Steve,

In my earlier reply I said that the solder balls stick more readily to a smooth 
high gloss surface and less to a micro matt surface. Curtain coating produces a 
micro-smooth surface whether the finished appearance is gloss or matt and so you 
will not see an appreciable difference.

The problem will be more apparent around connector runs if you use foam fluxing 
due to the increased amount of flux and also solderable surface. Are you fluxing 
by foam or spray? Air knives would certainly help if there is too much flux in 
these areas.

Foam fluxing will apply much more flux in holes than spray fluxing which tends 
to apply a uniform amount regardless of hole and lead density. Also, a spray 
flux system allows rapid changeover of flux without any down time making it 
easier to trial different fluxes. We have also found significant cost savings 
using spray fluxing.

I assume you are not post cleaning as this would tend to remove solder balls 
since it is the flux residues which bond them to the surface of the solder mask.

As you know, no-clean fluxes are much higher in volatiles and lower in solids 
than conventional fluxes which makes them more suitable for spray application as 
they don't foam so easily. Conversely, the higher solids content in conventional 
fluxes makes them more suitable for foaming since spray nozzles get gummed up.

Thinking about the mechanism, more volatiles will produce more solder balls, 
higher solids will increase the number which adhere to the solder mask. Reducing 
the amount of flux and the solids content is therefore desirable. Finding a low 
solids flux which produces acceptable joints is the next step.

Another factor is the type of wave produced on your machine. Our Electrovert 
MiniPak with Omega wave has a rapidly moving fall on the leading edge which 
tends to carry off the volatiles early on before entering a relatively static 
almost drag solder environment where the panel is lifted off the solder surface 
at a shallow angle. The wave can be made more dynamic with mechanical vibration 
for surface mount.

Last but not least is the PCB itself. Is there any outgassing from the holes? 
How good is the solderability?

Hope these thoughts help. 
-- 
Paul Gould
[log in to unmask]                                                

PS
What does N2 refer to?
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