The formula is conservative and assumes the lead is completely on the land. Surface tension on the wetted sides and heel of leaded devices add to the forces to that in cases where the leads occupy about 85% of the land pattern it is okay to use the land pattern in the formula. If the land patterns are unusually large you might have trouble. Then, use the surface area of the component lead. For, BGAs use the land pattern.
 
 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of bbarr
Sent: Wednesday, March 27, 2002 12:41 PM
To: [log in to unmask]
Subject: Re: [TN] Double Sided Reflow Flux and surface tension

A question regarding the "total pad mating area" in the formula below. Does one take into consideration only the area where the bottom of the lead contacts the pad, or the entire pad area? And, is the contact area of the bottom of the lead and the contact area of the top of the pad in contact with the lead added together to get the total contact area for that lead? Hope this question makes sense!
 

Bob

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Howard Watson
Sent: Wednesday, March 27, 2002 12:12 PM
To: [log in to unmask]
Subject: Re: [TN] Double Sided Reflow Flux and surface tension


Ed,

I can't answer your questions regarding the OA flux and BGA's, but I can help with your other questions:

"Is there a rule of thumb for the pad surface area / part weight ratio to
maintain enough surface tension to correctly hold the part?"  Yes, as stated in previous TechNet threads on the subject, the "rule of thumb" formula is the weight of the component in grams / Total pad mating area in square inches.  The resulting value in grams per sq. in. must be less than 30.