User-Manual-NeoDen-IN12-Reflow-oven.pdf - 第16页

15 always lower than the melting point of the all oy, and the peak temperat ure is always at the m elting point. The typical peak temperature range i s 205 ~ 230 ℃ . if the temperature i s set too high in this area, the …

melting point of alloy and the desired maximum temperature of reflow.

Before you start, you must have a basic understanding of the ideal temperature

wave. Theoretically, the ideal wave consists of four parts or sections, the first three

regions are heated and the last one is cooled. The more the temperature zone of the

oven, the more accurate and close the profile of the temperature wave can be

achieved. Most pastes can be successfully reflowed with four basic temperature

zones.

(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

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 tempera ture settin g

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

example, the automatic trigger of 38°C (100°F) allows the thermometer to start

working as soon as the PCB is put on the conveyor chain and into the furnace, so

that the thermocouple will not be triggered by mistake when it is handled by the

human.

Once the initial temperature profile is generated, it can be compared with the

profile recommended by the solder paste manufacturer or the profile shown in

Figure 2.

First, it must be verified that the total time from the ambient temperature to the

peak temperature of the reflux is compatible with the desired stay time of the

heating curve. If it is too long, increase the conveyor speed proportionally; if it is

too short, decrease the conveyor speed proportionally.

In the next step, the shape of the temperature profile must be compared with the

desired one (Figure 2). If the shape is not compatible, then compare it with the

following figures (Figures 3-6). Choose the most compatible temperature profile

compared to the actual shape of the profile. Should consider the deviation from left

to right (process sequence). For example, if there is a difference between the

preheating and soldering zones, first adjust the difference in the preheating zone

correctly. Generally, it is best to adjust one parameter at a time and test in run

before making further adjustments.Because a change in a given zone will also

affect the results of subsequent zones. We also recommend that novices had better

make small adjustments. Once you gain experience on a particular oven, you will

have a better "feel" to make more big adjustments.

Figure 3:Preheat too much/little soldering profile