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August 19, 2013

Bow and Twist Requirements

We've recently received analysis showing our PCBA's are well within the IPC-A-610 10.2.6 dimensional requirements for Bow and Twist.

My question is about "considering product reliability" and "it may be necessary" to "confirm through testing" that damage to the components and solder joints have not occurred.

Does this essentially mean the end user can define the test or reject them? What would the contractual obligations be considering we easily pass the dimensional measurement requirements?


Experts Comments

The short answer is, whatever your contract with the customer says. If I assume that it says nothing about bow & twist, but does refer to meeting requirements of IPC-A610, then I tend to agree that the language in A-610E implies that a "Defect condition" for all classes can be encountered even if the CCAs are well within the suggested limits. Emphasis on suggested. The wording in A-610 is "should" which technically means you do not have to meet even this criterion.

Practically, it is your customer's responsibility to evaluate what bow/twist limits are acceptable for their design, and normally, the suggested limits in A-610 are well within what might cause harm. If you want to be safe, make sure that all your future contracts explicitly state that bow & twist will be held within the suggested limits per IPC-A-610. By signing the contract, you and the customer then agree that this is acceptable.
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
The limits of Bow and Twist have been a requirement since the initiation of the IPC documents. Historically the issue was with the installation of the circuit boards into backplanes and any bow or twist of the board would prevent its installation into the backplane connectors or would allow the board to short out with the adjacent board in the connector series.

Today there are many applications relative to the size of the circuit and where it is to be used. Today many boards are secured mechanically with hardware forcing a non-flat board to conform to the security of the hardware. This does cause tension and compression forces on both the solder joints and the components, which is why the 610 document now states to "Consider Form, Fit and Function" and "product reliability" as many subcontractor building the product have no idea or concept as to where the boards will be used or how they will be secured.

Additionally today we are also concern with the manufacturing process and how the bow and twists of a panel or circuit can impact the solder paste deposition process and the component placement process. Any bow and twist of the circuit board or panel in those processes will cause misplacement of the components and could impact the volume of solder paste applied to the product. Hence, it is important to have flat boards.  

From a subcontractor's perspective, if the flatness of the board meets the requirements of the specification called out in the contractual agreement, then the user should be satisfied with the product being received. If however the product does meet the requirements of the specification and that is not acceptable to the customer, then this has to be negotiated on the contract as to what the criteria should be and this is defined as an AABUS agreement, an agreement between user and supplier.  

As for reliability testing, the onus should be on the user to determine the flatness requirements which could be determined through a testing process which would then define the damages caused by installing boards which are bowed and twisted into their products.

For reliability testing one should start with the end product requirements and conduct engineering evaluations to make sure all the components being used and the aggregate of the components in the product meet their final requirements. All the components have to work together to make a reliable product and bow and twist in only one of the issues to consider.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
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.
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