Hi, Ezequiel,
First off, a 5 mils diameter hole is very small indeed to fill with
silver-loaded epoxy - the minimum I've heard of fab houses being able to
achieve comfortably (i.e. economically) is 10 mils. The particle size
within the fill material makes filling smaller holes difficult to
impossible, especially if your board is more than 20 mils thick (I presume
that it is ... more than 20 mils thick).
I suggest you talk very closely with your fab house, ask them to produce
samples if they can that they can handle the filling, and do
microsectioning of the samples to see how good the fill is.
DuPont 101 is the popular material for such hole filling, but there are
others and some may have a smaller particle size that will do your job. Ask
a lot of questions, though - e.g. who has filled 5 mils holes? What
material did they use? How were the results? What application were ther
resulting boards used for? etc., etc.
The "Gold Body" finish is properly called ENIG (Electroless Nickel
Immersion Gold). Nickel plating over copper should be 235 microinches min.
with the gold being about 5 microinches, but look at IPC 4552 for
"official" information. Reliability of this finish is now good, and is
preferred by many for BGA boards. Finish flatness is very good and
solderability is about the best for BGA's IMHO.
I can't see why you would have additional finish thickness for the
connector, or am I reading you wrong? When you say "mix" connector, I take
it that this is a Plated Through-Hole Connector used on an otherwise SMT
board (?). I haven't heard of different parts of a board having
deliberately different thicknesses of finish plating, or why it would be
necessary. You don't get additional mechanical support from a thicker
plating finish - in fact you could weaken it, depending on the finish. If
it's mechanical support you want, secure the connectors to the board with
screws and washers before you solder them down.
Peter
Ezequiel Cruz <[log in to unmask]> 01/04/2003 06:23 AM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Ezequiel Cruz
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Silver Fill Plugged Vias / Gold Body
Could someone help me with the following.
I have a BGA with 5 mil (.127 mm) holes through the entire stack.
I need these silver filled plugged.
I need the correct call out for my notes on this procedure.
I also have a requirement to have this design finished in gold. My regular
board house calls this finish "Gold Body". However, I really need the
thickness call outs since this finish is being used for manufacturing and
reliability issues per the above BGA. But I also have an area for a mix
connector which should increase the thickness of the finish in that area
since these will be surface contact pads.
If there is anyone out there who can help me with these issues, I really
appreciate the feedback.
Thank You,
Ezequiel Cruz
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