The circuit board is the foundation of our electronic products, and is the most critical component affecting long-term reliability. Since most companies depend on manufacturing partners to fabricate the bare boards, we must:
•Select suppliers who are capable of delivering the required level of quality
•Specify the minimum level of acceptability for circuit boards
•Perform checks to ensure that the boards meet the expected reliability
On the other hand, cost is an important issue, and we don't want to impose expensive and unnecessary tests to prove the acceptance criteria. Our goal is to derive the maximum level of confidence for the minimum amount of time and expense.
When determining the level of testing, we should err on the side of caution. One mistake can cause a line-down situation that results in an inability to ship, which can create scheduling and delivery problems throughout the organization. One defective product can damage our reputation and in severe cases, de-value the company brand.
The cost of recalling or scrapping assemblies is always higher than the cost of scrapping bare boards, and the cost of scrapping bare boards is always higher than the cost of proper inspection and process controls along the way. With this in mind, a small investment up front can prevent a catastrophic failure later on.
Circuit Board Requirements
The following three requirements are summarized from "The Printed Circuits Handbook" by Clyde Coombs, and form the foundation for everything that follows:
1.The physical form should match the intended design. Dimensions and placement position of interconnect points along with the coating on these interconnection points must facilitate the attachment of components
2.The circuit board must provide proper interconnection between components
3.The circuit board must provide adequate insulation between interconnection points that should not be connected
These three items should be acceptable and of high quality throughout the expected life of the product. There are many details related to those three requirements, and it is essential that we define what is required for acceptability and quality for circuit board suppliers to ensure that those three requirements are met.
Properly implemented, the quality and acceptance criteria will give all parties a clear picture of what is expected.
IPC
The IPC is an "Association Connecting Electronics Industries" that collects data from member companies and volunteers, and uses that data to create standards, guidelines and requirements.
The value of the IPC is that members can learn from the experiences of other companies, and avoid the cost of duplicating expensive research. It also creates a foundation for everyone to build upon. For example, if the IPC recommends that the plating thickness of hole walls should average .001 inch, all circuit board manufacturers should be able to meet that recommendation consistently, as a minimum.
There are IPC documents available for every phase of electronic development including design, fabrication, assembly, test and documentation.
Here are the publications that are directly related to this tutorial:
In addition to these, there are many other IPC documents related to bare board manufacturing on subjects such as materials, drilling, silkscreens, soldermasks, and final finishes. These detailed specifications are crucial for a good design and manufacturing process, but are beyond the scope of this document.
All of these documents are under revision control, but when the document number is used without a revision letter, it is intended to mean "as of the latest revision".
Circuit Board Specification
What is the minimum required to QUOTE a circuit board?
A supplier does not need a complete data package to provide a rough estimate of the cost. Quite often, purchasers are involved in budgetary quoting before the circuit design is even finished. This is the minimum amount of information needed to generate an estimate:
If we called a supplier and said, "We are designing a 3 x 5 inch two layer FR4 board with 8mil traces and 8mil spaces, about 600 holes, smallest 12mil, no SMT, HASL, the data will be ready on Monday and I need 100 boards by Friday, How much will they cost?"
We could have an answer in minutes, certainly within the hour. Some bare board fabricators will even provide a web-based "instant quote", where you can enter the parameters online and get an estimate without delay.
Circuit board suppliers are willing to provide an estimated quote with the understanding that when the actual order is placed, if there are features that we didn't mention or that we stated incorrectly, they have a right to revise the quote.
What is the minimum required to MANUFACTURE a circuit board?
The answer to this question might be different than you expect.
Using the same example above, if we sent two layers of circuitry in the correct format (Gerber) and a drilling file (and absolutely nothing else) we could get a board from them, ready to use.
Would it work? The answer, in theory, is YES.
Would it work under the conditions we need it to work?
Maybe not...
