More - german placed company:
Soldering BGAs by Hook or by Crook
Dr. Hans Bell, rehm Anlagenbau GmbH + Co. KG and Marco Kämpfert, TechnoLab:
http://www.technolab.de/_en/downloads/bgabellkaempferte.pdf
Quote: "...
In addition to the popcorn effect, non-uniform heat input to the PCB or
the PBGA may cause delaminating or twisting, as well as warping of the
PBGA substrate and the PCB. The substrate is warped like a bimetallic
strip. Shorts and opens often result, as is shown very nicely in
Rupprecht’s photos [5].
...
Heat from below, encompassing the entire surface of the PCB, is ideal
for rework. The amount of heat input from below may even exceed the
amount from above in some cases. If (nearly) eutectic tin-lead solder
pastes and soft balls are utilized (BGA balls made from soft solder),
the customary reflow parameters are recommended for ball solder joints:
30 to 60 seconds at 200 to 220° C.
The best measuring method involves the use of a thermocouple which is
placed directly at the ball by inserting it through a suitable
drill-hole, as shown in figure 5.2.
...
In order to avoid the popcorn effect (the formation of fissures inside
the BGA during exposure to heat), it is absolutely essential to observe
the specified humidity rating (see JEDEC), as well as the working
conditions which result therefrom. It is often advisable to temper BGAs,
and in particular PBGAs, before they are processed (see IPC – SM – 786).
Formation of Voids
A frequently observed phenomenon is the formation of voids in the solder
joints, as is shown in figure 5.4.
...
Lee provides an excellent overview regarding the formation and
prevention of voids when soldering PBGAs [8]. Voids can usually be
traced back to outgassing which is caused by flux in the solder paste. A
small number of voids does not effect solder joint reliability. To a
certain degree, it has even been observed that reliability is improved
by voids because they prevent the spreading of fissures. However, under
no circumstances should any faith be placed in this phenomenon.
..."
br,
Glenn
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