No,
The basic reasons for warp after reflow are down to:
1) The layout of your board as far as copper distribution is concerned
2) The layer stack of your PCB

Re: 1)
If there are large areas of local copper flooding, not distributed across a
layer as a plane, then this can cause localised bowing.
If there is uneven distribution of copper, some layers have lots of copper,
but this isn't balanced to form a symetric pattern through the layers.
If the significant copper flooding/planes are on surface layers of the PCB
but not in the centre.

Re: 2)
The seperation between layers isn't symmetric about the centre layers. There
may be an unbalanced stack giving the effect of a bi-metallic strip.

Layer separation is most probably the cause.
Feel free to ask me more about this.
Regards
Edward Brunker
Principal Process Engineer

-----Original Message-----
From: Burtt, Nigel [mailto:[log in to unmask]]
Sent: 20 October 2000 12:19
To: [log in to unmask]
Subject: [TN] PCBs warping after SMT reflow


Colleagues,

We have occasional boards (just a few from a batch of 200 or more) warp
quite badly following reflow. The boards concerned are about 270mm x290mm
(10.5" x 11.4")

Is there any logical explanation why this happens? The boards are supplied
flat, so it would appear there is a built in stress factor which gets
relieved when the board is heated or there is some unequal
expansion/contraction occuring across the layers. Can this be down to a
mixed warp and weft of the glass in the outers during the layer build?

> Thanks,
>
> Nigel Burtt
> Production Engineering Manager
> Dolby Laboratories Inc - European HQ
> Email: [log in to unmask]
> Tel:     +44 (0)1793 842132 [direct line with voicemail]
> Fax:    +44 (0)1793 842101
>
>

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