|Ask the Experts|
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?
|Expert Panel Responses|
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). Also, see attached article I wrote for Circuitree covering this subject.
Senior Applications Engineer
Flexible Circuit Technologies
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.
I think 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.
PWB Interconnect Solutions
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.
Application Technology Manager
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.
Vice President, Technical Director
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