AN10365_3
© NXP B.V. 2008. All rights reserved.
Application note
Rev. 03 — 22 April 2008
21 of 24
NXP Semiconductors
AN10365
Surface mount reflow soldering description
5.2.1 Device removal
In order to remove an IC package from the board, it must be heated; if possible, this must
be done as locally as possible, to avoid heating the surrounding board and components.
Packages with leads at a relatively large pitch may first be removed from the board by
cutting the leads, after which only the leads must be de-soldered. This can be done with a
soldering iron.
IC packages without leads must be heated entirely, for removal. Heat can be supplied
using a hot air gun, a soldering iron, or focused infrared energy, depending on the
package type and availability. If necessary, the bottom of the board can also be heated.
The temperature to which the package solder joints should be heated depends on the
solder that was originally used, and it is best to keep the temperature as low as possible,
just above the melting point of the solder alloy used.
As soon as the solder has melted, the IC package is lifted from the board using a vacuum
wand or tweezers; note that package removal should not be initiated until the solder has
melted entirely.
Re-use of removed IC packages is not recommended.
5.2.2 Site preparation
After the device has been removed, the board area must be prepared for the new device.
Prepare the site by removing any excess solder and/or flux remains from the board.
Ideally this can be done on an appropriate de-soldering station, using solder wick or an
alternative method.
After most of the solder has been removed from a solder land, a very thin layer of solder
will be left, on top of a few intermetallic layers. In the case of Cu boards, for example, there
will be layers of Cu
3
Sn, Cu
6
Sn
5
, and finally solder, on top of the Cu. The top layer of
solder is easily solderable.
If, however, the pad is heated too much during removal of the rejected IC package, and
during site preparation, the top two layers will also be converted into Cu
3
Sn; in that case,
there will only be the Cu
3
Sn intermetallic layer on top of the Cu. Unfortunately, Cu
3
Sn is
hardly wettable. As a result, it will become very difficult to solder the replacement package
at this location. Therefore, care must be taken during reject package removal and site
redress, that the solder lands are heated not more than necessary.
Fig 20. Overheating and Cu
3
Sn formation
001aac848
Cu
Cu
3
Sn
Cu
6
Sn
5
solder
Cu
Cu
3
Sn
too hot for too long
normal
