User-Manual-NeoDen-IN6-Reflow-oven.pdf - 第10页

8 The solder paste characteristic parameter table is also necessary. The information it contains i s very important to the temperature c urve, such as the desired temperatur e curve duration, s older paste a ctivity t em…

time is short, it will affect the welding quality, and serious is causing the solder paste does not

melt. If the peak temperature is high or the reflow time is long, the metal power will be

oxidized and affect the welding quality and serious is causing the component and PCB

damaged.

5.2 The set of the temperature wave

Set according to the temperature curve of solder paste and the welding principle provided

above. Solder paste with different metal content should use different temperature curve, and set

specific reflow temperature curve according to the temperature curve provided by solder paste

manufacturer. In addition, the temperature curve is also related to the density and size of the

heated PCB and components.

In the assembly of printed circuit board (PCB) using surface mount components, an optimized

reflow temperature curve is one of the most important factors to obtain high quality solder

joints. The temperature curve is a function of the temperature on the circuit assembly to the

time. In the reflow process, at any given time, it represents the temperature on a specific point

on the PCB to form a curve.

Several parameters affect the shape of the curve, the most important of which is the belt speed

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

exposed to the set temperature in each zone. Increasing the duration allows more time for the

circuit assembly to approach the temperature setting in that 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 the PCB and the zone. The set

temperature of the increase zone allows the board to reach the set temperature faster. Therefore,

a graph must be made to determine the temperature curve of PCB. Next is the outline of this

step, which is used to generate and optimize graphics.

Before starting the curve making process, the following equipment and auxiliary tools are

required: high precision temperature curve instrument (in6), thermocouple (in6), tool for

attaching thermocouple to PCB (mainly high temperature tape) and solder paste parameter table.

There are several ways to attach thermocouples to PCB, the better way is to use high-

temperature solder such as silver / tin alloy, and the solder joints are minimized.

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

amount of thermal compound (also called thermal conductive paste or thermal grease, which is

often used on computer CPU or graphics card CPU) spots cover the thermocouple, and then use

high-temperature tape (such as Kapton) to stick it.

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

cyanoacrylate adhesives, which are generally not reliable.

The attachment position should also be selected. It is usually best 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 solder paste characteristic parameter table is also necessary. The information it contains is

very important to the temperature curve, such as the desired temperature curve duration, solder

paste activity temperature, alloy melting point and the desired maximum reflow temperature.

Before we start, we must have a basic understanding of the ideal temperature curve. In theory,

the ideal curve consists of four parts or intervals, the first three zones are heated and the last

zone is cooled. The more the temperature range of the furnace, the more accurate and close to

the setting of the temperature curve. Most pastes can be reflowed successfully in four basic

temperature zones.

(Figure 2. Theoretically, the ideal reflow curve consists of four zones, the first three zones are

heated and the last zone is cooled.)

Preheating area, also known as slope area, is used to raise the temperature of PCB from the

ambient temperature to the required active temperature. In this area, the temperature of the

product rises continuously at the speed of no more than 2-5 ° C per second. If the temperature

rises too fast, some defects will be caused, such as the tiny crack of ceramic capacitor. If the

temperature rises too slowly, the solder paste will be over sensitive to the temperature, and

there is not enough time to make the PCB reach the active temperature. The preheating area of

the furnace generally accounts for 25-33% of the total length of the heating channel.

The active area, sometimes called drying or wetting area, generally accounts for 33-50% of the

heating channel. It has two functions. The first is to sense the temperature of PCB at a fairly

stable temperature, allowing different quality components to be homogeneous in temperature

and reducing their considerable temperature difference. The second function is to allow the flux

to be activated and volatile substances to evaporate from the paste. The general active

temperature range is 120 ~ 150 ° C. if the temperature of the active zone is set too high, the flux

does not have enough time to activate, the slope of the temperature curve is an upward

increasing slope. Although some solder paste manufacturers allow some temperature increase

during the activation, the ideal curve requires relatively stable temperature, so that the PCB

temperature is equal at the beginning and end of the active region. Some furnaces on the market

can not maintain a flat active temperature curve. Choosing a furnace that can maintain a flat

active temperature curve will improve weldability. Users have a larger processing window.

Reflow zone, sometimes called peak zone or final heating zone. The function of this area is to

increase the temperature of PCB assembly from the active temperature to the recommended

peak temperature. The activity temperature is always a little lower than the melting point

temperature of the alloy, and the peak temperature is always at the melting point. The typical

peak temperature range is 205-230 ° C. if the temperature setting in this area is too high, the

temperature rise slope will exceed 2-5 ° C per second, or the reflow peak temperature will be

higher than the recommended one. This may cause excessive crimping, delamination or

burning of PCB and damage the integrity of components.

Today, the most commonly used alloy is Sn63 / Pb37. This ratio of tin and lead makes the alloy

eutectic. Eutectic alloy is an alloy melted at a specific temperature, while non eutectic alloy has

a melting range, rather than a melting point, sometimes called plastic loading state. All

examples described in this paper refer to eutectic tin / lead, which is widely used and has a

melting point of 183 ° C.

The ideal cooling zone curve should be a mirror image of the reflow zone curve. The closer the

mirror image relationship is, the closer the solid structure of the solder joint is, the higher the

quality of the solder joint is, and the better the integrity of the joint is.

The first parameter to be considered in making the temperature curve is the speed setting of the

transmission belt, which will determine the time spent by the PCB in the heating channel. A

typical solder paste manufacturer's parameters require a heating curve of 3-4 minutes. Dividing

the total heating channel length by the total heating sensing time is the accurate transmission

belt speed. For example, when the solder paste requires a heating time of 4 minutes, using a six

foot 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 zone temperature is not necessarily the display temperature of this

zone. The displayed temperature only represents the temperature of the thermistor couple in the

area. If the thermocouple is closer to the heating source, the displayed temperature will be

relatively higher than that in the area. The closer the thermocouple is to the direct channel of

PCB, the displayed temperature will be more able to reflect the temperature in the area. It is

advisable to consult with the furnace manufacturer to understand the relationship between the

clearly displayed temperature and the actual interval temperature. In this paper, we will

consider interval temperature rather than display temperature. Table 1 lists the interval

temperature settings for typical PCB assembly reflow.

Table 1 temperature setting of typical PCB return section

Area

Temperature Setting

Actual plate temperature at the end of zone

Preheat

210°C(410°F)

140°C(284°F)

Active

177°C(350°F)

150°C(302°F)

Reflow

250°C(482°C)

210°C(482°F)

After the speed and temperature are determined, they must be input to the furnace controller.

Once all parameters are input, start the machine, and after the furnace is stable (that is, all the

actual displayed temperatures are close to the set parameters), the curve can be made. The next

PCB is put into the conveyor belt, triggering the thermometer to start recording data. For

convenience, some thermometers include a trigger function to automatically start the

thermometers at a relatively low temperature, which is slightly higher than the human body

temperature of 37 ° C (98.6 ° f). For example, an automatic trigger at 38 ° C (100 ° f) allows

the thermometer to work almost as soon as the PCB is put into the conveyor belt and enters the

furnace, so as not to cause false triggering when the thermocouple is handled by hand.

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 confirmed that the total time from ambient temperature to reflow peak

temperature is in line with the desired heating curve residence time, increasing the conveyor