Hi 'sdu,'
In a message dated 01/15/2002 13:38:05, [log in to unmask] writes:
<< Can someone/anyone please send me or direct me to a study that concluded
that removing "a land that is not connected electrically to the conductive
pattern on its layer" decreased reliability in any way? How about a study
that concluded a maintained or increased reliability by keeping or adding
non-functional pads? ...I cannot find anything in the IPC specifications
discouraging the removal of non-functional pads. >>
I guess you have not been looking very hard. IPC-D-279 refers to IPC-TR-579
where it was found that "functional and nonfunctional lands appear to improve
reliability". IPC-TR-579 concentrated on barrel cracking and thus on
small-diameter holes; the most benefit from nonfunctional lands comes in the
prevention of innerlayer separation [ILS--formerly know as post-separation],
which of course plagues larger diameter holes more than small-diameter holes
[see IPC-TR-486].
<< From practical fabrication experience, removing a non-functional pad does
provide benefits:
1) It is easier to drill through less copper, causing less wear on the bit,
thereby increasing the potential quality of the hole.
2) Most importantly, removing the pads does not change connectivity but does
reduce/eliminate a real potential to short to adjacent connections.>>
A: both true
<< Other effects:
2) For some non-plated holes, if the pads are left on and it is drilled
after etch, the land/pad may be ripped off of the surface if there is not
enough ring to anchor the pad. This only happens on the outer layers.>>
A: why would anybody have lands of any kind in non-plated holes?
<< 3) For some applications, a land may be used to anchor the hole connection
to the material (PTFE, flex)>>
A: That is the main effect in MLBs as well--it prevents ILS as well as
hole-wall separation.
<< 4) Are there other effects? Thermal, impedance, pth reliability? >>
A: It prevents ILS as well as hole-wall separation, and to a lesser degree
replaces resin with copper thereby reducing the z-direction thermal expansion
mismatch [a small effect]
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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