Theshort answer is, whatever your contract with the customer says. If I assumethat it says nothing about bow & twist, but does refer to meetingrequirements of IPC-A610, then I tend to agree that the language in A-610Eimplies that a "Defect condition" for all classes can be encountered even ifthe CCAs are well within the suggested limits. Emphasis on suggested.The wording in A-610 is "should" which technically means you do not have tomeet even this criterion.
Practically, it is your customer's responsibility to evaluate whatbow/twist limits are acceptable for their design, and normally, the suggestedlimits 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 & twistwill be held within the suggested limits per IPC-A-610. By signing thecontract, you and the customer then agree that this is acceptable.
Fritz Byle
Process Engineer
Astronautics
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 theinitiation of the IPC documents. Historically the issue was with theinstallation of the circuit boards into backplanes and any bow or twist of theboard would prevent its installation into the backplane connectors or wouldallow 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 whereit is to be used. Today many boards are secured mechanically with hardwareforcing a non-flat board to conform to the security of the hardware. This doescause tension and compression forces on both the solder joints and thecomponents, which is why the 610 document now states to "Consider Form, Fitand Function" and "product reliability" as many subcontractor building theproduct have no idea or concept as to where the boards will be used or how theywill be secured.
Additionally today we are also concern with the manufacturingprocess and how the bow and twists of a panel or circuit can impact the solderpaste deposition process and the component placement process. Any bow and twistof the circuit board or panel in those processes will cause misplacement of thecomponents 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 boardmeets the requirements of the specification called out in the contractualagreement, then the user should be satisfied with the product being received.If however the product does meet the requirements of the specification and thatis not acceptable to the customer, then this has to be negotiated on thecontract as to what the criteria should be and this is defined as an AABUSagreement, an agreement between user and supplier.
As for reliabilitytesting, the onus should be on the user to determine the flatness requirementswhich could be determined through a testing process which would then define thedamages caused by installing boards which are bowed and twisted into their products.
For reliability testing one should start with the end product requirements andconduct engineering evaluations to make sure all the components being used andthe 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 andtwist 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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