IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第44页

in the test ﬁxture. The test system can then drive each device on the assembly and quickly locate defective devices or identify assembly process problems. T o insure precise alignment of the probe contact pins with the p…

Determining the percentage of nodal access to design into

a board layout requires trading off all the issues discussed

previously: defect rate, test development cost, test opera-

tion costs including manual troubleshooting costs, and, of

course, impacts on real estate. Short of having no defects

at all, full nodal access remains the most desirable option.

As with through-hole technology boards, once the board is

designed (nodal access ﬁxed) and its tests are designed

(test methods ﬁxed), the defect rate becomes the primary

key to reducing test costs. Therefore, defect reporting,

analysis, and correction/prevention are imperative. This

may involve closer supplier relationships to reduce compo-

nent and board level problems, and in-house action to

reduce process-induced problems.

5.2 Nodal Access In the early stages of product develop-

ment cycles, test philosophies and strategies are often

undeﬁned. This is especially true when a company is mov-

ing from one level of packaging technology to the next

higher level of packaging technology, for example, from

through-hole technology to surface mount technology or

from ﬁne-pitch lead-frame packaged ICs to BGA or CSP.

During these transition periods, the concurrent engineering

approach is essential for designing nodal access for test-

ability into the product. Concurrent engineering is the prin-

ciple vehicle by which test priorities can and should be

moved up to the beginning of the design cycle and

addressed with a higher priority. In the early stages of a

design, a test philosophy should be clearly deﬁned, then a

strategy for executing the tests can be implemented. An

ideal philosophy to adopt is one that identiﬁes all of the

different test types and the level of test that each type

requires.

5.2.1 Test Philosophy The test philosophy should be

written to encompass whatever combination of tests are

necessary for the product. Then, a simple strategy for

implementing the required tests can be deﬁned prior to

beginning the design process. Planning testability at the

beginning of a product development cycle instead of the

end can result in signiﬁcantly lower test costs per node and

provide higher nodal accessibility throughout the entire

process from initial design to ﬁnal test.

The best test philosophy to adopt is one that will make

provisions for executing every test method available. Even

when the product testing procedure is well deﬁned at the

beginning of the development cycle, it may change after

the design is complete. Some things to consider in outlin-

ing a test philosophy:

a) Strategic placement of all component vias.

b) Provide access to every node of every net.

c) Access of every node from one side of the board is pre-

ferred.

d) Correct test pad geometries and clearances.

Even in the higher density designs, the philosophy of pro-

viding 100% access to every node of every net from either

side of the board can be accomplished. However, this deci-

sion must be made at the beginning of a design.

5.2.2 Test Strategy for Bare Boards After the product

test philosophy has been established, a test strategy or pro-

cedure can be deﬁned. For an overview of several elements

of a procedure, consider the following:

a) Vision inspection of inner layers using AOI.

b) Vision inspection of O/L land/via connections.

c) Probe only vias on either side for bare board test.

d) Do not damage SMT lands with probe tips.

e) Probe secondary side vias for loaded test board.

f) Screen paste on vias for airtight board.

The actual product test strategy must be organized by all of

the concurrent engineering team members who will be

involved in the testing process. This will ensure that the

integration of the various test types and procedures will not

have too much redundancy, or create gaps that may endan-

ger test integrity.

5.3 Full Nodal Access for Assembled Board The num-

ber of test probes needed to test the board is equal to the

total number of device nodes or common connection

between devices. However, in the case of most dense sur-

face mount designs, this often requires the use of a double-

sided, or clamshell test ﬁxture because all of the nodes are

not accessible from one side of the board.

In-Circuit Test (ICT) only needs to have access to one node

per net. Every net has at least two nodes. Some nets have

many nodes, for example, on memory boards one net may

be connected to many nodes. In order to achieve full integ-

rity at the ICT level, access to only one node of each net is

all that is required. Therefore, the total number of test

probes required to perform the ICT is signiﬁcantly less

than the number required for the bare board test.

For ﬁne-pitch components, it is good design practice to

distribute approximately half of the test vias to the inside

of the land pattern and the other half to the outside of the

land pattern as shown in Figure 5-1. This accomplishes two

objectives:

1. The maximum density of test points established for a

given piece of test equipment is not exceeded.

2. Wider distribution of test points reduces the high-

pressure point areas which cause ﬁxture bowing during

vacuum or mechanical actuation.

5.3.1 In-Circuit Test Accommodation Speciﬁc via lands

and holes can be reserved and accessed for automatic

in-circuit test (ICT). The via land location for each com-

mon network in a circuit is matched to a test probe contact

February 2005 IPC-7351

in the test ﬁxture. The test system can then drive each

device on the assembly and quickly locate defective

devices or identify assembly process problems.

To insure precise alignment of the probe contact pins with

the printed board, exact x and y probe position and speciﬁc

networks must be furnished to the ﬁxture developer. Iden-

tifying the test locations as components in the CAD data-

base will allow for easy transfer of ﬁxture drilling data.

