Your customer is partially correct. The best storage is flash electrolytic gold on nickel, you can store at low temp for long time. You can use thicker electrolytic gold of 20 micro inches (if your flash got porosity need thicker to obtain full coverage), and pre tin the surface before use to avoid gold brittle IMC. The HASL got a thick IMC during process, if it is not clean properly, you can get excessive growth of IMC during storage (much more than Au finish). My 1,94 cents.
--------------------------
Sent using BlackBerry
----- Original Message -----
From: Uppina Nagaraj [mailto:[log in to unmask]]
Sent: Thursday, June 17, 2010 07:29 AM
To: [log in to unmask] <[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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