IPC-TM-650 EN 2022 试验方法.pdf - 第750页

4.6 Other Dedicated Fixtures Hard-wiring is the default connection method. Other dedicated fixtures may be used, provided that the fixture does not change the resistance by more than 0.1 decade compared to a comparable h…

each approximately 0.7 in x 5 in. Layers 5 and 6 have no trace

routing, so removing them provides an 8 layer coupon. Con-

versely, by duplicating layers 5 and 6, a 14-layer or higher

layer count coupon can be obtained. Note: Inner layer copper

filling (similar to external layer copper thieving) can be applied

to inner layers. These two CAF test coupons have additional

features for identifying root cause failure site(s). Design data

should show the drill sizes to be used (example: 0.37 mm or

0.0145 in), but not the finished hole sizes after plating. Solder-

mask application is not required for these CAF test coupons.

• IPC-9256 CAF coupon evaluates the A2, A3 and A4 hole

wall to hole wall structures, with controlled spacings

between the adjacent plated through holes in both X and Y

in-line dimensions so good CAF test data is obtained even if

the laminate material machine direction lay-up is done incor-

rectly.

• IPC-9255 CAF coupon evaluates the A2, A3 hole wall to

hole wall structures, with controlled spacings between the

adjacent plated through holes in both X and Y in-line dimen-

sions (so good CAF test data is obtained even if the laminate

material machine direction lay-up is done incorrectly). This

coupon also evaluates the B2 structure where hole wall to

hole wall spacings are diagonal and useful for determining

the quality of the CAF testing performed (reference CAF test

method user guide).

[NOTE: These coupons can be run on production board lots

on unused portions of the working panel, allowing more cost-

effective on-going process and/or product monitoring of CAF

resistance.]

As a general rule, there should be enough CAF test boards

run within each sample test lot to have at least the equivalent

number of potential CAF failure sites as on a single targeted

specific application PWB.

4 Equipment/Apparatus or Material

4.1 Environmental Test Chamber

A clean test chamber

capable of producing and recording an environment of 65 ±

2°C[149±3.6°F]or85±2°C[185 ± 3.6 °F] and 87 +3/-2%

relative humidity, and that is equipped with cable access to

facilitate measurement cables to be attached to the speci-

mens under test.

4.2 Measuring Equipment A high resistance meter

equivalent to that described in ASTM D-257, with a range up

to 10

ohms and capable of yielding an accuracy of ± 5% at

ohms with an applied voltage of 100 ± 2 VDC, or an

ammeter capable of reading 10

-10

amps and capable of yield-

ing an accuracy of ± 5% in combination with 100±2VDC

power supply. The values of resistors used shall be verified

by reference resistors traceable to known industry or national

standards such as NIST.

4.3 Power Supply A power supply capable of producing a

standing bias potential of 10 VDC up to 100 VDC with a

tolerance of ± 2 VDC, and current supply capacity of at least

1 Ampere (Amp).

4.4 Current Limiting Resistors Tight control of the total

current limiting resistance value is critical for this test method.

One 10

ohm resistor in series shall be used for each current

path. Insert the current limiting resistors in series with the ter-

minating leads going to each test pattern. Note that some test

equipment has current limiting resistors built into the testing

systems. For the purposes of this standard test, excluding the

current limiting resistor and for each CAF test circuit, the total

series resistance of the measuring equipment and wires shall

not be more than 200 ohms. A lower total resistance value

will increase potential for damage to the test board when a

CAF failure occurs. A higher total current limiting resistance

value for each test net removes test conditions further from

actual field conditions and is not recommended.

4.5 Connecting Wire Use PTFE- or PFE-insulated copper

wires and solder the copper wire directly to the board to con-

nect test points for each test board to the measurement

apparatus. The insulation material should not outgas during

testing.

IPC-2625-3

Figure 5 Photo of CAF Test Coupon IPC-9256

IPC-TM-650

Number

2.6.25

Subject

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

Date

02/21

Revision

Page5of11

4.6 Other Dedicated Fixtures Hard-wiring is the default

connection method. Other dedicated fixtures may be used,

provided that the fixture does not change the resistance by

more than 0.1 decade compared to a comparable hard-wired

system when measured at the test conditions. These fixtures

should be checked for their resistance values frequently.

