Here is an undimensioned, simplified answer to questions re difference HASL
/Imm Sn
Immersion tin is just a thin coating of tin (possibly with some non metal
brighteners or grain refiners). It just lays on tracks/pads and is supposed
to protect them ready for soldering. In use the Imm Sn dissolves into the
molten solder which then forms a normal solder joint to the underlying
track. Tin oxides are more difficult to remove than those of lead, so
Immersion tin oxides would be no different to lead free solder. Both equally
more difficult to remove than those from Sn/Pb HASL. This is simply because
there is less tin oxide in tin/lead.
However they are probably more relevant in immersion tin as the flux
activity is really needed to prepare the surface under the immersion tin
ready to make a solder joint, whereas with HASL the joint is already done.
Any surface oxide will mostly float out from mechanical action of the flux
as the solders merge.
Imm Tin is usually applied directly onto copper. The problems with immersion
tin are
1) It is very thin and can therefore be porous allowing the underlying metal
to tarnish
2) It is usually applied on to copper and if stored too long will diffuse
into the copper generating an unsolderable Cu/Sn intermetallic.
3) It can whisker
4) Sometime the copper tracks are coated with Ni before immersion tinning.
This is intended to reduce the probability of Whisker formation. Refer 1,
which is worse with nickel.
5) The quality of immersion tin can be very variable and especially with
supplier.
6) It's cheap. All the above was known 40 years ago, so invisibly to search
engines. Before Pb free it had periodic local outbreaks/rediscoveries, but
advent of PB-free turned those back into a more general epidemic. I assume
the periodic resurrections are due to its cheapness, this makes it again
attractive to introduce once the re-immunised engineers have moved on.
Hot air levelling applies solder to the bare copper tracks by making a
proper solder joint, an intermetallic bond. This solder layer is melted on
reflow and simply blends with the additional solder from the paste or wave.
The solder joint has already been made using the very powerful levelling
fluxes.
The problems with HASL can be
1) AS the protective layer is soldered it can have the usual meniscus of a
solder joint so the pads are not flat.
2) IF the air knives are adjusted on the vigorous side the pads will be very
flat because the meniscus has been blown off. Unfortunately this exposes the
intermetallic layer and this oxidises to unsolderable.
Lead free HASL is usually flatter than Sn/Pb, and if done by a competent
supplier is excellent for protection and soldering. It's a minority sport
these days because HASL was already in decline at intro of Pb free and with
sentiment already against it mostly faded out.
It's likely that just heating a PCB on a hot plate would reveal whether the
finish is solder nickel, or immersion Sn.
If its solder it will quickly melt, possibly dewetting as it does so.
It is Ag it will darken in minutes
And if nickel nothing much should happen at all. Possibly a slight coffee
stain.
Regards
Mike Fenner
Bonding Services & Products
M: +44 [0] 7810 526 317
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Wednesday, October 02, 2013 3:09 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Hi,
The following is not exactly a "Rigorous" description, I would appreciate
and additional comments.
ImSN is simply tin. HASL is solder and it typically thicker. My observation
is both tin and HASL will form oxides but tin does so faster and the oxides
on InSN do not wet as easily as a HASL surface. (Can anyone confirm / deny
this?). With HASL the layer of solder melts during reflow and mixes with the
applied solder. Oxides on the HASL surface are mechanically moved.
With ImSn the oxides are firmly on the surface, flux must remove them
chemically before a wetting process can take place.
It is one thing to have a thin layer of metal to which solder can flow and
attach while it is another thing to already have solder on the board.
Solder doesn't "Solder To" HASL. HASL is solder. It has been my experience
nothing solders better than Solder.
Now of course there are a lot of different solder types, and if solder is
completely removed from a pad leaving behind an inter-metallic surface then
that surface is no longer "Solder". I am biased towards HASL, it has simply
worked out so well for me. It is not "Flat" but who cares after the paste is
printed. And I have never see a board with ALL the solder removed.
I still don't understand why folks would use InSN when they could have used
HASL? Please explain. (Other than InSN is FLAT.)
