Thank you Tom for the great information.
-----Original Message-----
From: Hausherr, Tom [mailto:[log in to unmask]]
Sent: Friday, June 03, 2011 11:28 AM
To: (Designers Council Forum); Varughese, Renjith
Subject: RE: IPC - 7351B Landpattern for class 3 designs
Renjith,
Yes, the IPC-7351B produces Class 3 Land Patterns.
Solder Fillets:
The solder fillet shape has been used for many years as evidence of solder wetting, which implied that a good metallurgical bond had been formed. This was true for thru-hole technology where inspectors examined the solder side and component side (destination side) of component holes and vias. Many joints were re-touched unnecessarily if the shape of the fillet did not have the traditional concave appearance, contained a small void or wasn't bright and shiny.
With the advent of Surface Mounting inspectors still examine the fillet shape, however a lot depends on the component termination as to what represents a shape that implies proper solder wetting. The ceramic leadless chip carrier was a component that challenged the industries performance requirements related to the solder joint. The solder fillet was to be evident from the land into part castellation. When the solder joints kept cracking IBM suggested adding more solder to the joint. This changes the shape from concave to bulbous. After the DoD suggested the "Surface Mounting was a Technology at Risk" it became obvious that the problem was not the solder joint fillet but the differences in the coefficient of expansion of the ceramic part and the organic printed circuit board.
Today the solder fillet is still used as a visual inspection attribute. A lot of the shape depends on the metallization and shape of the component lead or termination, the shape of the land pattern, the solderability of the land surface (surface finish), the solderability of the component lead or termination, and the volume of solder used to form the joint.
A great deal has been learned about fillet formation during the development of the IPC-7093 on "Design and Assembly Process Implementation for Bottom Termination Components". Even though some of these parts are made using metal lead frames there are those OEMs that insist on seeing a small fillet at the edge of the metal lead frame. This edge has usually been cut by the component manufacturer in order to remove it from the lead frame sheet, however the edge surface must be solderable in order to achieve this solder fillet.
The land pattern shape and size is based on the component lead or termination. The principles are relatively different for each part. In many instances a lead or termination may have a Toe, Heel or side fillet based on the shape of the termination. This is not true for array parts where the fillet is formed all-around the termination and is examined for its location based on the diameter of true position (DTP) of each individual location.
Where a toe heel and side filet is possible the reliability of the solder joint and the fillet itself is based on the size that the individual land exceeds the dimension of the termination. The differences between the component "foot print" and the land pattern determine the shape of the solder fillet provided that the part is properly centered on the land pattern cluster. The term "Land Protrusion" is used to describe the amount of excess copper land is available for the formation of the base of the solder fillet. The size at the base also influences the height of the fillet.
The IPC land pattern standard, IPC-7351, offers 3 different basic conditions. These are identified as level A, B, and C matching the IPC identification for level complexity. Level A has the largest variation between the land size and the component termination; this results in the largest solder joint and fillet provided that all the other factors (solderability, solder volume, process control etc.) are in place.
Level C was been developed for the highest density products. The land to termination differences are the smallest, the land protrusion is the least and the solder fillet is the smallest. A solder fillet is still visible however the base and the height are greatly reduced in size. Attachment of the part and thus the joint reliability is based on the surface to surface are contact and metallurgical bond between those surfaces. A case in point is the Land Grid Array where the pitch is below 1.0 mm.
Visual evidence of a fillet, no matter how small, is good acceptance criteria unless X-ray is required since visual observance is not possible. The height of the solder fillet is not a measurable acceptance criterion in today's electronic products.
Tom Hausherr
Tom Hausherr
EDA Library Product Manager
Mentor Graphics Corporation
Ph: 858.592.4826 | Cell: 858.705.5096
[log in to unmask]
13730 Sorbonne Ct | San Diego, CA 92128
http://www.mentor.com/go/LPWizard
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Varughese, Renjith
Sent: Thursday, June 02, 2011 10:44 PM
To: [log in to unmask]
Subject: [DC] IPC - 7351B Landpattern for class 3 designs
Hello,
Does the IPC - 7351B LP Wizards land pattern support Class 3 type
design?
Thanks and regards,
Renjith Varughese, CID+
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