5 Procedure

5.1 Test Specimen Preparation

5.1.1 Sample Identification

Use a method for identifying

each test board that does not cause contamination, such as

a scribe, making marks away from the biased area(s) of the

specimen. Test boards shall be handled by the edges of the

board only, and the use of noncontaminating gloves is recom-

mended. Each board shall be clearly marked to identify the

simulated assembly reflow and reword processing, and other

differentiating parameters, completed prior to CAF resistance

testing.

5.1.2 Prescreen for Opens and Shorts Perform

as-received insulation resistance measurements using a mul-

timeter to make connection to each net, and check for gross

defects. Check for shorts at a 1.0 megohm setting. No opens

are allowed in connected nets.

5.1.3 Simulated Assembly & Rework CAF test samples

shall go through representative assembly and rework thermal

cycling for the application. Default lead-free simulated assem-

bly and rework is 6X at 260 °C. Default simulated eutectic tin-

lead assembly and rework is 6X at 230 °C. For reference see

IPC-TM-650, 2.6.27 (method for assembly simulation).

5.1.4 Cleaning Entirely clean each sample (CAF test

board) per IPC-TM-650, Method 2.3.25, Detection and Mea-

surement of Ionizable Surface Contaminants by Resistivity of

Solvent Extract by immersion washing until the level of ionic

contamination is reduced to less than 1.0 microgram NaCl

equivalent per square centimeter and for a maximum of 20

minutes. Boards not achieving this level of cleanliness within

20 minutes shall be scrapped for the purposes of this test.

See IPC-TM-650, Method 2.6.27, Thermal Stress, Convection

Reflow Assembly Simulation.

5.1.5 Connecting Wire Plated through holes near one

edge of the board may be used for connecting wire to each

test circuit. The test board shall be covered with noncontami-

nating film to prevent flux spattering during the wire attach

process. After stripping back the wire insulation, use water

white rosin (per J-STD-004, Type B) and best soldering tech-

nique (per J-STD-001, Class 1 or 2) to solder (per J-STD-006,

Type Sn63) PTFE- or PFE-insulated wires to the connection

points on each test board. Ensure against damaging PWB

laminate material adjacent to the plated holes during soldering

by using appropriate time/temperature parameters for the sol-

dering iron.

5.1.6 Cleaning After Attachment Perform appropriate

local cleaning and rinsing after the attachment of the connect-

ing wires. Isolation resistance between connecting wire

attachment sites should remain excellent through 96 hours

conditioning. Note: Each CAF test failure that does occur dur-

ing subsequent testing should be checked to determine

whether the connecting wire attach area is the low resistance

site. If the connecting wire attach area rather than the daisy

chain area is the low insulation resistance site, then that test

sample is no longer valid for data analysis.

5.1.7 Dry Bake sample boards for a minimum of 30 min-

utes in a clean oven at 105±2°C[221.0 ± 3.6 °F].

5.1.8 Precondition Precondition test board samples in a

bias-free state (no electrical potential applied to any test pat-

tern) for 30 minutes minimum at 23±2°C[73.4 ± 3.6 °F] and

50 ± 5% relative humidity prior to any initial insulation resis-

tance measurements [measuring insulation resistance of each

daisy-chain net on each test board before starting the first 96

hours (± 30 minutes) of bias-free temperature and humidity

conditioning].

5.1.9 Temperature/Humidity/Bias ( T/H/B ) Chamber

Place the specimens in the environmental test chamber in a

vertical position such that the air flow is parallel to the direc-

tion of all test boards in the chamber. Allow at least approxi-

mately 2.5 cm [nominally 1.0 in] between each test board.

Place the test boards, as much as possible, toward the cen-

ter of the chamber to help ensure against nonoptimum air flow

and/or drops of condensation falling onto the test boards.

Dress all wiring away from the test patterns, keeping the wires

away from the test patterns as they are routed to the outside

of the chamber. Also, wire should not impede airflow around

the samples. Set the chamber temperature and humidity with

a ramp rate of one hour.

