design for in-circuit test (vintage 1994)

Design for In-Circuit Test Advisor

DICTADICTA

Rules to be included in phase 1

of the DICTA system

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button to begin

RF 1994

Component Mounting

• Preferably, all components will be on the top side of the circuit board

• Chip resistors and other small components can be placed on the secondary side of the board, but they must be located away from test pads

• Any flying wires should be located on the top side of the PCB, away from test probes

best - all components on one side of the PCB

moderate - passives only on underside

worst - both sides populated with ICs

Board Shape• To facilitate sealing for vacuum fixturing, boards

should have straight sides and no internal cut-outs

• Any cut-outs must be surrounded by an area clear

of all obstructions.

Bad shapeLess good

shapeGood shape

Edge Clearance• An area around the board perimeter should be free of

components, test locations and other features

• This should extend for at least 0.100, and preferably

0.200 inches from any edges or cut-outs

Clamping or sealing the board edge becomes difficult

Tooling Hole Clearance

• The clear area around tooling holes should extend

for 0.375 inches

Locating Tooling Holes• Tooling holes should be provided on at least two

opposite corners

• If space allows, a tooling hole on a third corner will improve accuracy

• Holes should be offset so the board cannot be placed in a fixture the wrong way round

• All tooling holes must be unplated

• Minimum diameter is 0.125 inches

Board Thickness• To avoid distortion during testing, the board

should have a minimum thickness of 0.062 inches

Board Thickness• To avoid distortion during testing, the board

should have a minimum thickness of 0.062 inches

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Board Thickness• Thin board distorted during probing...

Via Holes• To ensure a reliable vacuum seal between the

assembled PCB and the test fixture, via holes

should be of sufficient diameter (0.062 inches or

more) so they will fill during flow soldering.

Likely to fill Unlikely to fill

Component Height

• Components on the bottom side of the board must

not obstruct the operation of the bed-of-nails fixture

• It is best if components have heights of less than

0.160 inches

• No components with heights greater than 0.360

inches should be mounted on the bottom of the

board

Component Height

• Components on the bottom side of the board must

not obstruct the operation of the bed-of-nails fixture

• It is best if components have heights of less than

0.160 inches

• No components with heights greater than 0.360

inches should be mounted on the bottom of the

board

Cut-out in top

plate required

Recess in vacuum

well required

Testability

unimpared

60 mil

Component / Test Land Spacing• The space between the centre of a test access point

and the edges of any adjacent components must exceed 0.060 inches

• This distance must be increased to 0.200 inches where the height of adjacent components exceeds 0.160 inches

200 mil

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Test Land Size• Test lands should be as large

as possible to ensure reliable probing

• Recommended size is greater than 0.060 inches

• Minimum size is 0.035 inches for a bed-of-nails type fixture

• If the PCB requires two-sided probing, the minimum size for test lands on the top side is 0.040 because probing here is less accurate

Small test pads may well be missed as

probes wear

Land Spacing• Test access points should be spaced with 0.100

inches between centres

• This can be reduced to 0.075 or even 0.050 where necessary

• The number of probes at smaller pitches should be kept to a minimum as these are less accurate

End of phase 1 rules

Other information to be included in the multimedia

design advisory system follows...

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to begin

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to continue

Board Probing

• If possible the board should be probed from one side only

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Board Probing

• If probing from both

sides of the board is

necessary it will be

found that probing

accuracy is reduced

on the top side

• The location of tall

components may also

make testing difficult.

Component Orientation• All packages of the same style should have the

same orientation so that time will not be wasted trying to identify pins during manual testing

• Clear, permanent marking of pin numbering for each component is also necessary

1

14

Probing Components

• Avoid probing the leads of the components

• Probing component leads can mask problems with bad solder joints

• Test pads should be included in the design instead

Probing Components

• Avoid probing the leads of the components

• Probing component leads can mask problems with bad solder joints

• Test pads should be included in the design instead

joint closed by probe

Probing Components

• Avoid probing the leads of the components

• Probing component leads can mask problems with bad solder joints

• Test pads should be included in the design instead

joint closed by probe

Probing Components

• Avoid probing the leads of the components

• Probing component leads can mask problems with bad solder joints

• Test pads should be included in the design instead

joint closed by probe

Test Land Geometry

• Square test pads offer a greater area to hit so the

testing process is more reliable

Square pad

offers 27%

greater contact

area.

Test Land Geometry

• Solder on test pads may make probing difficult

• Mounting pads and test pads should be separated to prevent solder flowing onto the test pad during manufacture

• This also ensures the test probe will not strike a component skewed during placement or reflow

Good Bad

Edge Connectors

• In terms of testabiity, the best design for testing would achieve all the necessary contacts for testing at the edge connectors

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Modularity• Ideally, complex products will be split into

functional modules which can each be tested individually

Test Probes• Available in a variety of

sizes and head

configurations

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications

plunger

wire wrap connection

springbarrel

fixture

head

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• Available in a variety of

sizes and head

configurations.

• Larger probes have a

longer reach but may not

be practical in fine pitch

applications.

Test Probes• A large probe should be

used where possible for

better electrical contact

and improved fixture

reliability.

• Smaller probes must be

used where the test pads

are closely packed or

access is a problem.

Design for In-Circuit Test Advisor

DICTADICTA

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