Hi Richard: I have to take issue with some of your comments.
First of all I'm not a proponent of HASL. Secondly, I built fused
tin/lead (FTL) boards for over twenty five years so I probably know a
little bit about it.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, May 08, 2007 1:25 PM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
And in rebuttal, also because fused tin/lead allows a very flat surface
without some of the drawbacks of immersion silver, ENIG, OSP, and of
course, HASL. A FTL surface is not flat. It suffers from uneven
thickness.
This is caused by variations in current density during the
electroplating of the tin/lead.
High current density areas have more plating than the LCD areas. Thus
they end up with thicker deposits
There are a lot of companies using FTL finish on PWBs, and for very,
very good reasons. Although it may cost slightly is a pretty non
descript term.
I have found if you want FTL without the TL under the mask i.e. with the
additional
selective masking and stripping it is a 20% adder.
IF you can find someone to do it.
more than HASL, its
application process is much easier on the PWB than HASL, and it can
effectively be applied the first time without a bunch of rework, The
reason there is not a
bunch of rework on a FTL board is you cant rework one, once it's been
etched.
It can't be re-plated and if it didn't wet to the surface copper during
fusing there's nothing you can do to fix it.
which is not always true of HASL.
As illustrated in all of these postings, FTL is a very misunderstood
plating. Properly applied, it does NOT leave tin/lead under the
soldermask. The standard FTL process DOES leave TL on the circuitry
unless
you go thru a process intensive set of masking, stripping and cleaning
steps.
It does NOT leave copper slivers along the tops of features.
It does NOT present a surface that is difficult for soldermask to adhere
to. "Why not just build a SMOBC/HASL board?" There are a lot of reasons.
First of all, I have yet to see a board house that can consistently,
over time, provide HASL boards where the HASL was not reworked at least
one time before the boards ever leave the fabricator, and usually it is
done many times. If your board supplier has to run back thru HASL many
times
you've got the wrong board supplier.
Evidence of this is manifest in the number of postings
on this forum where HASL PWBs were received that could not be soldered
properly due to dewetting issues. This is because after a certain number
of HASL rework cycles there is so much Cu diffused into the HASL that
during the initial soldering processes there is already a high level of
dewetting taking place.
Secondly, the HASL deposits are extremely non-uniform,
As are certain FTL designs. See above on current density.
with some areas/pads retaining much more of the HASL solder than others,
and very
poor gasketing between the stencil and the board occurs. This is the
reason other plating methods were looked at as components developed
finer pitches, along with the advent of the BGA. Trying to place a
component with thin compliant leads less than .020" apart or round balls
on top of a pad covered with a single dome of hard solder and covered
with thousands of tiny little solder marbles (paste) just does not work
very well. It usually leads to bridging and non-uniform paste deposits
in both volume and shape on all but the most basic, 50mil-pitch PWBs.
Thus the development of ENIG, IAg, OSP, etc., which are all flat
finishes. I don't think you'll see much wide spread use of FTL in
commercial, industrial,
telecommunications, etc. but som how they get it done.
FTL is still commonly used, especially in military, avionics, and space
programs, by quite a few companies. I would question about it being
commonly used.
If it was widely used you would find a lot more shops offering it. I
don't have the number
but I would guess it is very small percentage of all boards fabricated
world wide.
Like any other finish, your successwith it is directly dependent on the
performance of the board fabricator who applies it. Properly done, it
can be one of the best, most reliable
finishes you can possibly solder to. And just like you said, Ramon, it
works.
Yes but at a definite $cost penalty that's why you see it still being
used in military, avionics and space.
They don't care what the price is and are still running on 30 year old
drawing and call outs.
FTL provides excellent solderability, without all of the harshness and
variability and stresses to the PWB present with HASL.
FTL has it's own issues on harshness, variability and stresses.
The boards are normally run thru a heat cycle such as an IR oven
to melt the co-deposited electroplated TL. Ask someone from the military
how they like their high frequency Teflon multilayer board run thru the
fuser.
That's why they use not melting final finishes
I am not saying HASL has no place, either. There are different finishes,
all have their strengths and drawbacks. HASL is simply not my first
choice as a surface finish when I am determining what to use for the
guidance system of a nuclear missile, a flight control computer, an
airport tower communications system, or a traffic control system for a
high-speed train, for example. I would certainly feel comfortable
designating FTL if the design warranted it, however.
Selective FTL has its place if you really need it and want to pay for it
but I see it as a dying technology that will have limited availability.
A dinosaur of the past, living on due to ghosts walking the hall.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Tuesday, May 08, 2007 1:47 PM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Hi Michael: HASL is much less expensive and less time consuming
to plate. Selective solder reflow is used by very few companies that are
not ready to change the way things were made long time ago just because
it works.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Barmuta, Michael
Sent: Tuesday, May 08, 2007 11:37 AM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Hi Ramon: As I said "conventionally built" fused Sn/PB boards have the
fused Sn/Pb on the circuitry under the mask. This is how standard fused
Sn/Pb board were built in the 60's, 70's and beyond. A more
unconventional approach is what you are referring to. Boards can be
built this way but are more complicated and expensive due to the
additional processing. However it will take care of the melting metals
under the mask condition.
