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December 2001

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Subject:
From:
"Mcmaster, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 20 Dec 2001 15:55:12 -0800
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Bill

Can you give the specifics of the construction?  The board may be all FR4
but not all FR4 is created equal.  Are the design and the materials of
construction symmetric about the board centerline?  Any deviation from that
could contribute to warpage.  It's not just the relative position of the
plane layers and signal layers.  Depending on the cores and prepregs used to
build the board, the resin % can vary from the low 40s for 7628 glass to the
high 70s for 106.  I've seen boards that are all the same type of material
but have a preponderance of lower resin cores and prepregs on one side and
higher resin materials on the other with warpage.  Not 2.4% like you're
seeing, but more than was originally anticipated.  The industry usually only
publishes a single value for X/Y CTE for materials like FR4 but there's
actually a significant difference between FR4 that is 40% resin and 70%.

If the stackup is balanced, then you are probably looking at fab-related
issues.  Cool down as mentioned.  Or possible cross grain.  I think another
post alluded to this.  The process of making prepreg imparts a stress in the
fabric as it is pulled through the treater tower.  This is usually called
machine or grain direction.  The induced stress means that the material has
different CTE in the machine and transverse directions.  Even a balanced
board can have warpage if the grain direction is mixed in an unsymmetric
way.

There are two schools of thought on this.  One says that you just avoid
mixing grain direction.  The other says that you intentionally mix it,
though you have to maintain symmetry.  The latter is a lot more difficult to
do correctly and much more prone to mistakes.  There are also a lot of
constructions that don't lend themselves to maintaining the symmetry.

If it is cross grain, I'd also consider it the responsibility of the
fabricator.

> ----------
> From:         Safavi-Bayat Shahed[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.
> Sent:         Wednesday, December 19, 2001 5:44 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Warp age in FR-4
>
> Bill
> Are you sure that the material is FR4 on top and bottom layer?
> The reason I ask is I once used Rogers on component side and FR4 on
> circuit
> side and the board had
> warp age and the reason was mixing the two material.
> Other than that I think your fab shop kind of missed it this time.
>
> -----Original Message-----
> From: Kasprzak, Bill (sys) USX [mailto:[log in to unmask]]
> Sent: Wednesday, December 19, 2001 8:26 AM
> To: [log in to unmask]
> Subject: [TN] Warpage in FR-4
>
>
> Holiday Greetings to one and all,
>
> I have an 8 layer FR-4 board that was supposedly built IAW IPC-6011, Class
> 3
> and IPC-6012, Class 3. The board itself appears to be like any other
> board,
> .062" thick, no unusual ground plane layers etc. After wave soldering this
> assembly severely warped. Looking at the bare boards, they are also
> warped,
> but can be shaped into a straight position by bending a little. (I
> shouldn't
> have to do this.)
>
> Not being up to date on board fabrication techniques, was there anything
> in
> the fab process that could lead to a warped board? We're talking a 3/16"
> warp over an 8" length.. Or was there something at my assembly end that is
> introducing the warp. The boards were run in a titanium picture frame
> pallet.
>
> Your thoughts and ideas are greatly appreciated.
>
> Bill Kasprzak
> Moog Inc., Electronic Assembly Engineering
>
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