Ask the Experts
September 6, 2018
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 differencesbetween classes would be IPC-2222/IPC-6012 Design/Performance for rigid PCB)and IPC-2223/IPC6013 (Design/Performance for flexible PCB). Also, seeattached article I wrote for Circuitree covering this subject.
Senior Applications Engineer
Flexible Circuit Technologies
Mark Finstad has over 30 years in the flex circuit industry in both design and manufacturing. He is a regular speaker at IPC APEX (Professional development courses) and PCB West (flex circuit design courses). He is also vice chair of IPC-2223 and active member of IPC-6013. Finstad has extensive experience with both domestic and off-shore manufacturing.
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
Paul Reid has over 35 years experience in bare board fabrication, quality and reliability. Working for PWB Interconnect Solutions, which does thermal cycle evaluations (IST) of representative coupons, Paul provides failure and root cause analysis of how PWBs fail. His area of expertise includes how circuit board's copper interconnections and material fails in assembly, rework and in the field, as a result of thermal cycling.
The first difference between the two classes of products isbased upon the definition itself. Class 2 products are defined asproducts where continued performance and extended life is required, and forwhich uninterrupted service is desired but not critical.Whereas, Class 3products demand continued high performance and equipment downtime cannot betolerated 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 performancespecifications that provide information on how materials are evaluated on electronicassemblies and define how materials must perform especially for Class 3, highperformance electronics.
IPC-A-600 - Acceptance of PrintedCircuit Boards
ANSI/J-STD-001 - Requirements forSoldered Electrical and Electronic Assemblies
IPC-A-610 - Acceptability ofElectronic Assemblies
IEC-61189-5 - Test Methods forElectronic Materials, Interconnection Structures and Assemblies - Part 5: TestMethods for Printed Board Assemblies
The major differences between Class 2 & Class 3 is found incomponent placement for surface mount components, cleanliness requirementsbased on residual contaminants on the assemblies, plating thicknesses asdefined in plating through hole and on the surface of PCBs.
Although, there are many other differences, these are the ones thatwould primarily impact the performance requirements of Class 3 product.
Application Technology Manager
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
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
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.