This data will reduce ﬁxture development time and elimi-

nate the drilling of excessive, nonfunctional holes in the

ﬁxture base. For low volume assembly, or high component

density assembly, ﬁxtureless testing by way of ﬂying probe

equipment is an option.

5.3.2 Multi-Probe Testing Some test probe systems can

exert considerable deformation forces on the assembled

boards and are a known source of premature service fail-

ures. An essential part of the printed board layout is to

ensure that the location of probing points on the board are

staggered at sufficient distances to avoid excessive defor-

mation during multi-probe testing. When the probe point

locations are highly concentrated, additional support may

be needed in the test ﬁxture design in order to counter the

effect against the high probe pressure concentration. The

area on the board where the support is to be provided

should be located where it is clear of conductors and com-

ponents.

5.4 Limited Nodal Access Provided the designer has

allowed sufficient room for access to the test land(s), lim-

ited nodal access (less than 100%) still allows the use of

spring probe (bed-of-nails) testing, but not as effectively as

full nodal access does. When nodal access is less than

100%, shorts, defects and in-circuit testing cannot be per-

formed completely, and some faults may not be detected.

A greater burden is therefore placed on functional or sys-

tem test to detect and diagnose shorts, defects, and bad

devices. This burden varies inversely with the nodal access

percentage. The extra effort at functional test may consist

of additional recurring manpower cost to diagnose failing

boards, or it may mean developing a more detailed func-

tional test (nonrecurring cost) than would have been

planned otherwise.

5.5 No Nodal Access No nodal access (0%) prohibits

bed-of-nails testing and defers all assembly defects and

component testing until the functional or system test bed.

This can only be cost-justiﬁed if the much higher cost-per-

defect repair is performed so infrequently that the total cost

is less than the cost of developing and operating an ATE

bed-of-nails test. In other words, the ﬁrst pass yields must

be extremely high to justify this approach.

5.6 Clam-Shell Fixtures Impact Probing the printed

board from both sides requires a ‘‘clam-shell’’ type of ﬁx-

ture. These are expensive, take more time to fabricate,

require larger test lands on the primary side to protect

against registration problems due to tolerance stack-ups,

and they are more difficult to maintain.

IPC-7351-5-01

Figure 5-1 Test Via Grid Concepts

Test Via Grid Concepts

1.25 mm Pitch Component

2.5 mm Via Test Grid

1.25 mm Pitch Component

1.25 mm Via Test Grid

0.63 mm Pitch Component

1.25 mm Via Test Grid

Preferred

Acceptable

Poor Design

IPC-7351 February 2005

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5.7 Printed Board Test Characteristics

5.7.1 Test Land Pattern Spacing

Design for testability

is as much a part of the schematic design process as it is a

part of the board layout process. Ideally, the printed board

would have 100% of the nodes accessable from the second-

ary side of the PB assembly. In-circuit testers must have

access to at least one node per net. Probe spacing is

optional; however, standard probe spacing is typically

2.0 mm [0.0787 in] to 2.5 mm [0.0984 in] while miniature,

needle type probes can be spaced as close as 1.0 mm

[0.0394 in] to 1.25 mm [0.04921 in].

The drawbacks to the 1.0 mm [0.0394 in] to 1.25 mm

[0.04921 in] grid-based test lands are the following. The

miniature, needle type probes are more expensive and they

do not hold up as well in high-volume production. Also,

any via sites that are to be used as test points should be

solder ﬁlled for better contact and increased probe life.

5.7.2 Test Land Size and Shape Lands or vias should be

0.9 mm [0.0354 in] to 1.0 mm [0.0394 in] for probing. As

land sizes decrease, misses increase dramatically as shown

in Figure 5-2. The use of square via lands may provide a

larger target zone for the test probe to contact.

5.7.3 Design for Test Parameters The following other

considerations are important to the general land pattern

design that should be incorporated into the printed board.

a) Two unplated tooling holes should be available on

diagonal corners of the printed board.

b) Test lands should be 2.5 mm [0.0984 in] minimum from

the edge of the printed board to facilitate gasketing on

vacuum ﬁxtures.

c) When using vias for test points, caution should be taken

to insure that signal quality is not degraded at the

expense of testing capability.

d) Test lands should be 0.63 mm [0.0248 in] minimum

from mounting land areas.

e) Where possible, provide numerous test lands for power

and ground.

f) Where possible, provide test lands for all unused gates.

Free running gates sometimes cause instability during

in-circuit testing. This will provide a means of ground-

ing these spurious signals.

It is sometimes desirable to provide drive and sense nodes

test lands to perform six-wire bridge measurements during

IPC-7351-5-02

Figure 5-2 General Relationship Between Test Contact Size and Test Probe Misses

Probability of at Least One Miss

Test/Via Probe Contact Size

100%

80%

60%

40%

20%

0.25 mm 0.35 mm 0.5 mm 0.6 mm

Diameter

0.75 mm

0.9 mm

1.0 mm

1.0 mm Dia

[0.04 in]

0.9 mm Square

[0.036 in]

February 2005 IPC-7351