in place along with the traceability of the materials throughout the process.
From an assembly perspective, the materials all have to be qualified and documented, process
documentation has to be in place and the people need to be proficient in their jobs as defined by
J-STD-001.
Secondly the visual requirements as defined in IPC-A-610 only address a few issues for the
differences between class 2 and class 3, such as PTH hole fill, smt component placement, heel
fillet etc.
The most significant documents are the IPC 2220 series for 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.
As an anecdotal example, the product has to be designed from the ground up, you cannot put
Pirelli Tires on a Volkswagon and expect it to be a Ferrari, it won't work.