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BGA welding process principle
BGA welding process principle
different manufacturers of BGA rework station takes a slightly different, but roughly the same. The concept
of temperature curve is first introduced here. The ball on BGA, divided into two kinds of lead-free and lead.
A melting point lead solder at 183 to 220 DEG C, melting point lead-free solder ball at 235 to 245 degrees
Welding is divided into preheating, insulation, return, cooling four interval (different BGA rework do stations
with slightly different) regardless of lead solder and lead-free solder, solder melting stage is in the
recirculation zone, just the temperature vary, before the return curve can be as a slow rise of temperature
and heat insulation of the process. To understand this basic principle, any BGA rework station can and so on.
Here, a few warm areas:
Preheat zone: also called the slope zone, which is used to raise the temperature of the PCB from the ambient
temperature to the required temperature. In this area, heat circuit board and components are different, the
actual temperature to improve their rate of different. Circuit boards and components of the temperature
should be no more than 2 ~ 5 degrees per second rate of continuous rise, if too fast, will have a thermal
shock, circuit boards and components are likely to be damaged, such as ceramic capacitors of minor cracks.
The temperature rise is too slow, will paste temperature excessive, solvent evaporation is not sufficient,
affecting the quality of welding. The preheating zone of the furnace generally accounts for 15 ~ 25% of the
length of the heating area.
Insulation: sometimes called dry or wetted area, the area accounted for the general heating area of 30 ~ 50%.
The main purpose of the active region is to make the temperature of the components on the PCB tend to be
stable, as far as possible to reduce the temperature difference. In the region given enough time the large heat
capacity of the components of temperature catch smaller components, and solder paste flux are guaranteed
sufficiently volatile. At the end of the active zone, the oxide on the pad, solder ball and element pins is removed,
and the temperature of the entire circuit board reaches a balance. It should be noted that all components on
the PCB should have the same temperature at the end of this area, otherwise it will enter the return flow area
will be due to the different parts of the temperature is not produced a variety of bad welding phenomenon.
Generally common active temperature range of 120 to 150 DEG C, if the active region temperature setting is
too high, the flux (paste) don't have enough time activity, the slope of the temperature curve is a to increase
the slope. Although some solder paste manufacturers allow activity during the temperature increase, but a
desired temperature profile should be stable temperature.
Reflux zone: sometimes called the peak area or the final temperature rise. The area is to increase the
temperature of the PCB to the recommended peak temperature. Active temperature is always lower than the
melting point of the alloy, while the peak temperature is always on the melting point. The peak temperature
range of typical solder alloy melting point temperature plus 40 degrees Celsius, 20 - is the recirculation zone
work time 50s. The temperature setting is too high will make the temperature rise slope longer than every 2
to 5 DEG C, or the peak reflow temperature is higher than the recommended, or to work on time too long can
lead to PCB's too curly, delamination or burning, and damage to components of integrity. The peak temperature
of reflux ratio recommended low, work time is too short may appear cold welding defects.
Cooling zone: solder paste in the area of tin alloy powder has melted and sufficient wetting connected surface,
should be exhausted as soon as possible speed for cooling, so that there will be a help in the formation of crystal
alloy, bright spot, and has good appearance and low contact angle. Slow cooling can cause the circuit board to
more breakdown of impurities into the tin, resulting in a dark and rough solder joints. In extreme cases, it may
cause a tin to be caused by a tin, and a decrease in the binding force of the solder joint. The cooling rate is
generally 3 ~ 10 / S.
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