IPC-TM-650 EN 2022 试验方法.pdf - 第753页

e) Organic Solderability Preservative (OSP): A surface finish for fine pitch featured PCBs that are often assembled using surface mount components (SMCs). The OSP surface finish provides very flat/co-planar land areas fo…

5.3 Data Handling and Analysis

5.3.1

Lognormal plots are recommended for plotting per-

cent of samples above an insulation resistance value, versus

insulation resistance. Use the log value of the insulation resis-

tance.

5.3.2 If lognormal plots are not used, a test circuit failure

shall be determined by more than a decade drop in insulation

resistance as a result of the applied bias. The baseline for the

decade drop shall be the average insulation resistance at 96

hours for each coupon (A-1, A-2, etc.).

5.3.3 Test board nets with less than 10 megohms insulation

resistance (high resistance short) after the 96-hour stabiliza-

tion shall be excluded, since these failures are due to poor

PTH hole quality or laminate capability.

5.3.4 The insulation resistance baseline (before bias condi-

tioning) value for a given daisy-chain net (same design spac-

ing) shall be the average resistance of those un-shorted

daisy-chain nets on all test boards in the valid sample group

as measured after the 96-hours stabilization period.

5.3.5 The percent failure rate for a given sample group and

subsequent test condition is the percent of test boards that

show more than a decade drop in resistance compared with

the baseline value for daisy-chain nets with the same design

spacing.

5.3.6 For a given sample lot, there may be binomial failure

distributions where assignable causes exist along with differ-

ent levels of capability.

5.4 Visual Examination After completion of the test, the

test boards shall be removed from the environmental cham-

ber and examined at 10X magnification for evidence of sur-

face insulation resistance failure (i.e., discoloration, corrosion),

handling or processing defects other than CAF.

5.4.1 Assignable Cause Where an assignable cause of

low insulation resistance can be properly attributed to a han-

dling or processing defect other than CAF (i.e., contamination

on the insulating surface of the board, scratches, cracks, or

other obvious damage affecting the insulation resistance

between the conductors), then such a value should be

excluded.

5.4.2 CAF Microsections Since CAF filaments form along

the interface between resin and the woven reinforcement,

these filaments can be very small and easily disrupted by a

relatively low current flow or other causes. Microsectioning to

observe CAF filaments can be a tedious process with a low

success rate.

5.5 Reporting Results

5.5.1

The percent failure rate at 500 hours for each spacing

in sections A and B are the results of interest. Generally PWB

processing has the greatest impact on reduced CAF resis-

tance at smaller plated-through hole-to-plated-through hole

(PTH-PTH) spacings, while the laminate material has the

greatest impact at larger PTH-PTH spacings. However, the

laminate material used can also affect the extent of fracturing

and copper wicking near a PTH.

5.5.2 There are several additional factors that can affect

CAF resistance. See APPENDIX A for the list of PCB manu-

facturing parameters that may affect CAF resistance and that

should be documented.

6 Notes

6.1 Definitions [Only those terms not already included in IPC-

T-50.]

a) Conductive Anodic Filament (CAF) Formation:

The growth of metallic conductive salt filaments by means

of an electrochemical migration process involving the

transport of conductive chemistries across a nonmetallic

substrate under the influence of an applied electric field,

thus producing Conductive Anodic Filaments.

b) Electrochemical Migration (ECM):

The growth of conductive metal filaments across or

through a dielectric material in the presence of moisture

and under the influence of voltage bias.

c) Electroless Nickel / Immersion Gold (ENIG):

This is a multi-functional surface finish wherein the electro-

less nickel layer is capped with a thin layer of immersion

gold. It is applicable to soldering, aluminum wire bonding,

press fit connections and as a contact surface. The

immersion gold protects the underlying nickel from

oxidation/passivation over its intended life.

d) Maximum Operating Temperature (MOT):

An underwriters Laboratories Inc. (UL) requirement value

of laminate materials as determined by the results of tests

performed by UL.

IPC-TM-650

Number

2.6.25

Subject

Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis

Date

02/21

Revision

Page8of11

e) Organic Solderability Preservative (OSP):

A surface finish for fine pitch featured PCBs that are often

assembled using surface mount components (SMCs). The

OSP surface finish provides very flat/co-planar land areas

for the placement and attachment of SMC devices.

f) Printed Circuit Board Fabrication (PCB Fab):

This phrase alludes to the manufacturing of a bare printed

circuit board that is not populated (assembled) with any

discrete components.

6.2 Reference Documents

IPC-TR-476

Electrochemical Migration: Electrically Induced

Failures in Printed Wiring Assemblies

IPC-TM-650, Method 2.6.14.1 Electrochemical Migration

Resistance Test (note: covers only surface electrochemical

migration)

IPC-9201 Surface Insulation Resistance Handbook

IPC-TM-650

Number

2.6.25

Subject

Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis

Date

02/21

Revision

Page9of11

APPENDIX A

Additional CAF Resistance Factors Checklist

Document for every CAF resistance test the several additional factors

that can affect CAF resistance. These critical factors include:

Parameters Comments

Test board part number and revision level.

Drilled Hole Size for each Hole-Hole and Hole-Plane spacing tested (also

recommend drilling feed rate, speed of rotation, chip loading data, backup

material type, etc.).

Desmear & Cleaning Process (ex: permanganate, plasma).

Whether glass microetch was used (and if so, the controlling process

parameters).

Board finish type (HASL or specific OSP, immersion silver, immersion tin,

ENIG, etc.).

Laminate material type that was used (manufacturer and material name or

number).

Woven Glass Manufacturer (silane treatment application).

Type of soldermask (if used).

PCB Fab manufacturer and facility.

Method of separating test board from working panel.

Number of circuit layers in test board.

Copper thickness in plated through holes in test board.

Dielectric Thickness and Construction (same construction throughout is

recommended for CAF testing). Examples:

Core 2/2 3.0 mils thick 1 ply 1080, Prepreg 1 ply 1080

Core 1/1 4.3 mils thick 1-106/1-1080, Prepreg 1-106/1-1080

Core H/H 5.0 mils thick 1-2113, Prepreg 1-2113

Failure Location (with examples)

Glass-Resin Boundary

Migration Between Glass Fibers

Migration Between Fiber Bundles

Void within Fiber Bundle

Incomplete Wetting Against Fiber

Hollow Fiber (report glass cloth supplier name, name of fiber source)

Test Interruptions:

Cause (example: power interruption)

Length of Interruption

Test Re-Start Parameters

Frequency of Measurement. Examples:

24 Hours

20 Minutes

6 Minutes

Time of Failure: Examples

200 Hours (hollow fiber)

300 Hours (void within fiber bundle)

21 Hours (migration between fiber bundles)

IPC-TM-650

Number

2.6.25

Subject

Conductive Anodic Filament (CAF) Resistance Test: X-Y Axis

Date

02/21

Revision

Page 10 of 11