Hello, I have several questions to ask regarding bow and twsit. They are : 1. How were the value of 0.75% or 1.5% determined for Bow and Twist? If my boards consist of SMDs, must 0.75% apply for both bow and twist or we can have one value for bow and another value for twist. 2. John Lau did share with me that 0.5% should be used if the printed board have BGA. However, to date, this value has yet been offically adopted in the IPC, why?. 3. What is the rationale to measure bow on the short side if bow / warp has the tendency to occur on the longer side? 4. Very often, people specify "bow & twist shall not exceed 0.75%". In general, am I correct to say, the statement is interpreted as 0.75% on the side that experience bow or twist. If I specify my statement as " bow and twist shall not exceed 0.75% on the short side", how would the general public interpret this statement? Is this statement practical? 5. IPC Test Method : For bow & twist, why IPC states that we need press the 4 corners with the convex facing upward for bow and 3 corners down for twist? Why can't we let the sample rest on the reference plate without constraint? What is the reason behind such procedure? 6. Should not board thickness be included in the calculation for bow and twist? I have not see any of this parameter being mentioned. I have notice that for the same board size with 0.4mm and 1.6mm board thickness, the former will be more prone to twist. If I apply the same twsit formula to both, the thinner board will definitely fail. As it is natural, I cannot reject it. Does it not sound contradicting. 7. I have a board with width = 100mm and length = 300mm. Board thickness = 1.6mm. Applying the IPC twist formula, the pin gauage diameter = 4.74mm. Assuming the maximum twist is found to be 4mm. This mean the board pass the twist requirement. If it is mounted with components and later mounted down flat with mounting holes along its length including the 4 corners, will the pressure pose solder joint reliability? If yes, which area or part of the board will be stress out? I am very sorry that this message is pretty long but I really need the information to discuss with my manger. A million thanks in advance to all who answer Regards, Wee Mei --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------