IPC-7095C-2013.pdf - 第151页

T able 8-6 Accelerated T esting for End Use Environments Worst-Case Use Environment Accelerated T esting Use Category Tmin °C Tmax °C Δ T (1) °C t D hrs Cycles/ Y ear T ypical Y ears of Service Approx. Accept. Failure Ri…

solder joint reliability testing. There are multiple models

available and people use this to estimate the Pb-free solder

joint reliability, for their products and applications. There is

no universal agreement since these model are product-

speciﬁc.

The validation and qualiﬁcation tests should follow the

guidelines given in IPC-SM-785, Guidelines for Acceler-

ated Reliability Testing of Surface Mount Solder Attach-

ments. However, for large components with signiﬁcant heat

dissipation, for components of asymmetric construction,

and for small global CTE-mismatches, temperature cycling

tests are inadequate to provide the required information;

full functional cycling including external temperature and

internal power cycling is necessary.

8.8 Screening Procedures

8.8.1 Solder Joint Defects

The solder joint defects of

greatest reliability concern are those involving inadequate

wetting for whatever reason. Properly wetted solder joints

have adequate strength even for severe mechanical loading

conditions, without diminishing thermal fatigue resistance.

However, solder joints not properly wetted, can prema-

turely fail both as the result of mechanical and thermal

cyclic loading.

Voids in the solder joints are generally regarded as not

constituting a reliability threat. Possible exceptions are

large voids reducing the solder joint cross-section enough

to reduce a required thermal heat transfer function, and

voids in high-frequency applications where the voids can

cause signal deterioration.

BGA components having noncollapsible balls (high tem-

perature solder 90% Pb 10% Sn, with a melting point of

302°C) typically will have few or no induced voids

because the ball solder never melts during the reﬂow pro-

ﬁle.

8.8.2 Screening Recommendations Effective screening

procedures need to be capable of causing the failure of

latent solder joint defects, i.e., weak inadequately wetted

solder joints, without causing signiﬁcant damage to high

quality solder joints. The best recommendation is random

vibration (6-10 grams for 10-20 minutes), preferably at low

temperature, e.g., -40°C. This loading does not damage

good solder joints, but overstresses weakly bonded ones.

Thermal shock can also be successfully used; however,

some damage to good solder joints can be expected, par-

ticularly for larger components.

8.9 Accelerated Reliability Testing The validation and

qualiﬁcation tests should follow the guidelines given in

IPC-SM-785, Guidelines for Accelerated Reliability Test-

ing of Surface Mount Solder Attachments and/or IPC-9701,

Performance Test Methods and Qualiﬁcation Requirements

for Surface Mount Solder Attachments. Although acceler-

ated temperature cycling (ATC) is often conducted, for

some products, ATC needs to be combined with mechani-

cal shock and/or vibration testing. From a single set of ATC

test conditions or inadequate ATC test conditions may not

provide valid conclusions.

Accelerated reliability testing is carried out on design pro-

totypes, typically to failure or until a predetermined reli-

ability goal is achieved. The appropriate reliability goal can

be determined with an appropriate acceleration model (see

IPC-D-279 Design Guidelines for Reliable Surface Mount

Technology Printed Board Assemblies).

Once failure occurs, the resulting failure modes are ana-

lyzed as to the underlying failure mechanism(s). If it fails

expectations then corrective action is necessary. Either the

assembly process needs to be improved or the product

needs to be redesigned. In either case retesting may be

necessary after the corrective action has been implemented.

Recognizing that a matrix was needed to determine the

exact requirements and the testing necessary for perfor-

mance under various conditions, the IPC Product Reliabil-

ity Committee developed the following table, ‘‘Product

Categories and Use Environments.’’ Table 8-6 attempts to

relate seven product categories by typical application to the

thermal, mechanical, atmospheric, and electrical perfor-

mance requirements that they must meet during typical

manufacturing processes, storage, and during operation.

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Table 8-6 Accelerated Testing for End Use Environments

Worst-Case Use Environment Accelerated Testing

Use Category

Tmin

°C Tmax °C

ΔT

(1)

°C

hrs

Cycles/

Year

Typical

Years

Service

Approx.

