MMBD717LT1

http://onsemi.com

4

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

100°C

150°C

160°C

140°C

Figure 5. Typical Solder Heating Profile

DESIRED CURVE FOR HIGH

MASS ASSEMBLIES

170°C

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 7 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.

SOLDER STENCIL GUIDELINES

Prior to placing surface mount components onto a printed

circuit board, solder paste must be applied to the pads. A

solder stencil is required to screen the optimum amount of

solder paste onto the footprint. The stencil is made of brass

or stainless steel with a typical thickness of 0.008 inches.

The stencil opening size for the surface mounted package

should be the same as the pad size on the printed circuit

board, i.e., a 1:1 registration.

TYPICAL SOLDER HEATING PROFILE

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.