...but the point is this: 
The Fabrication Drawing
(This was highlighted briefly in the DATA section, but is repeated here)
The fabrication drawing is the master document for manufacturing a circuit board, and has four main purposes:
1.It can be used by the purchasing department for quoting purposes
2.It communicates critical characteristics to the circuit board manufacturer
3.It is a tool for quality inspection, defining acceptance or rejection criteria
4.It serves as a record of a product in document control
In this section we are concerned about item number two, but in the section on verification (below), we will be discussing item number three. The other items are self-explanatory.
These are the common specifications found on circuit board fabrication drawings:
•A way to identify the circuit board, part number and revision
•Traceability information about who created/checked/approved the drawing and what design control it belongs to
•A picture of the circuit board with dimensions for the overall size as well as dimensions for any features that are important
•Construction details that describe the layer stack with copper thicknesses, and may include specific laminate details
•A hole chart identifying plated vs. non-plated and sizes
•A method to determine appropriate materials for soldermask, silkscreen and final finish
•Other documents that serve as "attachments" to the drawing
•Any notes that are meant to over-ride the attachments
Regarding the "other documents as attachments" listed above, there are benefits to using separate documents to list the acceptance criteria for bare board acceptability. We don't have to clutter every drawing with a large number of "boilerplate" notes, and by having these as a separate document under revision control, we can change these criteria without having to edit every drawing affected by them.
These documents are generally in the form of internal company specifications or internationally recognized industry standards.
Industry Standards
The advantage of using industry standards is they establish a common foundation, they create a "level playing field" that all participants should be able to adhere to as a minimum. It is much easier for everyone to become knowledgeable about a common specification than it is to develop and interpret an endless number of individual company specifications.
The disadvantage of relying on industry standards is that, since they are created by a consensus and a consensus takes time to achieve, they are usually behind the times and represent the lowest common denominator. Many companies are forced to move beyond the standards for various reasons as their design needs advance beyond the scope of the standards available.
Even with disadvantages, if it is possible to adhere to industry standards, many chances for failure will be avoided. For example, if all of the surface mount equipment makers in the world have agreed to recognize a camera target that is a 1mm (.040") round circle, it would be foolish for a board designer to use anything other than a 1mm (.040") circle, and even more foolish for an equipment manufacturer to develop a machine that required anything other than a 1mm (.040") circle.
(Unless, of course, a company found a significant advantage by doing it another way. That's how standards evolve.)
It is up to the customer to specify the level of acceptability.
The criteria for evaluating circuit board quality has been divided up into three categories called "classes". Declaring a circuit board as Class 1, 2 or 3 will automatically identify the minimum set of requirements for acceptability of that product.
Again, it is up to the customer to determine which Class is appropriate for a product. If a toy manufacturer wants circuit boards that meet class 3 requirements and they are willing to pay for the extra level of reliability, that is their decision to make.
IPC-6011 Generic Performance Specification for Printed Boards
This is the root document of a set that covers all types of circuit board technologies (the entire set is labeled IPC-6010).
The first requirement in this document (3.1 General) specifies three things worth repeating here, paraphrased as:
•boards shall meet or exceed all the requirements of the specific class of this specification (ours are "class 2"),
•boards furnished under this specification shall be as specified on the procurement documentation, and
•boards shall be fabricated from design information that includes sufficient coupons to facilitate testing, with the intent that the coupons are as similar as possible to the boards they represent.
Additional coupons may be added at the discretion of the board supplier for product and/or process verification and control, as needed.
IPC-6011 also specifies that (3.3) Verification of compatibility with specifications, master drawings and patterns, and the specific manufacturing facilities and processes used are the responsibility of the supplier. The supplier shall ensure that the end-product board meets the requirements of the printed board procurement documentation.
In addition, IPC-6011 defines the (4.1) Responsibility for inspection. The supplier is responsible for the performance of all inspection requirements. The user reserves the right to perform any of the inspections when deemed necessary to assure that circuit boards and services conform to prescribed requirements.
