>>> "Guy Ramsey" <[log in to unmask]> 09/19/01 12:00PM >>>

IPC workmanship standards do not allow end overhang on this type of
component. This is because the endpoint fillet gives much of the strength to
the connection. A leadless component stacked on top of another leadless
component will have end overhang. It will be weak. The connection to the
board on the bottom component will see the mass of the component stacked on
top of it.   IPC-D-275 and IPC-2222 are design guidelines. It would not be a
good practice to design with the intent of using this type of modification.
Consider the CTE mismatch the bottom component will see. Top to bottom. Will
both components heat the same? Will the board expand at the same rate as the
component on top?

Guy Ramsey
Senior Lab Technician / Instructor


E-Mail: [log in to unmask] <mailto:[log in to unmask]>
Ph: (610) 362-1200 x107
Fax: (610) 362-1290



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Jack Crawford
Sent: Tuesday, September 18, 2001 1:17 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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