the space around CSP is very small.
d. Lay on BGA Flux Paste
Laying solder paste on the PCB has important influence on the result of BGA
Rework. It is convenient to lay solder paste on the PCB by selecting mould matched with
BGA. For CSP, there are three solder pastes for choice, including flux paste, clean-free and
water-soluble. If we choose the flux paste, the reflow time should be longer, if choose the clean-
free solder paste, reflux temperature should be lower.
(1) Mounting: The main purpose of mounting to make every BGA solder align to the PCB pad
with special equipment.
(2)Hot air reflow: Hot air reflow soldering is the key to the whole BGA Rework.
①
The curve of reflow soldering of BGA Rework should be similar with the original soldering
one. Hot air reflow soldering curve can be divided into four zones, including preheat zone, heating
zone, recirculation zone and cooling area. These four zones’ temperature and time parameter can be
set respectively, when it connects with the computer, these programs can be saved and transferred at
any moment.
②
In reflow soldering process, we must choose the right heating temperature and time of different
zones; at the same time, we should notice the heating speed. Commonly, the maximal heating rate
is not more than 6
℃
/s before 100
℃
, and the maximal heating rate is not more than 3
℃
/s
after 100
℃
. In the cooling zone, the maximal cooling rate is not more than 6
℃
/s, because
both of the exorbitant heating rate and cooling rate may cause damage to PCB and BGA,
which sometimes cannot be observed by unaided eyes. For different BGA and
different solder pastes, we should choose different heating temperature and time. For example,
reflux temperature of CBGA BGA should be higher than that of PBGA…choose the
higher reflux temperature. For no clean solder paste, its activity is lower than non-no clean solder
paste, so the soldering temperature should not be too high, and the soldering time should not be too
long, so as to prevent the oxidation of solder particle.
③
In hot air reflow soldering, the bottom of PCB board should be heated. There are two
purposes for this kind of heating. First, avoid the warping and deformation in one-side
heated PCB board; second, shorten the time of the solder paste melting. For the dimension pad
of BGA rework, this kind of heating on the bottom of PCB is especially important. There are
two ways of heating on the bottom of BGA rework equipment. One of them is hot air heating,
and the other is infrared ray heating. The advantage of hot air heating is the homogeneous heating,
which common Rework Techniques suggest to adopt this way of heating. What opposite with it is
the disadvantage of infrared ray’s inhomogeneous heating.
④
We should choose the right hot air reflux suction nozzle. The hot air reflux nozzle is non-
contact heating, which depend on the high temperature air current make every solder of joint on
BGA melt synchronously. For that reason, it ensures the steady temperature circumstance in the
whole of reflux process, and it protects the adjacent parts from damaging by the convective hot air’s
regeneration.
C. Installation
1.
Place
In order to ensure the validity of BGA Rework Station, the installation should meet the following
requirements.
a.
Away from inflammable and explosives;
b.
Away from water and other liquids;
c.
Ventilated, dry place;
d.
Stable and flat, free from tremor.
e.
Less dust;
f.
No heavy objects on the controlling box;
g.
Not affected by airflow of air conditioner, heater or ventilator.
6
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