David
The technique is adapted from one used for making transmission electron
microscopy (TEM) samples rapidly compared to older (pre-1990) methods.
Applying it to cross-section preparation for PWB's is straightforward.
In the TEM world, after mounting the sample on a stub bonded with wax, the
back side of sample to be subsequently sanded down to 5-10 um (or less)
thickness with 1um accuracy. In TEM, one would then chemically or ionically
polish the sample to under 0.1 um in places before doing microscopy.
For PWB, the only shared value is to get the sample sanded to a location
with 1 um accuracy. I've developed it for cross-sections of PWB's using
commercial tripods from TEM paraphenalia suppliers. All said, I think it
was set up for ~$600 (I'm hazy on this, it could be $1k or it could even be
$400) plus the polishing wheel. One doesn't need a polishing wheel...just a
long flat Al block for placing sand paper on. The key is using a good
transparent wax that also covers the surface of the microvias so you can
inspect them (periodically under a microscope) while you sand away at the
cross-section.
In brief: 1) cut part out with a punch or hand shears, 2) mount the sample
on stub with the vias upward and the cross-section side exposed using a hot
plate and wax, and 3) mount on a tripod with micrometers for feet (at least
one micrometer is required, two or three help the angle control), 4) sand,
adjust micrometers, sand, inspect with microscope and repeat sequence. It
takes about 20 minutes per sample. The tripod allow excellent (sub-micron)
control in sample thickness/cross-section depth.
The advantage is that you can see the microvias because the wax is thin over
the surface as you grind away cross-sectionally. In epoxy/resin mounts, you
only see the vias as you start cutting them because the epoxy is round in
shape. I have to plead some ignorance... I never did epoxy mounts for
microvias in detail. So, don't flame me if those are obviously easier. It
didn't seem so to me when I did do them.
Carey
ps- Besides being a thin film person, I'm a microscopist too....(goes with
the territory)
-----Original Message-----
From: Smith, David V. <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, December 10, 1999 6:55 AM
Subject: Re: [TN] microsectioning equipment for microvias
>Carey,
>
>I'd like to hear more about your sample preparation technique. What are the
>advantages/disadvantages of encapsulating your samples in wax as opposed to
>the standard epoxy or acrylic resins? Also, what purpose does the polishing
>tripod serve?
>
>Thanks,
>David
> -----Original Message-----
> From: Carey Pico [mailto:[log in to unmask]]
> Sent: Wednesday, December 08, 1999 9:57 PM
> To: [log in to unmask]
> Subject: Re: [TN] microsectioning equipment for
>microvias
>
> Edward
> Nice to hear from you again. If the standard sectioning
>equipment is not satisfactory, I have developed a wax-mounted technique.
>Using a wax called Crystal Bond (it is a transparent wax with a good
>hardness), I mound a piece of board with the vias on a cross-section mount.
>Using a high accuracy polishing tripod, use 600 grit sand paper and inspect
>periodically using a microscope. When you get close, use a higher quality
>sandpaper (e.g., 1,000 or whatever it is, I can't remember). It takes
>about 20 minutes per sample. It is labor intensive, but it works nearly
>100% when you are in practice. I found the parts at South Bay Technology
in
>Anaheim, CA. It is a microscopy supply shop.
> Carey
>
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