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Subject: PCB Raw card Bow & Twist calculation Ref: IPC-TM-650
Date: Tue, 28 Sep 2004 14:43:02 -0500
Message-ID: <[log in to unmask]>
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Thread-Topic: PCB Raw card Bow & Twist calculation Ref: IPC-TM-650
Thread-Index: AcQnAqTwpm94C6bcSP65pcf/eVu10QAF5I/gAACCvuAfaG8OkAA07L2g
From: <[log in to unmask]>
To: <[log in to unmask]>
Cc: <[log in to unmask]>
Fellow TechNetters,
=20
Anyone out there cares to put in their 2=A2 worth. I sure would =
appreciate the CLARIFICATION.
=20
Given the following physical dimensions of a raw card I proceeded to =
apply the IPC-TM-650 Bow and Twist Method 2.4.22 step 5.2.2 for maximum =
allowable twist percentage for an SMT, ,75%, technology card. Then =
confirmed that calculated thickness by applying 5.2.7 formula to confirm =
the SMT tolerance, .75%, guideline. I also conducted the 5.3 Referee =
method (Twist) for a second correlation. The 2 methods do not yield =
similar data. Can you please help to explain where I went wrong.
=20
Sample #1. Length: 1.5 inches,
Width: 10.4 inches,
Diagonal: 14.75 inches,
Thickness: 0.062 thousands.
Technology: SMT
Target: 0.75%
=20
5.2.2 Maximum Allowable Twist R =3D 2(D)(T)/100 =3D =
2(14.74)(.75)/100 =3D 0.22125
5.2.7 Percentage Twist R =3D R/2(D)x100 =3D =
.22125/29.5x100 =3D .75
Therefore, the max allowable twist Go/No-GO thickness =
is 0.221125
I proceeded with the exercise.
=20
I place the sample on a granite rock. While penning one edge of =
the sample I slide a Go/No-Go thickness gage on the corner onsite the =
pin edge which measured .042 thousands. Now applying the formula for =
actual percentage twist.
R =3D R/2(D)x100 =3D .042/29.5x100 =3D 0.14237
The value of .14% is well below the maximum allowable =
.75% percent twist, acceptable product.
=20
I proceeded with the exercise:
=20
5.3 Referee Method.
Actual: R1 =3D 0.0955
R2 =3D 0.0590
5.3.6 Percentage Twist =3D R1-R2/Lx100 =3D 0.0955 - =
0.0590/10.5 =3D 0.0034761 =3D .34%
=20
The results of 5.2.7 actual does not correlate to the results of =
5.3.6. What's missing.
=20
In advance I thank you for your timely response in this matter.
=20
Victor,
=20
-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]=20
Sent: Tuesday, April 20, 2004 4:16 PM
To: 'TechNet E-Mail Forum.'; Hernandez, Victor G
Subject: RE: [TN] PCB Raw card Bow & Twist Tolerance
=20
=20
For bow it is the length of the board. For twist it is the diagonal. But =
these are not "requirements" they are "should" best manufacturing =
practices. Bow and Twist shall not be sufficient to damage the =
components or solder connections in subsequent assembly operations. =20
=20
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G.
> Hernandez
> Sent: Tuesday, April 20, 2004 5:00 PM
> To: [log in to unmask]
> Subject: Re: [TN] PCB Raw card Boe & Twist Tolerance
>=20
> Ramon,
>=20
> I have seen the 1.5% and 0.75% criteria before. What I=20
> am not sure
> is % of what base number.
>=20
> Victor,
>=20
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
> Sent: Tuesday, April 20, 2004 1:07 PM
> To: [log in to unmask]
> Subject: Re: [TN] PCB Raw card Boe & Twist Tolerance
>=20
>=20
> Hi Victor:
> It should definitely be less than 1.5% for THT and
> less than 0.75% for SMT, which that is what it is for a board after it =
> has been soldered.
> JStd.001B 9.2.3
> Regards,
> Ramon
>=20
> > -----Original Message-----
> > From: Victor G. Hernandez [SMTP:[log in to unmask]]
> > Sent: Tuesday, April 20, 2004 1:02 PM
> > To: [log in to unmask]
> > Subject: [TN] PCB Raw card Boe & Twist Tolerance
> >
> > Steve,
> >
> > What has been your experience with measuring Bow &
> Twist on a raw
> > card. Is there a RULE Of Thumb that can apply generically.
> >
> > Victor,
=20
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