15
(Fig. 2. Theoretically, the ideal reflow wave consists of four zones, the first three
zones are heated and the last zone is cooled.)
It is also used to warm up the ambient temperature of the PCB from the active zone.
In this region, the temperature of the product rises continuously at a rate of no
more than 2-5
℃
per second
If the temperature rises too fast, it will cause some defects, such as tiny cracks in
ceramic capacitors. If the temperature rises too slowly, the solder paste will feel
too much temperature, and there is not enough time for PCB to reach the active
temperature. The preheating zone of the oven generally accounts for 25 ~ 33% of
the total length of the heating channel.
The active area, sometimes called dry or wet area, generally accounts for 33 ~ 50%
of the heating channel, and has two functions. The first is to sense the temperature
of PCB at a relatively stable temperature, allowing components of different quality
to be homogeneous in temperature, so as to reduce their considerable temperature
difference. The second function is to allow the flux to be activated and volatile
substances to volatilize from the solder paste. The general active temperature range
is 120 ~ 150
℃
. if the temperature of the active zone is set too high, the flux does
not have enough time to activate, and the slope of the temperature wave is an
upward increasing slope. Although some solder paste manufacturers allow some
temperature increase during activation, the ideal wave requires a fairly stable
temperature so that the PCB temperature is equal at the beginning and end of the
active zone. Some ovens on the market can not maintain a flat active temperature
wave. Choosing a oven that can maintain a flat active temperature wave will
improve the weld performance, and users will have a larger processing window.
The reflow zone is sometimes called the peak or final heating zone. The function
of this area is to increase the PCB assembly temperature from the active
temperature to the recommended peak temperature. The activation temperature is
always lower than the melting point of the alloy, and the peak temperature is
always at the melting point. The typical peak temperature range is 205 ~ 230
℃
. if
the temperature is set too high in this area, the temperature rise slope will exceed 2
~ 5
℃
per second, or the reflow peak temperature will be higher than recommended.
This situation may cause excessive crimping, delamination or burning of PCB, and
damage the integrity of components.
Today, the most commonly used alloy is Sn63 / Pb37, and this proportion of tin
and lead makes the alloy eutectic. Eutectic alloy is an alloy that melts at a specific
temperature. Non eutectic alloy has a melting range, not a melting point,
sometimes called plastic loading. All the examples described here refer to eutectic
tin / lead, which has a melting point of 183
℃
, because it is widely used.
The ideal cooling zone wave should be a mirror image of the reflow zone wave.
The closer to this mirror relationship, the closer the solid structure of solder joint is,
the higher the quality of solder joint is and the better the integrity of solder joint
is.
The first parameter to be considered for the temperature wave is the speed setting
of the conveyor belt, which will determine the time spent in the heating channel of
the PCB. Typical solder paste factory parameters require a heating wave of 3-4
minutes. Dividing the total heating channel length by the total heating temperature
sensing time is the accurate belt speed. For example, when the solder paste requires
four minutes of heating time, using six feet of heating channel length, the
calculation is: 6 feet
／
4 minutes = 1.5 feet per minute = 18 inches per minute.
Next, it is necessary to determine the temperature setting of each zone. It is
important to understand that the actual interval temperature is not necessarily the
display temperature of the zone. The display temperature only represents the