If you are going to go to all the trouble of maintaining a Sn/Pb plating
line, plating Sn/Pb, selectively masking, selectively stripping the
plating, stripping the masking, fusing, washing, cleaning the exposed
Cu, applying soldermask, etc, why not just build a SMOBC/HASL board.
It's much simpler.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: Tuesday, May 08, 2007 7:37 AM
To: TechNet E-Mail Forum; Barmuta, Michael
Subject: RE: [TN] What is the difference between Fused SnPb and HASL?
Michael:
Traces or other geometries that are not to be soldered
and are going to be under the solder mask do not need to have fused
solder at all, just pure cleaned, bright copper. If solder is left over
the traces that are under the solder mask, it will melt when board is
reflowed or wave soldered as you said. That would not be a good thing.
Solder mask should adhere to the copper well and not be affected by
process temps much at all.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Barmuta, Michael
Sent: Tuesday, May 08, 2007 10:21 AM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Wee: A fused Sn/Pb board is normally covered by soldermask. If not you
could obviously have a real mess at wavesolder.
The problem with a conventionally built fused Sn/Pb board i.e. circuitry
is fused Sn/Pb, is that during reflow and/or wavesolder the Sn/Pb melts
under the mask. This can cause adhesion failure of the soldermask to the
Sn/Pb. It is especially true for larger features, ground planes, etc. If
the board is then washed the condition can be aggravated by the
chemistry and spray impingement resulting in additional loss mask or
entrapment of wash residues.
This was one of the drivers for going to a SMOBC style of construction,
no melting metal under the mask.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lum Wee Mei
Sent: Monday, May 07, 2007 5:37 PM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Thanks for the responds. From what I have gathered, fused SnPb can also
have solder mask on them. My initial understanding or interpretation was
that HASL finish is for SMOBC while fused SnPb finish is for board
without solder mask.
Regards,
Wee Mei
________________________________
From: TechNet on behalf of Dehoyos, Ramon
Sent: Mon 5/7/2007 10:15 PM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Wayne:
Mostly copper with SnPb over it on some slivers. Pure copper
from geometries such as traces and with SnPb such as pads. The slivers
can be very small or very large. It depends on how it was removed. Large
by hand and small by pumice spray. If they break off and wash away, no
problem. If they partially break off and stay, they may shorten
geometries. Other name for them is inclusions.
Ramon
>>>>> --------------- <<<<< The edges at the top will break
off as slivers.
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, May 07, 2007 9:53 AM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Ramon-
But it was the SnPb which had slivers, and if that is chemically etched
away, then what are the slivers made from?
Wayne
>>> [log in to unmask] 5/7/2007 9:16 am >>>
Wayne:
That is affirmative. SnPb is removed from everywhere except the
pads, lands and anything else that needs solder over the top of copper.
At this time bare copper is oxidized and needs to be cleaned by hand or
spray so that SMOBC can adhere to it and slivers brake off.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, May 07, 2007 8:56 AM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Ramon-
What is it that needs mechanical removal? The oxidation of the copper
traces due to selective SnPb removal?
Wayne
>>> [log in to unmask] 5/7/2007 8:46 am >>>
Fused solder is more expensive and it takes about 5 more steps.
When the SnPb is used as etch resist, as Wayne mentions, to pattern the
outer layers and plate the pads and lands, the etching is not vertical
so there is some underetching.
slivers are formed at the top edge of the geometries. Later the traces
have the SnPb taken off. So prior to having the solder mask applied,
the copper traces are dull due to oxidation from etching the SnPb off
which needs to be mechanically removed, leaving possible inclusions
partially broken off. It can be a messy situation. Pads and lands are
left with the solder over them which is later on fused by reflow oven.
Thus the name fused solder. Most companies use HASL very few use Fused
solder.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, May 07, 2007 8:14 AM
To: [log in to unmask]
Subject: Re: [TN] What is the difference between Fused SnPb and HASL?
Werner is quite correct but only mentioned the metallurgy, not the
ramifications for processing.
HASL is a quite obnoxious process in terms of board stress during
application, but most non-aggressive board constructs can handle it:
The panel is plunged into a vat of molten solder and then withdrawn
while being blown off with very hot air. The deposits are lumpy, and
start giving problems for smaller parts.
Fused SnPb is just convection heated to slightly above the melting
point--no thermal shock. Also the deposits are extremely even, so very
fine pitch work is no problem. However, the normal way of achieving
Fused SnPb is to use the plated SnPb for the final etch resist. This
means that if you have a board with a solder mask, then the SnPb will be
under the mask, which will greatly reduce its effectiveness.
Wayne Thayer
>>> [log in to unmask] 5/6/2007 9:39 pm >>>
Hello,
I come across QML vendors that state their finish system : HASL, Fused
Solder, while some only states : HASL. I know that HASL stands for "Hot
Air Solder Leveling", what then is fused solder? There must be some
difference between them - what are they?
Thanks and regards,
Wee Mei
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