Jim,
See my answers/comments below.
-----Original Message-----
From: West, Jim [mailto:[log in to unmask]]
Sent: August 24, 2001 3:09 PM
To: [log in to unmask]
Subject: [TN] Determining the Inner metallic bond on a surface mount
connector
Hi,
I had Steve put a picture on his web-site.
Go to: <http://stevezeva.homestead.com/>
Labeled "Connector footprint"
Sorry for the picture not being a closer look. Hopefully you'll understand.
In this picture, you will see the leftovers of me removing a 41 position
connector with pliers. What I was trying to do was see what kind of solder
connection I had. I wasn't sure what was going to happen when doing this,
but what you see in the picture is that most of the pads came away from the
substrate with the connector leads,
>This USUALLY a sign of pretty strong solder joints. Of course they could
be strong and brittle, but not usually.
and 14 positions had solder left behind that looked porous.
>See below answer to number 3.
Before I removed the connector, all 41 positions had evidence of wetting
from the pad up the sides of the leads. I had some doubt of the joint
because the solder did not show as good of a wetting action between the pad
and the toe of the lead (by the way, this is a gull-wing style).
>Pad to toe fillets will only provide you about 5% of your solder joint
strength, so not an issue in that regard. If you >were getting good toe
fillets and now are not, then you need to check and see if something changed
in the process or in the >lots of components.
We use IPC class II inspection criteria. This same board passed our
electrical continuity test.
Questions:
1. Does pulling of the connector tell me anything about the inner-metallic
bond between the lead and pad?
>No, at least not at this magnification! And this is a pretty gross method
(two of the several meanings of this word implied >:) ) If you were
dealing with an Alloy 42 lead device where they had not removed the stamping
oil from the lead frame >before putting on the tin/lead finish, you
certainly could tell with this method, but that's about it. Perhaps with an
SEM >someone might be able to tell something from the fractography of the
joint, but I couldn't. Other than that I don't think >this tells you much.
Comments anyone?
2. Is this a fair way to determine the bond?
>No. It gives you no strength values, force/torsion etc. differ from one
time to the next... A cross-section would >certainly tell you if
the intermetallic was there and how thick it was.
3. Because the left over joint looked porous, does this mean the solder did
not flow completely?
>No
Or, did the pulling of the lead away from the pad cause the porous look?
>Absolutely not.
>If the solder join is mushy like solder paste or you can still move
individual solder balls within the solder joint, then it did not completely
reflow. Voiding in solder joints is very common. You need to have a
standard about the amount of voiding you will allow and where in the joint
it is. If 80% of the area of your heel fillet is one big void then you have
a problem. If it were 5% or you had 25% of your foot area voided I would
not be overly concerned. I am not able to tell you exactly what your
standard should be.
I'm just trying to determine if I have a good joint, but we don't have any
way of verifying the joint except by using the IPC guidelines.
>Well either stick to the visual standard or invest in some good
cross-sectioning equipment and then the next step goes from there $$$ !!
Thanks for your help!
>Hope this was worth reading.
Jim
regards,
Bev Christian
Research in Motion
----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
