7

SMT Assembly

Reliable assembly of surface mount components is a

complex process that involves many material, process,

and equipment factors, including: method of heating

(e.g., IR or vapor phase reflow, wave soldering, etc.) circuit

board material, conductor thickness and pattern, type of

solder alloy, and the thermal conductivity and thermal

mass of components. Components with a low mass, such

as the SOT-323/-23 package, will reach solder reflow

temperatures faster than those with a greater mass.

Avago’s diodes have been qualified to the time-

temperature profile shown in Figure 21. This profile is

representative of an IR reflow type of surface mount

assembly process.

After ramping up from room temperature, the circuit

board with components attached to it (held in place with

solder paste) passes through one or more preheat zones.

The preheat zones increase the temperature of the

board and components to prevent thermal shock and

begin evaporating solvents from the solder paste. The

reflow zone briefly elevates the temperature sufficiently

to produce a reflow of the solder.

The rates of change of temperature for the ramp-up and

cool-down zones are chosen to be low enough to not

cause deformation of the board or damage to components

due to thermal shock. The maximum temperature in the

reflow zone (T

These parameters are typical for a surface mount

assembly process for Avago diodes. As a general

guideline, the circuit board and components should be

exposed only to the minimum temperatures and times

necessary to achieve a uniform reflow of solder.

Figure 21. Surface Mount Assembly Profile.

25

Time

Tp

tp

t

L

t 25

Ramp-up

ts

Ramp-down

Preheat

Critical Zone

T

Lead-Free Reflow Profile Recommendation (IPC/JEDEC J-STD-020C)

Reflow Parameter

Lead-Free Assembly

Average ramp-up rate (Liquidus Temperature (T

S(max)

to Peak)

3°C/ second max

Preheat

Temperature Min (T

S(min)

)

150°C

Temperature Max (T

S(max)

)

200°C

Time (min to max) (t

S

)

60-180 seconds

Ts(max) to TL Ramp-up Rate

3°C/second max

Time maintained above:

Temperature (T

L

)

217°C

Time (t

L

)

60-150 seconds

Peak Temperature (T

P

)

260 +0/-5°C

Time within 5 °C of actual Peak temperature (t

P

)

20-40 seconds

Ramp-down Rate

6°C/second max

Time 25 °C to Peak Temperature

8 minutes max

Note 1: All temperatures refer to topside of the package, measured on the package body surface