IPC-7095C-2013.pdf - 第78页

6.4.4 Top Side Reflow for Lead-Free Boards Most lead- free solders targeted for use, including the popular solders in the SnAgCu system, have melting points higher than that for eutectic tin/lead solder . Hence, the pote…

IPC-7095c-6-26

Figure 6-26 Heat Pathways to BGA Solder Joint During Wave Soldering

Heat Pathways to the BGA Solder Joint During Wave Soldering

BGA

Wave

Printed

Circuit

Board

Heat Source

IPC-7095c-6-27

Figure 6-27 Methods of Avoiding BGA Topside Solder Joint Reﬂow

Methods of Avoiding Topside BGA Solder

Joint Reflow During Wave Soldering

BGA

Wave

Printed

Circuit

Board

Heat Shield

(Attached to Pallet)

Nonmetallic

Wave Shield

(Attached to Pallet)

Via Capping

Heat Source

January 2013 IPC-7095C

Provided by IHS under license with IPC

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No reproduction or networking permitted without license from IHS

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6.4.4 Top Side Reflow for Lead-Free Boards Most lead-

free solders targeted for use, including the popular solders

in the SnAgCu system, have melting points higher than

that for eutectic tin/lead solder. Hence, the potential for top

side reﬂow when wave soldering boards with surface

mount lead-free components already reﬂow soldered on the

top side of the board is signiﬁcantly reduced. For SnAgCu

solders, the maximum top side solder joint reﬂow tempera-

ture can reach 190°C without any impact on the BGA sol-

der joints.

6.5 Testability and Test Point Access The differences

in contact design have differing effects on the quality and

reliability of the contact, contactability, and later solder-

ability. The effects are negligible and insigniﬁcant when the

contact is minimal.

6.5.1 Component Testing As the BGA pitches and sol-

der ball size decrease, socket manufacturers face increasing

challenges in the design of sockets to adequately test BGA

packages. There are a myriad of tip designs socket manu-

facturers are working with to meet the needs of sub-mm

pitch BGAs.

One challenge is to be able to make contact to all BGA

individual balls. While attaching to a PCB, the solder balls

melt and self align to attach to the corresponding lands on

the PCB. Therefore, wider variations in the placements of

solder balls are tolerable with respect to BGA attach to the

PCB. But these variations need to be tightened in the case

of test and burn-in and thus there is no self-alignment of

solder balls to socket contacts.

The design of socket probes needs to take into account the

solder ball height variation. A larger variation in solder ball

height would require a wider reach range for socket probes,

and also when testing, the solder balls will be more vulner-

able. Burn-in of BGAs is conducted at an elevated tem-

perature. The burn-in time/temperature combination will

soften the solder balls to an extent dependent upon the sol-

der ball material. Under test probe pressure, the softened

solder ball will deform heavily and may affect the contact

quality during burn-in. The socket manufacturers need to

assure that the probes do not stick to softened solder balls

and the solder balls are not pulled away.

The socket test probe design is also critical in that the

probe does not gouge the solder ball in such a way or to

such an extent that the deformation becomes a quality or

reliability concern during or after BGA attach to the PCB.

Some contacts touch the solder ball on their sides, and

some on the tip of the solder balls. In some designs, the

individual pins are spring-loaded; in others, all contact

probes are in the same rigid plane.

6.5.2 Damage to the Solder Balls During Test and Burn-

In their pristine form, eutectic solder balls on a BGA

are shiny and quite round. Their attach, handling, and

subsequent BGA processing steps may induce deformation,

damage, pokes and dents.

The solder ball deformation during test and burn-in is an

expected phenomenon and, as such, is an acceptable

anomaly as long as it does not affect the usefulness of the

product. Many contact designs exist, each vying for a bet-

ter share of the market. Each contact will impart its unique

imprint on the solder ball. These probes contact the solder

ball at differing locations and impart unique imprints char-

acteristic of the contact design to the solder ball during test

and burn-in.

Solder balls have been known to come off in certain situa-

tions. Rather than trying to catch the problem by using

vision systems to detect the presence or absence of solder

balls or the damage to solder balls during testing, it is pru-

dent to optimize the solder attachment and to choose a test

socket which is benign to the device under test.

Some contacts disturb only the sides of the solder ball and

not the bottom. The bottom of the solder ball is untouched

during contact. One such example is shown in Figure 6-28.

