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March 5, 2012

Black Land Edges

Black Land Edges
After vapor phase assembly of a class 2, 10-layer HDI board, the inner edges of the lands of plated through holes have turned slightly black.

In the cross-sections of the board we cannot find any irregularities regarding the Cu and NiAu plating.

What might be the root cause for this?

A.V.

Experts Comments

Couple of questions. Does the black material clean off? What type of preheat are you using? Solder and flux type? I'd be happy to help as I can but I'll probably need to get a bit deeper into the process.
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David Suihkonen
President
R&D Technical Services Inc.
David is President at R&D Technical Services, a vapor phase reflow oven manufacturer. he has 20 plus years experience in the vapor phase reflow industry.
I'll make the assumption that the Ni-Au finish is ENIG, meaning a very thin Au layer, less than 6in. I can think of two scenarios that might lead to the localized black color:
  1. Exposed Ni which has reacted, perhaps with oxygen, to form a black oxide or other compound
  2. An organic material was present on the surface, which was charred during heating to form the black deposit
I'd regard scenario 1 as the most likely. It's not normally possible to see the Au layer directly in a micro-section. The thickness of less than 6 in is equivalent to 0.15 m or 150nm, which is well below the accepted resolvable feature size for optical microscopy.

The fastest, easiest method to get to the bottom of this is to examine the area using a SEM (Scanning Electron Microscope) equipped with EDX (Energy Dispersive X-ray) capability. This will tell you rapidly what the elemental composition of the area is, and how it differs from the nearby areas. This will tell you whether there is exposed Ni, or whether there is  any organic material present on the surface. XRF (X-Ray Fluorescence) will not work due to the size and position of the blackened area.

A good metallurgical lab can provide the SEM/EDX analysis as a service. Do be aware that SEM maximum sample sizes are typically small and you should consider this a destructive test, as a section of the PWB may need to be removed for analysis.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
The gold at the edge of pads and at through hole can be substantially thinner than over the rest of the pad. This would be very difficult to see in an x-section due to the extremely thin plating thickness. The black color is probably the nickel oxidizing after the reflow process.
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Richard Boyle
Global Product Champion
Henkel Electronics
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.
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