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bga Ball Grid Array Package 2
The 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
Here are the lead solder balls and no temperature curve by shot welding.
From the above two curves can be seen, welding can be roughly divided into preheating, heat preservation,
reflux, 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 as a slow heating and heat preservation 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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