MCR08B, MCR08M

http://onsemi.com

8

SOLDERING PRECAUTIONS

The melting temperature of solder is higher than the rated

temperature of the device. When the entire device is heated

to a high temperature, failure to complete soldering within

a short time could result in device failure. Therefore, the

following items should always be observed in order to

minimize the thermal stress to which the devices are

subjected.

• Always preheat the device.

• The delta temperature between the preheat and

soldering should be 100

°C or less.*

• When preheating and soldering, the temperature of the

leads and the case must not exceed the maximum

temperature ratings as shown on the data sheet. When

using infrared heating with the reflow soldering

method, the difference should be a maximum of 10

°C.

• The soldering temperature and time should not exceed

260

°C for more than 10 seconds.

• When shifting from preheating to soldering, the

maximum temperature gradient should be 5

°C or less.

• After soldering has been completed, the device should

be allowed to cool naturally for at least three minutes.

Gradual cooling should be used as the use of forced

cooling will increase the temperature gradient and

result in latent failure due to mechanical stress.

• Mechanical stress or shock should not be applied

during cooling.

* Soldering a device without preheating can cause

excessive thermal shock and stress which can result in

damage to the device.

TYPICAL SOLDER HEATING PROFILE

For any given circuit board, there will be a group of

control settings that will give the desired heat pattern. The

operator must set temperatures for several heating zones,

and a figure for belt speed. Taken together, these control

settings make up a heating “profile” for that particular

circuit board. On machines controlled by a computer, the

computer remembers these profiles from one operating

session to the next. Figure 19 shows a typical heating

profile for use when soldering a surface mount device to a

printed circuit board. This profile will vary among

soldering systems but it is a good starting point. Factors that

can affect the profile include the type of soldering system in

use, density and types of components on the board, type of

solder used, and the type of board or substrate material

being used. This profile shows temperature versus time.

The line on the graph shows the actual temperature that

might be experienced on the surface of a test board at or

near a central solder joint. The two profiles are based on a

high density and a low density board. The Vitronics

SMD310 convection/infrared reflow soldering system was

used to generate this profile. The type of solder used was

62/36/2 Tin Lead Silver with a melting point between

177 –189

°C. When this type of furnace is used for solder

reflow work, the circuit boards and solder joints tend to

heat first. The components on the board are then heated by

conduction. The circuit board, because it has a large surface

area, absorbs the thermal energy more efficiently, then

distributes this energy to the components. Because of this

effect, the main body of a component may be up to 30

degrees cooler than the adjacent solder joints.

STEP 1

PREHEAT

ZONE 1

“RAMP”

STEP 2

VENT

“SOAK”

STEP 3

HEATING

ZONES 2 & 5

“RAMP”

STEP 4

HEATING

ZONES 3 & 6

“SOAK”

STEP 5

HEATING

ZONES 4 & 7

“SPIKE”

STEP 6

VENT

STEP 7

COOLING

200

°C

150

°C

100

°C

50

°C

TIME (3 TO 7 MINUTES TOTAL)

TMAX

SOLDER IS LIQUID FOR

40 TO 80 SECONDS

(DEPENDING ON

MASS OF ASSEMBLY)

205

° TO

219

°C

PEAK AT

SOLDER

JOINT

DESIRED CURVE FOR LOW

MASS ASSEMBLIES

DESIRED CURVE FOR HIGH

MASS ASSEMBLIES

100

°C

150

°C

160

°C

170

°C

140

°C

Figure 19. Typical Solder Heating Profile