No, Inge, please don't jump into the pot. You are absolutely correct for chip components, as you mentioned. What Herminia is referring to is the heel fillets on compliant leads, such as SOIC or QFP leads. It does not apply to chip caps/resistors.
For SOIC and QFP leads, we are bound to the criteria in J-STD-001 that Herminia was referring to (F= solder thickness (G) + 50% lead thickness). A class 3 fillet is defined as having a heel fillet that is wetted up the heel at least to the line extending through the center of the lead thickness. The wetting can be perfect, but if it does not meet that minimum height, it is defined as a defect, and it gets reworked. It is not defined as a process indicator.
-----Original Message-----
From: Hfjord [mailto:[log in to unmask]]
Sent: Thursday, February 14, 2008 11:56 AM
To: 'TechNet E-Mail Forum'; Stadem, Richard D.
Subject: SV: [TN] Minimum Heel Fillet Height
Guru Richard,
I don't quite understand your philosophy. We don't pay much attention to the fillet heel HEIGHT, but rather the wetting quality. A 'high' fillet has sometimes proven to be false, with nearly no true adhesion to the component (in this case we had issues with ceramic caps). Furthermore, high fillets are negative when soldering chip components on RF Teflon boards with metal back, i.e. the CTE mismatch can cause component cracks. We have produced hundreds of thousands boards with the requirement, that the fillets should be as 'low' as possible, the reliability and joint strength based on good adhesion between the component's projection on the board, rather than fat fillets. If the physical conditions of the two parts, board pad and component termination, are good, the capillary force will provide for a suitable and nice fillet, very often only some 25 percent of the total height. I have a whole book based on wetting mechanisms, and the authors underline what I said above. There is no math for deciding fillet heel height in the book. If all I say here is wrong, my soldering universe will disappear under my feet, and I'll fall free in the cyber solder pot.
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Stadem, Richard D.
Skickat: den 14 februari 2008 14:26
Till: [log in to unmask]
Ämne: Re: [TN] Minimum Heel Fillet Height
Herminia,
Because the heel fillet is the only portion of the solder joint
(typically) that is made to the finished (flat) portion of the component lead, it is the most critical. The toe and side fillets are formed to exposed copper. While it is desirable to have good toe and heel fillets, the heel is the most important.
With that in mind, a decision was made to require a heel fillet at least half a lead thickness in height.
Having sufficient amounts of solder paste certainly improves one's chances of obtaining a Class 3 fillet, but certainly does not gaurantee it.
Just because you have proof that your solder thickness sufficient to form a good solder fillet isn't good enough. Solder does not always wet where it is supposed to, or this forum would never have existed.
JEDEC standards exist for coplanarity of component leads.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Herminia Guevarra
Sent: Wednesday, February 13, 2008 5:01 PM
To: [log in to unmask]
Subject: [TN] Minimum Heel Fillet Height
Hi, folks,
Can anybody please explain in details how the minimum heel fillet height criteria for Class 2 was computed and specified?
F= solder thickness (G) + 50% lead thickness
Can solder thickness be solder paste height prior reflow?
What about if leads are not coplanar? What would be the spec for co planarity? How can they be measured?
Need reply...
Thanks,
Herminia
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