NeoDen IN12, Руководство пользователя

Страница: 16 / 24

15
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
temperature of the thermocouple in the area. If the thermocouple is closer to the
heating source, the displayed temperature will be relatively higher than the interval
temperature. The closer the thermocouple is to the direct channel of PCB, the
displayed temperature will be more able to reflect the interval temperature. It is
advisable to consult the oven manufacturer to understand clearly the relationship
between the displayed temperature and the actual interval temperature. In this
paper, the interval temperature rather than the display temperature will be
considered. Table 1 lists the interval temperature settings for typical PCB assembly
reflow.
Table 1. Typical PCB return zone temperature setting
I n t e r v a l Interval temperature setting Actual plate temperature at the end of interval
Preheating
210
℃
140
℃
Activity
177
℃
150
℃
Reflow
250
℃
210
℃
The interval temperature is set to the actual plate temperature at the end of the
interval
After the speed and temperature are determined, they must be input to the oven
controller. Once all parameters are input, start the machine, and after the oven is
stable (i.e., all the actual displayed temperatures are close to the set parameters),
the wave can be started. The next PCB is placed in the conveyor belt, which
triggers the thermometer to start recording data.
For convenient using,some thermometer includes a trigger function to
automatically start the thermometer at a relatively low temperature, which is
typically slightly higher than the human body temperature of 37°C (98.6°F). For