User-Manual-NeoDen-IN12-Reflow-oven.pdf - 第15页

14 melting point of alloy and the desired m aximum temperature of reflow. Before you start, you m ust have a basic understanding of the ideal temperature wave. Theoretically, the ideal wave consists of four parts or sect…

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

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