Sandy -
As others have asked, it will be helpful to tell us where in the hole the
circumferential voids occur, and when you see them. I have outlined some of
the possible locations and causes below.
1. Circumferential voids in the center of the hole
- Something is interfering with the deposition of copper in the hole - air
bubbles in a key step like smear removal (keeps epoxy from wetting later on),
air bubbles in copper plating this high aspect hole, poor solution movement
of Blackhole(TM) itself or the Blackhole(TM) conditioner thru the hole
leaving too little conductivity to knit the copper flash in the hole.
- Copper got deposited in the hole, but was etched out of the center of the
hole in alkaline etch - something deposited over the copper in the center of
the hole (dry film developer residue dried on, hard water dried on) and there
was no further copper electroplate or tin plate, the tin bath does not throw
into the center of the hole allowing alkaline etchant to create center hole
circumferential voids
2. Circumferential voids near a copper layer in the hole -
- Wedge voids from some process like under-cure of resin and excess
permanganate attack, wedge voids from attack on the black oxide of the inner
layer, nail heading in drilling with microetch after Blackhole exposing epoxy
now not having a carbon coating,
3. Circumferential voids near the knees of the holes -
- Dry on pattern of hard water leaving a mineral crust interferring with
copper and tin electroplating resulting in alkaline etch of flash copper,
dry film being "sucked" down into the holes making removal by developer
impossible (leaves deposit that won't electroplate with copper or tin, and
etches out).
To help us trouble shoot, let us know:
A. Where in the hole is the circumferential void
B. Was copper plated at the void at one point in the process or does the
copper plating taper down to a void? Can you tell if the tin plating could
be so thin it is not a good alkaline etch resist?
C. Post a cross section picture on one of the recommended free internet sites
common to TechNet - Driveaway?
D. Refer to the IPC Process effects manual - 740, I think
MacDermid has propagation test boards to tell if the Blackhole(TM) and the
acid copper plating baths are functioning normally, or if either is now out
of specification for direct metallization processing of boards.
Please contact me off line for further help -
Dennis Fritz, MacDermid
[log in to unmask]
In a message dated 1/25/01 5:41:09 PM Eastern Standard Time,
[log in to unmask] writes:
<< I am looking for information on the possible causes of circumfrential
voids (occuring inside the hole barrel), that I am seeing in the cross
sections of multilayer FR4 boards with 10 mil holes with aspect ratio of 10.
After drilling, these boards go through Blackhole graphitic carbon / Cu flash
/ image / Cu pattern plate with Sn as etch resist / strip dry film, etch Cu
and strip the Sn/ LPI and HAL. >>
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