Other topics address noncompliance, auditing and Statistical Process Control (SPC)
NOTE: It is not my intention to abbreviate or summarize IPC-6011, or any other specification mentioned here. This is only a brief overview for the purpose of introducing you to the relevant specifications, but there is much more information in the documents themselves. Please refer to the actual documents for a more thorough explanation.
IPC-6011 applies by default to all circuit board types, but must be supplemented by a performance specification that contains the requirements for the chosen technology.
Here are the technologies currently in the IPC-6010 set:
Since the majority of electronic products contain rigid boards, IPC-6012 is the most common specification in documentation packages.
IPC-6012 Qualification and Performance Specification for Rigid Printed Boards
The first requirement in IPC-6012 (3.1) is that "boards furnished under this specification shall meet or exceed the requirements of IPC-6011 and the specific performance class as required by the procurement documentation."
This requirement allows us to specify only IPC-6012, but get the acceptance criteria of both IPC-6011 and IPC-6012.
Here is a single page of the IPC-6012 Appendix, which is a summary of the requirements specified in the main body of the IPC-6012 document, and a paragraph reference where it can be found. This page is intended to give you a general idea of the type of criteria that it specifies:
Again, this is only shown for educational purposes. You must refer to the latest revision of IPC-6012 for the most current set of requirements.
IPC-A-600 Acceptability of Printed Boards
It is sometimes difficult to establish non-conformance criteria from verbal descriptions alone. For this reason, the IPC-A-600 was developed as a visual guide to supplement most of the requirements described in IPC-6012.
Most of the illustrations and photographs in IPC-A-600 represent three levels of quality for each specific characteristic; i.e., Target Condition, Acceptable and Nonconforming.
Characteristics are divided into two general groups:
Externally observable conditions are features or imperfections which can be seen and evaluated on or from the exterior surface of the board.
Internally observable conditions are features or imperfections that require microsectioning of the specimen for detection and evaluation.
Here is a typical example of visual acceptance criteria, taken from IPC-A-600:
Here is another example from IPC-A-600 using photographs:
Resolving Specification Conflicts
There are various methods for communicating requirements for circuit board acceptability, primarily:
•Purchasing/Procurement Documentation
•The Fabrication Drawing
•Internal Company Specification
•Industry Standards and Requirements
What happens when they don't agree?
A circuit board manufacturer considers the purchaser of the product as the "highest authority" when comparing multiple sources of criteria. Think about it this way; Industry standards provide a "foundation" for everyone to agree on in the absence of any specifics, but a particular company may have decided that its products need "a little extra", and creates a document containing one or more modified requirements for all of its circuit boards. This company-specific document takes precedence over any other generic industry standards or supplier "best practices". Now, let's say for one specific design there needs to be an extra requirement. This extra requirement can be listed on the fabrication drawing in a note, and it will only be applied to that one design. A note like this can over-ride all the other industry and company-specific documents combined. Now let's pretend it is ten years later, the original designer is not avaiable, a purchaser wants to order more boards, but wants something different. Rather than changing the design or the fabrication drawing, the change can be incorporated into the procurement documentation, and that will take the highest precedence.
For example, let's assume that most of the world is using green soldermask, but a company decides it would rather use blue. The company may create a general document that declares the default soldermask color to be blue. On a particular design from the same company, a note on the fabrication drawing calls for a red soldermask. The circuit board will be constructed with a RED soldermask. Now let's assume that a board purchaser wants another order of boards, but would rather have black soldermask. All reference to soldermask color in all other documentation will be ignored, and for that one order, the boards will be fabricated with BLACK soldermask.
In this manner of thinking, customer preferences in the form of a default company specifications, fabrication notes, or procurement instructions take priority over any industry recommendations or supplier "best practices". It is the philosophy that "the customer is always right", provided the customer is willing to accept the consequences.
In some cases a supplier will put a job on hold to verify a discrepancy, but in the end, the customer decides what criteria is appropriate for its products.
