Tombstoning is not generally associated with which end of the component
enters the reflow zone first. The parts in question are generally too small
for this to be a concern.

Rather, the problem is when the wetting forces act on one end of the
component. This might caused by a thermal imbalance, more copper on side of
the land pattern. But poor solderability can cause this too.

If you are concerned about thermal imbalances the best approach is to slow
the conveyor down and give your convection oven a chance to do its magic. Do
not exceed the preheat times suggested for your paste but work low and slow
using long side of the paste profiles.

As for the cool down theory, IMHO, it shouldn't be causing problems. The
lead and land of the component should have wetted and the forces should not
be acting on the part as the assembly passes into the cooling zone. Again,
the slower conveyor will work in your favor and give the solder time to act
on the component and land.

Guy Ramsey
Senior Lab Technician / Instructor


E-Mail: [log in to unmask] <mailto:[log in to unmask]>
Ph: (610) 362-1200 x107
Fax: (610) 362-1290



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Vinit Verma
Sent: Monday, August 27, 2001 7:17 AM
To: [log in to unmask]
Subject: [TN] Tombstoning


Hi Technetters!

A posting from my side after a long time.

I wish to get some information from those who have experienced tombstoning
and that too because of improper reflow profiles. My query is that an
improper profile can affect tombstoning in two ways! One: During the
entrance of the PCB into liquidous. That
is, the entrant pad enters liquidous first and hence due to surface tension
lifts the component since the other pad has still not entered liquidous.
Two: During the entrance to cooldown.  The earlier exiting pad into cooldown
cools down first and hence
contracts the solder and hence lifts the component since the other pad is
still under liquid condition.

My question is: How to figure out as to which area of the profile is to be
attacked? Assuming I don't have a realtime cam to view both the situations!

Forgive me if my question sounds stupid.

Thanks in anticipation.

Regards
Vinit Verma
Philips India Ltd., 9th Floor, Ambadeep Building,
14, Kasturba Gandhi Marg, Connaught Place
New Delhi-110001
Tel: -+91-11-3738791, 3324547; Fax: +91-11-3322859
Mobile: +91-9810208557
[log in to unmask]

---------------------------------------------------------------------------------
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
---------------------------------------------------------------------------------