Follow-up to your questions:

Background Plating

Pretreatment Factors

While different supplier processes rely on a number of different process 
chemistries to achieve an active surface for the nickel bath to initiate 
plating, all processes share the following needs :

Control of chemistry with appropriate concentration, bath operating condition 
and bath age limits

Control of process rinsing quality and times

Beyond these factors, the relative process windows for resistance to background 
plating of #different suppliers systems may very well be different.


Nickel Plating Bath

The following observations apply to nickel baths in general.

The tendency towards background plating may be caused by operation of the bath 
at :

Excessive levels of plating rate / activity

Low levels of bath stabilizer

These conditions correspond to either excessive bath additions or inadequate 
frequency or quantity of replenishment materials. As a first step, baths should 
be operated within the control limits established by the product supplier.

Inadequate filtration might also lead to a build-up of fine particulate matter 
in the bath that could act as initiation sites for background plating.

In addition, there will be differences between the performance of different 
suppliers products. The desired combination of good plating rate and resistance 
to plating initiation on areas other than activated copper is a function of the 
specific bath formulation chosen.


Substrate

The substrate can influence the process in several ways.

Failure to completely remove all the copper from the areas between traces, even 
at a microscopic level may lead to plating on those particles. Treatment of a 
sample of the work being processed with oxide can help to make these fine 
copper particles more visible under inspection. If use of a longer microetch 
immersion time has a beneficial effect on background plating performance, this 
mechanism may be operational.

The texture of the substrate epoxy may also be an influence. Higher profile 
foils will leave behind a more deeply textured epoxy surface. This may in turn 
lead to greater difficulty rinsing residual activation chemistry from these 
areas, leading to inadvertent activation of resin areas. Longer rinse times or 
use of tempered rinses may be of benefit in this case.

Hope this helps.

Regards,

John S. Grosso
Shipley

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