PRECONDITIONING OF SMT PACKAGES - National Semiconductor

Preconditioning of SMT Packages

Introduction

National Semiconductor, as an integrated circuit manufac-

turer, is compelled to provide high quality, reliable surface

mount components to many industries such as automotive,

telecommunications, avionics, and computing. The inte-

grated circuit components are subjected to one common

process of surface mounting. Surface mounting entails plac-

ing components on a printed circuit board (PCB) with flux,

and then passing these boards through a furnace to a tem-

perature great enough to melt the solder on the leads. Upon

cooling the solder solidifies, attaching the components to the

PCB.

The surface mount process may seem simple, but many

problems can arise. For instance, moisture absorbed by the

plastic packages can induce cracking. Such cracking is

caused by extreme internal pressure created by moisture va-

porizing under the hot temperature of solder reflow condi-

tions. Once cracked, the surface mount package reliability is

compromised, leading to potential corrosion of the device by

moisture and ionic contaminants.

Preconditioning and Reliability

Preconditioning is a simulation of the surface mount process

that is typically done at the PCB assembly houses. At Na-

tional Semiconductor preconditioning is done prior to the fol-

lowing reliability tests: temperature-humidity bias, autoclave,

and temperature cycle. The preconditioning flow that is used

is given in

Figure 1

The moisture soak conditions for preconditioning are depen-

dant on the moisture sensitivity level (MSL), as defined by

the JEDEC/IPC joint industry standard, J-STD-020A. These

soak conditions are given in

Table 1

TABLE 1. Preconditioning Soak Requirements

Moisture Sensitivity

Level (MSL) Standard Soak

Conditions Accelerated Soak

Conditions Customer Floor Life

1 85˚C/85%RH, 168 hr Same as standard Unlimited

2 85˚C/60%RH, 168 hr Same as standard 1 year

2a 30˚C/60%RH, 840 hr

(Note 1) 60˚C/60%RH, 150 hr

(Note 1) 4 weeks

3 30˚C/60%RH, 336 hr

(Note 1) 60˚C/60%RH, 70 hr

(Note 1) 168 hours

4 30˚C/60%RH, 96 hr(Note

2) 60˚C/60%RH, 20 hr(Note

2) 72 hours

Note 1: A Manufacturer’s Exposure Time (MET) of 7 days is assumed for Levels 2a and 3. Note that this differs from the default value of 24 hours given in

J-STD-020A.

Note 2: An MET of 24 hours is assumed for Level 4.

For a given reflow profile, the small packages on the PCB

will reach a higher peak temperature than the large pack-

ages on the same PCB. This difference in peak temperature

is due to the fact that the large packages have greater ther-

mal mass (heat capacity) than the small packages; thus their

temperature rise is more limited. For preconditioning, the

Start

↓

Electrical Test

↓

TMCL: 5 cycles -40/+60˚C

↓

Bake: 16 hours at 125˚C

↓

Moisture soak (see Table 1)

↓

Solder Reflow (see Tables 2 & 3): 3

passes Vapor Phase

3 passes Convection/IR

↓

Flux Immersion Room Temperature, 10

seconds

↓

Rinse & Dry

↓

Electrical Test

↓

Reliability Testing

FIGURE 1. Preconditioning Flow for Surface Mount

Packages

August 1999

Preconditioning of SMT Packages

large packages are reflowed at a peak body temperature of

220˚C (or 215-219˚C if vapor phase reflow is used), whereas

the small packages are reflowed at a peak temperature of

235˚C.

Table 2

defines the peak temperature used for the

various packages at National Semiconductor.

Table 3

de-

fines the details of the reflow profiles for each of the two pos-

sible peak reflow temperatures and for each of the two reflow

methods.

TABLE 2. Peak Reflow Temperatures

Peak Reflow Temperature Package Types

220˚C (+5/-0˚C) - Conv./IR or 215 - 219˚C

- VPR PQFP/TQFP ≥2mm thick OR ≥20x20mm body size, PLCC, SO /=

300 mil wide, SOJ ≥20 leads, PBGA, Multi-chip Packages

235˚C (+5/-0˚C) - Conv./IR PQFP/TQFP <2mm thick AND <20x20mm body size, SO <300

mil width, SO-EIAJ, TSOP, TSSOP, SSOP, TO-263, SOT, MSOP,

SC-70, CSP

Note 3: Conv./IR refers to convection/infrared reflow; VPR refers to Vapor Phase Reflow

TABLE 3. Reflow Profiles

Convection or IR/Convection VPR (Note 4)

Average ramp-up rate (183˚C to Peak) 3˚C/second max 10˚C/second max

Preheat temperature 125(±25)˚C 120 seconds max

Temperature maintained above 183˚C 60-150 seconds

Time within 5˚C of actual peak

temperature 10-20 seconds 60 seconds

Peak temperature range 220 +5/-0˚C

235 +5/-0˚C

215 - 219˚C

Ramp-down rate 6˚C /second max 10˚C/second max

Time 25˚C to peak temperature 6 minutes max

Note 4: At National Semiconductor Vapor Phase Reflow (VPR) is only used for packages requiring the lower peak reflow temperature (see Table 3).

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whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect its

safety or effectiveness.

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Preconditioning of SMT Packages

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