This is one of those Doug Paul-type answers.

My experience is that flexure deflection radius is greatest in the
center of a curve between two points of stress for PWBs. Also,
consideration must be given to whether side rails or frames are used,
and points where the assembly is secured within the device. Placing a
BGA close to a cooling rail that, when attached with hardware, sometimes
places stress on the PWB will result in the BGA having stresses placed
on its solder ball connections. Stress applied to solder joints =
shorter time to failure.
 
1. For a square or rectangular board, if the PWB is physically bent
downward (or upward) along the top and bottom edges, the greatest point
of flexure deflection will be anywhere along a line going from left side
to right side, horizontal through the center of the PWB, including near
the left and right edges.

2. If the same board has those forces placed along the left and right
sides, then the greatest point of flexure deflection will be anywhere
along a line going from the top center to bottom center of the PWB,
again including near the top and bottom edges.

3. If the same board has those forces placed on two diagonally opposite
corners, the location of greatest flexure radius would be along a line
running diagonally between the two OTHER corners.

So there is no "magic location"; some assemblies will see greater
flexure near the center, while others near the edge or near mounting
hardware, depending on the final configuration and the location of the
forces causing the flexure. 

The pressure points may be localized to a any given section of the PWB,
depending on where the pressure points are. Pretty much the same is true
for deflection caused by CTE mismatches due to differing copper planes
within the PWB, or for flexure caused by the board flexing because it
wants to expand but is bound by a metal frame with a different CTE.

 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Maxwell
Sent: Monday, April 16, 2007 12:36 PM
To: [log in to unmask]
Subject: Re: [TN] Placement of large flatpacks and BGAs close to board
edge

Inge,
This is because board flexure or deformation is typically greatest at
corners and along board edges. The same rule should be applied to chip
capacitors as well.

Regards,

John Maxwell


>Dear collegues,
>
>I have heard of a thumb rule, never place them along and close to the 
>edge. Hence, we place them (when suitable) in the center. I have 
>forgotten why this rule. Was it because of vibration nodes, warpage 
>during soldering, tempcycle issues or what? Is there a IPC standard or 
>other reference?
>
>Inge
>
>  
>

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