Hi Wee,

Wicking is a condition where copper "wicks" from the edge of the drilled hole and along glass fiber bundles into the dielectric around the hole.  The cause is usually from etch back where the epoxy is aggressively removed during the "desmear" process.  Poor drilling and a number of material problems may contribute to wicking.  The net effect is that the bond between the epoxy and glass fibers is compromised enough to allow copper to be plated between the fibers in glass bundles.  

The worst-case condition is a short between holes drilled on a tight grid.  The most insidious condition that wicking is a sight for CAF (conductive anodic filament) growth

On microscopic examination this condition presents as copper going down the glass bundles that are parallel to the plane of the microsection.  The wicking in bundles that are perpendicular to the microsection are much harder to see but may look like copper rings, half moons or squiggles.  Dark field illumination usually shows that the glass to epoxy interface is distributed to a greater depth than the copper penetrates.  The color of the wicking tends to go from copper near the hole wall to silver at the end of the wick.  There are many times when the color is quite variable and hard to discern.  Lighting can play a role in evaluating this defect and illuminating the side of the section with a variable intensity fiber optic light may be helpful.

Wicking in microsection examination looks similar to "dry weave", measling, crazing and some of the more subtle forms of delamination than follow glass bundles.  They all have in common a break between the glass fiber and epoxy, but in the case of wicking, copper is also present. If delamination is present in the material before electroless and electrolytic plating wicking may be present due to delamination but I have found this to be rather rare.

If the microsection is not ground to the center of the hole the condition may be artificially exaggerated.  Assure that the drilled hole size and the measured hole size in the microsection are similar. This will assure the section is through the central region of the hole.  I found that the section needs to hit the center of the hole +/- 25%; so on a .010 your have about a .005" window in which all holes must fall. Anything greater than +/- 25% will produce significant artifacts by exaggerating copper thickness and wicking depth.  I just measured the hair from my beard to be .002" thick, hitting the center of as many as 18 holes +/- 2.5 beard hairs is a bit of a challenge and a common microsection artifact.   

Paul Reid
PWB Inc.
613-596-4244 ext. 229


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Wee Mei
Sent: Thursday, May 25, 2006 7:48 PM
To: [log in to unmask]
Subject: [TN] Wicking in Printed Circuit Board


Hello,

What is wicking in printed circuit board. What is the casue? How will the presence of wicking affect the board? It does not look like a common defects in PCB, right?

Thanks in advance and regard,

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Wee Mei

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