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September 2001

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Subject:
From:
Jack Crawford <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 18 Sep 2001 15:38:14 -0500
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>>> "Ashok Dhawan" <[log in to unmask]> 09/18/01 01:57PM >>>

Ken

IPC-HDBK-001 does specify the conditions when you can use piggy backing of
components." Except when required in high frequency applications, stacking
of components should not be permitted in initial design. ......Stacking
usually occurs when the existing design must be modified to incorporate
additional components on the assembly......Stacking should be documented on
the assembly drawing." Refer to Figure 6-24.

The pitfalls I can think of, is overheating of parts as they are stacked,
constriction of solder flux between parts, inconsistance process and also,
difficulty in automating the process. Thermal dissipation can be an issue.
If volume is too low and it is not economical to respin the fab and it does
not make functional problems , I will say why not use Stacking and document
it.

Ashok




> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Jack Crawford
> Sent: Tuesday, September 18, 2001 12:17 PM
> To: [log in to unmask] 
> Subject: Re: [TN] Piggy Backing of passives...Why one should go for
> addtional fab spin?
>
>
> >>> Ken Patel <[log in to unmask]> 09/18/01 11:06AM >>>
> All,
> What are the major reasons for not allowing piggy backing of the
> components? I know IPC-D-275 and it's replacement does not allow. I know
> there will be an additional rework cost but if the board is laid out and
> then engineering comes out with ECN that requires piggy backing of
> components -People here want to avoid costly fab spin on relatively low
> volume?
>
> There shall be some good quality and reliability reasons which I would
> like to know.
>
> re,
> Ken Patel.
>
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