additional:
most of our experience from dewetting is poor barrier plating, e.g. ENIG with bad nickel plating. The copper itself is seldom a problem. With 'poor' I mean porous, too thin, contaminated or other anomalies. Typically, the solder wets instantly on the gold, but when the gold is consumed and the next step starts, namely wetting on the nickel, the solder withdraws because of insufficient nickel wetting.
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: fredag 20 februari 2009 10:08
To: [log in to unmask]
Subject: Re: [TN] dewetting
It is true that dewetting can be time-dependent. I know of two potential
explanations:
1. Back to abrasive cleaning: when brushing with fibrous gritty brushes, such as Sc****br**e, or with pumice, abrasive particles get implanted into the copper that is then swaged over the embedded particles with a
<1 µm thick layer of metal. Subsequent power rinsing removes the surface particles but not the swaged over ones. When soldering, the copper wets perfectly initially but, as the thin copper over the particles is "eaten" away, forming the IMCs, dewetting can subsequently start. It takes only a second or so to pass from good wetting to dewetting.
2. Most PCB laminates are made with electrodeposited copper foil which, for reasons of speed of manufacture and thus economy, are deposited as rapidly as possible. This produces a coarse crystal structure in the copper, growing from the drum side (outer surface of PCB) to a rough (subsequently treated) side which promotes adhesion to the laminate.
This copper is horrendously porous, some microsectioned pores being visible under an optical microscope i.e. >0.4 µm across, most being smaller. Before pressing the laminate, the glass cloth is impregnated in an epoxy resin solution in e.g. a ketone solvent which is then dried in a heated tunnel or similar. This hardly cross-links the epoxy prepolymer which may not be 100% stoichiometric (i.e., it may have a small excess of monomer). This B-stage pre-preg is assembled with the copper foil and pressed for a given time at very high pressure and an appropriate temperature profile to ensure cross-linking of the epoxy. The crunch is that the B-stage still contains, say, 1% solvent. The heating to temps of 2-3 x the boiling point causes this to evaporate but the vapours have little place to go, except into the pore structure of the copper. In doing so, they may take with them any residual monomers or even some prepolymer. The pores therefore become lined with a soup of organic compounds of almost indeterminate composition. The larger pores may even have cross-linked polymer filling them (bleed-through). During board manufacture, chemical cleaning processes may remove these organics on the surface, providing a solderable surface, but the contaminants may lie untouched a µm or so under the surface, causing dewetting after a second or so of IMC formation.
A lesser-known addendum to this is that subsequent copper plating is often epitaxial to the original copper structure, so that the pores may extend through to the new surface and any volatile monomers may reach the new surface during soldering, still causing dewetting. This is sometimes also ascribed to organic additives (levellers, brighteners, throwing power additives etc.) co-deposited in the copper, but I'm sure that polymer residues can reach the surface in some cases, especially with sulfate baths, as opposed to pyrophosphates.
I researched these phenomena in quite good depth in the second half of the 1960s and published them in a paper in 1969. I used a home-built automated (including fluxing in 25% WW rosin, drying and solder-bath dross removal) edge-dip test with 0 dwell time in the range of 0-10 s in a carefully temp controlled pot for determining the point at which dewetting would start and, more importantly, how to push it up the time scale so that dewetting would not occur during wave soldering. One practical discovery was that with ultrasolderable boards from processing to reduce dewetting, we could solder ultra-fine line boards with no bridging. One such board, I remember, had 4 conductors passing between adjacent pads of a dual-in-line package and this always soldered without bridging (orientation in the wave soldering machine was important, though). Of course, this was in the days before solder masks were current. We manufactured the demo boards for three major soldering machine manufacturers at that time, one of which, I remember had 35 µm spacings, quite a challenge in the late 1960s!
Brian
Louis Hart wrote:
> Thanks, Brian, this explanation makes sense. Mike Sewell sent a link
> to an article claiming excess time and temp will lead to an
> intermetallic depletion and attendant poor solder joint. If true, that
> is something of which I was not aware. Louis
>
> -----Original Message-----
> From: Brian Ellis [mailto:[log in to unmask]]
> Sent: Thursday, February 19, 2009 12:02 PM
> To: TechNet E-Mail Forum; Louis Hart
> Subject: Re: [TN] dewetting
>
> The most usual cause of dewetting is myriad tiny unwettable areas in
> an otherwise wettable metal. A very common cause is abrasive cleaning,
> where abrasive particles are implanted into the copper surface, but
> there are other causes. What happens is that IMCs are formed where the
> e.g. copper is solderable and this bridges over the contaminants. When
> the solder flows away, the bridging breaks down, leaving areas with
> thickish solder and areas with just the residual IMC, which is very thin.
>
> Brian
>
>
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