Accept.

Failure

Risk %

Tmin

°C Tmax °C

ΔT

(2)

°C

min

1) Consumer 0 +60 35 12 365 1-3 1 +25 +100 75 15

2) Computers +15 +60 20 2 1460 5 0,1 +25 +100 75 15

3) Telecom -40 +85 35 12 365 7-20 0,01 0 +100 100 15

4) Commercial

Aircraft

-55 +95 20 12 365 20 0,001 0 +100 100 15

5) Industrial &

Automotive

Passenger

Compartment

-55 +95 20

&40

&60

&80

185

100

10 0,1 0 +100 100 15

& COLD

(3)

6) Military

Ground &

Ship

-55 +95 40

&60

100

265

10 0,1 0 +100 100 15

& COLD

(3)

7) Space

leo

geo

-55 +95 3

to 100

8760

365

5-30 0,001 0 +100 100 15

& COLD

(3)

8) Military

Avionics

-55 +95 40

&20

365

10 0,01 0 +100 100 15

& COLD

(3)

9) Automotive

Under Hood

-55 +125 60

&100

&140

1000

300

5 0,1 0 +100 100 15

& COLD

(3)

LARGE ΔT

(4)

& = in addition

ΔT represents the maximum temperature swing but does not include power dissipation effects; for power dissipation calculate ΔT; power dissipation can make

pure temperature cycling accelerated testing signiﬁcantly inaccurate. It should be noted that the cyclic temperature range, ΔT is not the difference between

the possible minimum, T

min

and maximum, T

max

, operational temperature extremes; ΔT is typically signiﬁcantly less.

All accelerated test cycles shall have temperature ramps, 20°C/minute and dwell times at temperature extremes shall be 15 minutes measured on the test

boards. This will give ~24 test cycles/day.

The failure/damage mechanism for solder changes at lower temperature; for assemblies seeing signiﬁcant cold environment operations, additional ‘‘COLD’’

cycling, from perhaps -40 to 0°C, with dwell times long enough for temperature equilibration and for a number of cycles equal to the ‘‘COLD’’ °C operational

cycles in actual use is recommended.

The failure/damage mechanism for solder is different for large cyclic temperature swings traversing the stress-to-strain -20 to +20°C transition region; for

assemblies seeing such cycles in operation, additional appropriate ‘‘LARGE ΔT’’ testing with cycles similar in nature and number to actual use is

recommended.

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9 DEFECT AND FAILURE ANALYSIS CASE STUDIES

The following clauses identify possible assembly anoma-

lies related to the assembly of BGA components. The

descriptions include post process failures related to the

mounting structure characteristics and the variation in the

solder ball used as the BGA termination. In many instances

attachment metallurgy are discussed especially if the char-

acteristics contributed to the joint failure. Final joint con-

ﬁgurations are also analyzed.

9.1 Solder Mask Defined BGA Conditions There are

two ways BGA Lands are deﬁned: solder masked deﬁned

(SMD) where the land size is larger than solder mask and

the molten BGA ball touches the solder mask after reﬂow.

The other method for designing the BGA land is called

etched or nonsolder mask deﬁned (NSMD) where the mask

opening is larger than the copper land and hence the ball

does not touch the solder mask after reﬂow. This condition

is shown in 9.1.1 and 9.1.2.

Solder mask-deﬁned lands may be used on corresponding

noncritical or functional pins, since SMD lands can help

minimize pad cratering defects. However, it should be

understood that solder mask deﬁned lands create additional

stress initiation sites and should be avoided on both the

interposer and the printed board land.

9.1.1 Solder Mask Defined and Nondefined Lands

Possible Cause

Interposer is solder mask defined; board is metal-defined. If the

two areas are very different, the stresses are not uniform and

cracks may occur at the solder mask defined side. Board land

pattern too large.

Potential Solution

Area of the two attachment conditions should be similar or

identical. In addition, solder mask defined lands create addi-

tional stress initiation sites and should be avoided on both the

interposer and the printed board land.

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