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June 2010

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From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Thu, 17 Jun 2010 13:47:03 +0100
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My initial instinctive response would be that HASL would normally give a
sufficiently long shelf life. Also that overall HASL (properly done) gives
the least solderability issues. Therefore a change from HASL probably would
not address their quality issues.
Of course they might have unreasonable concept of storage time. How long is
a long time for them?

Or they could have had a batch of PCB with improperly applied HASL, For
example the solder layer was levelled too thin, leading [in storage] to the
coating being all unsolderable intermetallics.  On an ENIG board the Au
layer is very thin and although intermetallic layers do not tend to form
between the Au and Ni, the Ni can oxidise below the very thin layer (<0.0X
um) of Au and become hard to solder.
In either case we should be talking long times here, up to years. So very
possibly their conditions of storage/packaging or time expectations are the
issue not type of finish. (Assuming this is not a one off batch related
issue).

When hand soldering you need to think of the heat input and not just
temperature. If you reduce tip temperature you will increase time to solder.
The thermal burden can therefore be more not less. 
You need a hand soldering specialist to answer this better, But a Sn/Pb with
MP of 183 needs a superheat to a min 210C preferably 220C to work. With a
tip temp of 210 you will be effectively waiting for the work to come to
equilibrium with the soldering iron, and will still not be hot enough. 
Solder flows to wards heat and onto solderable surfaces. If the component
leads are less massive than the circuit they will heat first. And solder
goes there. If you have pad solderability issues then that will not help.
Possibility a change in technique is required, but certainly a higher tip
temperature than 210. I would guess a min 250C, maybe 260 depending on the
quality of the iron temperature control. Tip shape is also important in
determining effective heat transfer and flow.


Regards
 
Mike



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Uppina Nagaraj
Sent: Thursday, June 17, 2010 12:30 PM
To: [log in to unmask]
Subject: [TN] Query on shelf life of PCBs & impact on solderability

Dear All,

I have 3 queries in all.....

1. One of our customer has changed the surface finish from HASL
   to ENIG. Their main reason for this change was that they
   have had a bad experience with storage of HASL finished boards
   (the reason for which is not shared with us). They had encountered
solderability problems on the HASL finished board which they
   had stored for a period of time.

   Their expectation with Gold plated surface is the storage
   of the PCBs can be for a longer period of time as compared to
   HASL finish.

   Initially they had a requirement for 1micron thick electrolytic gold, 
but later on they changed the requirement for an ENIG process with 
gold around 0.05 to 0.10 microns.

   Now my question......

   Is there any guideline on the shelf life of the PCBs (with regard to
solderability) of the following surface finishes....

   a) ENIG
   b) HASL
   c) Immersion Tin
   d) Electrolytic Ni and electrolytic gold with Gold thickness
      of 1.0microns

   Will vacuum packing help to extend the shelf life?

2. One of our customer is doing hand soldering of PCBs with ENIG finish.
   They're inserting a strand of tinned Copper wires from component side
and later on using soldering iron to do the soldering from the solder
side.

   The solder is able to flow up from solder side to component side, but
the solder is unable to complete wet the pad on the component side.

   I've attached the photos of example of the phenomenon. These photos are
without the wire, I've just used bare PCBs to do the soldering to
illustrate the phenomenon.


   Now my question....

   a) Will increasing the volume of the solder help in complete wetting
      of the pads on the component side?
   b) Any other way to get a full coverage on the pads on the component
      side.

3. This question is about the same customer which I've mentioned in
   para 2 above, is doing hand soldering. In one of the stage of
   component assembly, they're (hand) soldering a flexible PCB
   on the ENIG finished rigid PCBs.

   They have a unique practice during this stage of soldering. They've
explained to us that this particular component is very sensitive to
high temperature and hence the soldering iron temperature is set at
210Deg C. They're using Sn60/Pb40 solder for this soldering operation.
After the soldering iron achieves 210Deg C, they're  turning off the
power to the soldering iron and doing the soldering of the flexi PCB
onto the rigi PCBs. We'll not know the actual
   soldering iron temperature at the point of contact.

   They've complained to us that the soldering is poor.

   We know that the melting temperature of solder is around 183deg C. But,

   is there any guideline on the minimum temperature for soldering to get
a good flow of solder and completely wetted solder?

Thanks for your inputs in advance.

Best Regards,
U.Nagaraj
MICROPACK Ltd,
Bangalore, INDIA.






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