User-Manual-NeoDen-IN12-Reflow-oven.pdf - 第14页

13 components. An optimized reflow t emperature wave is one of the most im portant factors to obtain high quality solder joints in printed circuit board (PCB) assembly using surface mount components. The temperature wave…

5.Temperature wave setting principle

5.10 what is the heating unit temperature?

the real-time temperature of the heating plate.

5.11 what is oven temperature?

the air temperature between the chain surface and the heating plate.

5.12 what is PCB surface temperature?

the temperature of component soldering feet when PCB is soldered. (the guide

temperature on the soldering wave provided by the solder paste manufacturer

refers to the PCB surface temperature.)

When working, the temperature displayed in the temperature zone on the panel is

the actual temperature of the heating unit, which does not represent the temperature

in the oven and the actual temperature of the plate surface. Therefore, the

temperature displayed will be about 20-40 degrees higher than the temperature in

the oven. The actual use is related to the chain speed, PCB size, thickness, material

and component density.

5.1 Principle of reflow soldering and temperature wave

When the PCB enters the heating zone (dry zone), the solvent and gas in the solder

paste evaporate. At the same time, the flux in the solder paste moistens the pad,

component ends and pins. The solder paste softens, collapses and covers the pad,

isolating the pad and component pins from oxygen. When the PCB enters the heat

preservation area, the PCB and components are fully preheated to prevent the

damage of PCB and components caused by the rapidly raise of temperature.When

the PCB enters into the soldering area, the temperature rises rapidly to make the

solder paste melt. The liquid soldering tin enters the cooling area to solidify the

solder joint and complete the reflow soldering. The temperature wave is the key to

ensure the soldering quality. The temperature rise slope and peak temperature of

the actual temperature wave and the solder paste temperature wave should be

basically consistent. Before 160 ℃, the heating rate should be controlled at about

℃

/ s. if the heating rate is too fast, on the one hand, the components and PCB

will be heated too fast, which will damage the components and easily cause PCB

deformation; on the other hand, the solvent in solder paste will volatilize too

quickly, which will easily splash metal components and produce solder balls. The

peak temperature is generally set at 20 ℃ - 40 ℃ higher than the melting

temperature of solder, and the reflow time is 10s-60s. If the peak temperature is

low or the reflow time is short, the soldering will not be sufficient, and in serious

cases, the solder paste will not melt; if the peak value is too high or the reflow time

is long, it will cause metal powder oxidation, affect the soldering quality, and even

damage the components and PCB.

5.2 Setting of temperature wave

Set according to the temperature wave of solder paste and the soldering principle

provided above. Solder paste with different metal content should have different

temperature wave, and the reflow temperature wave should be set according to the

temperature wave provided by the solder paste manufacturer. In addition, the

temperature wave is also related to the density and size of the heated PCB and

components.

An optimized reflow temperature wave is one of the most important factors to

obtain high quality solder joints in printed circuit board (PCB) assembly using

surface mount components. The temperature wave is a function of the temperature

applied on the circuit assembly to time. During the reflow process, it represents the

temperature at a specific point on the PCB to form a wave at any given time.

Several parameters affect the shape of the wave, the most important of which is the

belt speed and the temperature setting of each zone. The belt speed determines the

duration of exposure of the board to the set temperature of each zone. Increasing

the duration allows more time for the circuit assembly to approach the temperature

setting of the zone. The total duration of each zone determines the total processing

time.

The temperature setting of each zone affects the temperature rising speed of PCB,

and high temperature produces a large temperature difference between PCB and

zone temperature. The set temperature of the increase zone allows the board to

reach the given temperature faster. Therefore, it is necessary to make a figure to

determine the PCB temperature wave. Next is the outline of this step to generate

and optimize the graphics.

The following equipment and Auxiliary tools are required before starting the wave

procedure

High precision temperature profiler ( with IN12), thermocouple (with IN12), tools

for attaching thermocouple to PCB (mainly high temperature tape) and solder paste

parameter table.

There are several ways to attach thermocouple to PCB. The better way is to use

high temperature solder such as silver / tin alloy, and the solder joint should be as

small as possible.

Another acceptable method is fast, easy, and accurate enough for most applications.

A small amount of thermal compound (also known as thermal paste or grease,

which is often used on computer CPU or graphics card CPU) spots cover the

thermocouple, and then stick it with high-temperature tape (such as Kapton).

Another way to attach thermocouple is to use high-temperature adhesives, such as

cyanoacrylate adhesives. This method is usually not as reliable as other methods.

The attachment position should also be selected. It is usually better to attach the

thermocouple tip between the PCB pad and the corresponding component pin or

metal end.

(Figure 1. Attach thermocouple tip between PCB pad and corresponding

component pin or metal end)

The table of solder paste characteristic parameters is also necessary, and the

information contained in it is very important to the temperature wave, such as the

desired duration of temperature wave, the active temperature of solder paste, 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