Phil,

Matching the pad on the board to the part helps with alignment. The part pad may be slightly larger due to tolerance, but it might be slightly smaller too. Best to aim for nominal.

Of course you could argue that you should take advantage of the largest solder joint possible to improve reliability, so you could make the board match the max pads size per tolerance?

If you are concerned about thermal transfer, I can't imagine that you'd get that much better performance by opening up the pad; the size of the solder joint probably isn't the biggest barrier in your thermal path, right?

Ben

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Thursday, January 12, 2012 3:05 PM
To: [log in to unmask]
Subject: EXTERNAL: Re: [TN] QFP center slug over signal vias

I am in the process of getting this design changed but have another question related to it:

This device data sheet does not have a recommended land pattern, only the mechanical drawing for the part.

If the center slug on the part is 3.5 mm, shouldn't the pad on the board be larger than 3.5?

My designer is saying that the IPC pad calculator is matching the size exactly but this just doesn't sit right with me.  It doesn't allow for any tolerance of the size and I have always seen pad center slugs larger than the component's slug.

I measured a part and found the actual size to be over 4 mm, and the xray shows that inside the component, the  heat spreader size is even larger (almost 5 mm).

Thanks in advance,

Phil

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