Subscribe free of charge current tips, trends and
Insider infor-mation. You can cancel the Newletter at any time - [ More
]
— OSP (Organic Surface Protection) —
for Printed Circuit Boards No Question of Continental Preference for
Procurement Trading Xchange for Interconection Carriers (pt_X_ic)
The answer to the lead-free
problem could sound this simple: Immerse a printed circuit board into a solution, and the result
is a final surface that can be stored and soldered.
Completely level thanks to direct
contact with the copper
In fact, such a thing does exist: a lead-free alternative to HAL
(Hot Air Levelling) with these
wonderful features, at least from the point of view of the PCB manufacturer.
OSP is an organic
solution based on an imidazole substitute which, by means of dipping
or rinsing can be selectively
applied to the copper surfaces, ready for soldering. A
transparent layer, max. 0.2 to 0.6 my thick,
covers the copper like a barely visible clear
varnish.
Purely as a sealing agent, OSP offers good preconditions to adhere components that
require an
absolutely level surface, thanks to the fact that the soldering paste is applied
directly to
the copper.
The common problem of twisted fine-pitch components due to raised “tin bubbles” on
the HAL surface is, therefore, a thing of the past. The impress technique can be more
precise
with OSP, if one considers how much firmer the copper is because of its direct
contact with the
component.
The varnish forms an airtight cover to the copper surface and “allows” the processed
boards a storage life of 6 months maximum. Increasingly, after this time, the encaps-
ulated surface
protection characteristic is gradually lost. A copper diffusion, as is the
case with chemical
tin, for instance, does not occur.
Should there be soldering problems due to processing errors, “de-wetting” or other
faulty
soldering points can be seen immediately, thanks to clear demarcation to the copper.
However, the
most important argument in favour of OSP is its price: in contrast to other
lead-free alternatives
with their complicated electrolytic or chemical procedures, OSP is
basically just a one-step process.
Chemical tin, chemical silver and chemical nickel-gold are therefore,
far more expensive;
HAL only marginally so.
Doubts - just a European attitude?
So, what are the disadvantages ? European users, in particular,
can list quite a few: the
predominant one being, that for mixed components and other multi-thermic
processes,
the organic protection layer breaks off at temperatures above 150° C .
All in all, the area of use for soldering at higher melting temperatures has not yet been
reliably
tested. In addition, OSP does not bond.
The ability to bond with soldering paste – in particular with lead-free soldering paste –
depends
greatly on the soldering procedure, such as convection ovens with or without a
nitrogen atmosphere,
high cooker ovens, etc Tests so far
show that bonding is not as
good as with the other named surfaces.
Therefore, the self-centralising effect is noticeably lower and requires greater precision
of
solder paste pressure. In addition, the test for soldering ability showed that emphasis
should
be placed on a suitable flux agent, as this, in addition to the thermic effect, also
leads to
the removal of the OSP layer.
In Europe, there is a lack of enthusiasm for OSP, especially due
to the great demands
made on the flexibility of lead-free surfaces, plus the need for multi- soldering
Different Industrial Structures
- Different Surfaces
It is, therefore, not surprising that, especially in Asia, OSP
is as popular as chemical tin is
in Europe, or chemical silver in the US.
An environment of mass production with long-term planning – often in-house – using
a
single thermic soldering process makes OSP the absolute favourite.
As long as Europe is oriented towards flexible, small series equipment with a variety of
special
technical requirements, OSP will remain an alternative niche.
