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

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