Here's a phenomenon that can occur Richard. On fine pitch QFPs with a generous volume of solder in the joint or a bulging joint, the presence of a reasonable thickness of acrylic conformal coating (say 100um) between the feet of the device, together with regular extended periods of time below zero degC (say -10 degC) can result in the solder being extruded out of the joint thus forming a short to the adjacent lead. The modulus of the acrylic is greater than that of the SnPb at sub zero temps so the solder is under compression and flows.

Been there and got that particular T-shirt

Regards
Ian

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: 11 September 2013 20:03
To: [log in to unmask]
Subject: Re: [TN] Gull wing solder joints and minimum electrical clearance

Joyce, how would it affect reliability?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Wednesday, September 11, 2013 1:18 PM
To: [log in to unmask]
Subject: Re: [TN] Gull wing solder joints and minimum electrical clearance

More likely effect your reliability than electrical clearance.  

Joyce Koo
Researcher
Materials Interconnect Lab
Office: (519) 888-7465 79945
BlackBerry: (226) 220-4760

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter Wong
Sent: Wednesday, September 11, 2013 2:06 PM
To: [log in to unmask]
Subject: [TN] Gull wing solder joints and minimum electrical clearance

We have assemblies where gull wing solder joints have so much solder that along the side joint, the solder connection extends out to the adjacent leads similar to Figure 5-1, Image C from IPC-A-610E.  "the solder connection to a termination may exhibit a wetting angle exceeding 90 degrees when it is created by the solder contour extending over the edge of the solderable termination area or solder resist" (We will be modifying our stencils to reduce the amount printed on these parts in the future).

There is no IPC-610 spec for this condition so we were thinking that if the spacing between the solder joints does not violate Minimum Electrical Clearance, they are acceptable.

Is this a correct acceptability criteria?

Thanks,
Peter

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