Hi Matt,
The method of producing inner layers on double sided laminates means that to
have an odd number of layers is unbalancing the construction. One of the
layers would have to be a single sided laminate, or etched off on one side.
To achieve an equal spacing of the odd layers, there would have to be an
asymmetric construction. Doubling up on a ground plane will not increase
cost and will make the construction more balanced with an even number of
layers.
These laminates also shrink in the press as they are taken above the glass
transition temperature and lose their stability. Thinner inner layers will
shrink more. This causes warpage if the inner layers are not balanced in
thickness about the centre of the panel.
Ground planes on the laminate will reduce the amount of shrinkage whereas
layers with tracking only will shrink more. On 8 mil laminate a shrinkage of
500 ppm is not unusual. The mass lamination technique is to use copper foil
for the outer layers so that on a standard six layer, the two inner layer
laminates would have a ground plane on one side and tracking on the other.
This is okay as long as it is symmetrical.
ie signal-gnd-gnd-signal
and not signal-gnd-signal-gnd.
Hope this helps
Paul Gould
Teknacron Circuits Ltd
-----Original Message-----
From: Matthew Sanders <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 06 July 1998 20:32
Subject: Re: [TN] Warpage
>Can someone explain why a 3 layer board would warp? It seems that the
>structure would be balanced around the middle if it was 1) ground 2) signal
>3) ground. I don't fully understand why an odd number of layers warps.
>
>Thanks,
>Matt
>
>Matthew Sanders
>PWB Procurement Engineer, Trimble Navigation Limited
>[log in to unmask]
>Phone: (408) 481-7817
>Fax: (408) 481-8590
>
>> -----Original Message-----
>> From: Charles Barker [SMTP:[log in to unmask]]
>> Sent: Saturday, July 04, 1998 9:04 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] Warpage
>>
>> Dear L. Ramesh,
>> Warpage can be caused by many different things. One frequent cause that I
>> have run into is poor board design. It is important to keep the amount of
>> copper on each layer balanced about the middle of the board when viewed
>> from the edge. As an example, a four layer board with plane layer(s)
>> should
>> be:
>>
>> Layer one, signal
>> Layer two, plane layer (Power of Ground)
>> ========================================= Middle of Board
>> Layer three, plane layer (Ground or Power)
>> Layer four, signal
>>
>> If the circuitry NEEDS only ONE plane layer (three layer board),
duplicate
>> that layer to provide the necessary balance (make it four layers).
>> There should always be an even number of layers in a multi-layer board.
>>
>> There can also be other issues such as the grain direction, size and
>> structure of the pre-preg material. One of the board fabricators on
>> TechNet
>> will have to explain those issues to you. Other causes could be how the
>> board is cooled after lamination.
>>
>> I hope this helps.
>>
>> Charlie B.
>>
>>
>>
>> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
>> respond
>> to "L. RAMESH" <[log in to unmask]>
>>
>> To: [log in to unmask]
>> cc: (bcc: Charles Barker/US/I-O INC)
>> Subject: [TN] Warpage
>>
>>
>>
>>
>> Dear Technetters,
>> I would like to know the possible causes for warpage and its control.
All
>> suggestions is gratefully acknowledged.
>> Regards,
>>
>> L. RAMESH, Tel: +91-8221-26618, 26619,
>> Indal Electronics Ltd., 26375, 26584 Extn: 345
>> No.12 A, Industrial Area, Fax: +91-8221-26641
>> Nanjanagud 571 301, email: [log in to unmask]
>> District Mysore, INDIA. web site:www.indalpcb.com
>>
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