IPC-TM-650

Number

2.6.25

Subject

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

Date

02/21

Revision

Page6of11

5.2 Test Procedures

5.2.1 Environmental Test Chamber Controls

Tight con-

trol of the test chamber temperature and humidity is critical for

this test method. A difference of 5% relative humidity can

result in a 0.5 to 1.0 decade difference in measured resis-

tance. If condensation occurs on the test specimens within

the environmental chamber while the samples are under volt-

age, other dendritic growth can occur. Water spotting may

also be observed in some ovens where the air flow in the

chamber is from back to front, when water condensation on a

cooler oven window can be blown around the oven as very

small droplets that deposit on test specimens. This contrib-

utes to dendritic growth.

5.2.1.1 Test Interrupt In the event that the environmental

test chamber deviates from the controlled environment, a

drop in relative humidity may be permitted for short periods

not to exceed 15 minutes, provided that the chamber air tem-

perature does not exceed board temperature by more than

5 °C. Boards or coupons being added to a chamber shall be

pre-heated to the temperature of the chamber.

5.2.2 Resistance Measurements Measure the insulation

resistance of each test board daisy-chain net using 50 VDC

per second rate of rise and minimum hold time of 60 seconds

at 100 VDC test voltage. The polarity of the bias (conditioning)

voltage and the polarity of the test (measurement) voltage

shall always be the same. 100 VDC applied voltage is used

as the test voltage for insulation resistance measurements.

5.2.3 After initial insulation resistance measurements are

taken, close the environmental test chamber and allow the

test boards to stabilize for 96 hours (± 30 minutes) at the

specified 65±2°C[149±3.6°F]or85±2°C[185 ± 3.6 °F]

with 87 +3/-2% relative humidity and no bias applied. After the

96 hour (± 30 minutes) stabilization period, insulation resis-

tance measurements shall be made between each daisy-

chain net and ground.

5.2.4 Ensure that all test board samples are connected and

that the appropriate current limiting resistor is in series with

each corresponding test circuit. Then, connect the test

boards to the power supply to begin the T/H/B portion of the

CAF testing.

5.2.5 Verify that the appropriate voltage bias is being

applied for the duration of the test. For comparing the CAF

resistance of different laminate materials and processes, the

10 VDC and 100 VDC bias are standards. For correlating test

results to expected life in the field, the bias voltage selected

should be two times the maximum operating voltage differen-

tial for a given application. Higher voltage almost linearly

affects time to failure, however higher voltage may offset the

impact of humidity due to localized heating and should be

avoided since humidity is a key part of the failure mechanism.

5.2.6 The bias polarity should always be the same as the

polarity used when measuring the insulation resistance after

the 96-hour stabilization period.

5.2.7 It is recommended that resistance monitoring mea-

surements be taken every 24 to 100 hours of bias (condition-

ing) voltage during the duration of the test, ensuring that the

polarity of the insulation measurement voltage and the bias

voltage are always the same. Decade drops in resistance,

observed when these intermediate measurements are taken,

also count as failures and improve the accuracy of the test

since CAF filaments are very thin and are easily destroyed.

Also when over 50% of the parts have failed, the test can be

stopped. As CAF forms, the voltage delivered across the CAF

failure site will drop as the resistance decreases. This

becomes significant as the resistance of the net approaches

the resistance of the current-limiting resistor, so adjustments

to the voltage during the test are not required.

5.2.8 After 500 hours of applied bias (596 hours total), per-

form the insulation resistance measurements, as before.

5.2.9 Additional temperature/humidity/bias conditioning may

be performed after 500 hours of bias, sometimes up to 1000

hours or more. However, the 500 hours bias testing results

shall provide a minimum standard for reporting CAF testing

results when using this procedure.

5.2.10 Suspect CAF test failures may be checked to deter-

mine whether the connecting wire attach area is the low resis-

tance site rather than the daisy-chain area. This requires

cutting the trace near the daisy chain (destructive). After all

testing is completed, if the connecting wire attach area rather

than the daisy chain area is then found to be the low insula-

tion resistance site, then that test sample is no longer valid for

data analysis.

IPC-TM-650

Number

2.6.25

Subject

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

Date

02/21

Revision

Page7of11