Thanks,
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez
Sent: Wednesday, October 02, 2013 7:09 AM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Folks,
I has a similar experience in which I questioned the use of HASL or ImSn
and did not receive any feedback. What is the difference between ImSn and
HASL besides the volume of solder left on the pads.
Victor,
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Wednesday, October 02, 2013 5:45 AM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Hi Steve-
From these limited pictures, the finish looks like ImSn: On the solder
sample picture, the parts of pads not wet by solder look matte to me. On
the close-up of the original picture, the "tarnish" area is more likely
copper being exposed as the thin layer of ImSn gets sucked into the solder
joint. (ImAg does the same thing if you don't have the right profile.)
If the pads on the boards "as delivered" are matte white, you're dealing
with either ImSn or ImAg. But most ImAg formulations wet a lot better than
most ImSn formulations because, as noted many times on this forum, Ag oxides
are easy to solder to, but Sn oxides can create lots of problems. However,
Ag oxides are visually trivial to detect, so if you wrap a fresh board up
with a peeled hardboiled egg, then you'll know in a few hours whether the
finish is ImAg.
Since Pb free HASL deposits mostly Sn, it may be difficult to discern
whether you're dealing with ImSn or HASL unless the finish is matte.
(Bright Sn plate is usually even less desirable than matte Sn since it grows
whiskers the fastest.) A cross-section should show a "considerable"
thickness of Sn if the finish is really HASL (>5 microns). I can't remember
if IPC specs have a minimum thickness for HASL coating--it's a finish I only
use rarely. And, of course, HASL should show "puddling".
An interrupted ENIG would usually have enough Ni on it that you could
clearly see it in a cross-section at around 200X. And, of course, there are
several analytic techniques and instruments available at external labs to
see whether you are looking at Sn or Ni.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, October 01, 2013 7:14 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Okay here's another picture. I printed a solder sample and sent it through
the oven and this is what it looked like:
http://stevezeva.homestead.com/SolderSample.jpg
You can see all the non-wetting going on. The board was profiled and it was
fine. We are using a FCT Assembly WS889 SN100C solder paste.
What is even more strange is that the first side of the board looked okay,
but the second side looks like in the photos, like the problem is confined
to one side of the board if that's possible.
The solder sample was only reflowed once, but I printed the paste on the
side that was to be built last. Weird huh?
I know you can't tell anything for sure just from some photos, but it's the
strangest thing I've seen in a while...
Steve
-----Original Message-----
From: Steve Gregory
Sent: Tuesday, October 1, 2013 4:46 PM
To: [log in to unmask]; Steve Gregory
Subject: RE: [TN] HASL question
Here's one picture:
http://stevezeva.homestead.com/NonWetting.jpg
Look at the left pad and the tarnish that you see there, does that look like
tarnished nickel?
This is after the board was washed. I have places all over the board that
looks like this.
I'll post a picture of what it looks like after I printed a solder sample
with paste and sent it through the reflow over
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, October 1, 2013 4:16 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Ahhhh....it's what I figured. I think that's what happened. I'll have
pictures later.
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Tuesday, October 1, 2013 4:15 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
No. Not unless somebody screwed up.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, October 01, 2013 6:11 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Hi all,
It's funny that the HASL topic has come up today, because we just ran a
board today that supposedly was lead-free HASL finished, and man it looks
like crap!
Question; does a PCB get nickel before it gets Lead-free HASL'ed?
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Vladimir Igoshev.
PhD
Sent: Tuesday, October 1, 2013 2:23 PM
To: [log in to unmask]
Subject: Re: [TN] HASL question
Hello Victor,
It wouldn't solder in a first place.
--
Best regards,
Vladimir Igoshev. PhD
mailto:[log in to unmask]
SENTEC Testing Laboratory Inc.
11 Canadian Road, Unit 7.
Scarborough, ON M1R 5G1
Tel: (416) 899-1882
Fax: (905) 882-8812
www.sentec.ca
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
|