Other contacts impact the bottom of the solder ball. Figure

6-29 shows a solder ball which has been contacted at the

bottom of the solder ball. The concern is that, during

reﬂow, ﬂux may get entrapped in the depressions formed

by the contact and may explode under reﬂow heat, splatter-

ing solder around, causing shorts, etc.

Some contacts are designed not to allow the entrapment of

ﬂux. Contact probes may contact the bottom of a solder

ball to create a pattern which provides a path for the ﬂux

to escape and will not entrap ﬂux during reﬂow.

Other contact impressions allow the entrapment of ﬂux. If

the impressions cause a hole at the bottom of the solder

ball then there is a greater chance of ﬂux entrapment. Such

entrapment may be quite benign for shallow depressions.

Figure 6-28 An Example of a Side Contact Made with a

Tweezers Type Contact

IPC-7095C January 2013

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To be damaging, the depressions have to be larger than

anticipated with current designs to entrap enough ﬂux to

cause problems.

Some contacts are designed with individual force mecha-

nisms for each contact pin. Other contacts come with com-

mon force mechanism applied to all contacts simultane-

ously. All different kinds of contacting mechanisms will

leave some imprint on the solder balls. Contact size should

be matched with the solder ball size to be tested to mini-

mize the solder ball deformation. Therefore, the size of the

contact needs to decrease with decreasing solder ball sizes

and decreasing array pitches. Too large of a contact size

may short and may subject the solder ball to undesired lev-

els of deformation.

The force mechanism to assure contact needs to match the

solder ball hardness, which is dependent upon the solder

constituents. Too much force will impart unnecessary

deformation. The force mechanism needs to provide

enough force to contact the smallest balls in an array. The

solder balls soften when subjected or exposed to elevated

temperatures and time durations. Force mechanism design

needs to take this change in consideration for their use at

varying temperatures. Even at ambient, continuous testing

of parts may increase the temperature at the contact site.

For all practical purposes, the solder ball needs to be sol-

derable with acceptable contact strength, contact area and

solder column shape after it has endured the rigors of test-

ing and burn-in. To assess the impact of processing through

test and burn-in, the items to look at should include solder-

ability, coplanarity, and general cosmetic shape of the sol-

der ball. The solder ball should not lose so much solder to

the test and burn-in process that enough solder is not left

for optimum connection. The ball should be able to

respond to the reﬂow process to produce an acceptable

contact. Ball corrosion and foreign material picked up dur-

ing test and burn-in should not adversely affect the quality

and long-term reliability of the solder ball. After the test

and burn-in cycle, especially for no-melt balls, meeting the

coplanarity expectations will be an essential requirement

for proper BGA attachment to substrate.

Various technologies contact different parts of the solder

ball. In fact, most of the contact designs touch the bottom

of the solder ball to make a contact. In view of these

options, it would be impossible to declare certain parts of

the solder ball untouchable during its processing. To keep

a certain area of the solder ball untouchable is neither prac-

tical nor necessary if it has no effect on solderability, copla-

narity, solder volume, quality and reliability of the end

product.

6.5.3 Bare Board Testing There are a number of issues

that arise when considering the test inspection and mea-

surement of increasingly complex substrate interconnec-

tions, especially when involved with the electrical evalua-

tion of the substrate. In order for the manufacturer to be

able to reduce cost, while adequately assuring the electri-

cal function of the substrate interconnection, the customer

will have to provide deﬁnitive test data, with the preference

being 100% net list testing. Compatibility of the data is

currently an issue as well. It is hoped that industry stan-

dardization efforts will help to solve this issue in the fore-

seeable future. One item which could prove key to achiev-

ing this is likely to be acceptance of a standard base grid

pitch, which would allow test equipment and socket manu-

facturers to examine and focus on creation of a universal

solution.

The use of ﬁxtures and bed of nails testing for opens and

shorts testing is quickly losing the ability to meet test

requirements as feature size decreases, coupled with

increased densities. Double density, or 1.77 mm pitch, test

beds seem to be adequate for 400 µm pitch and up. As the

density of the substrate increases beyond 400 µm pitch,

alternative techniques must be considered. Quad density

ﬁxturing is a possibility, with 62 test probes per square

centimeter, but concern increases about potential feature

damage due to the contact by the probes. In addition, the

cost of double and quad density ﬁxtures, as well as the cost

of test equipment, make it difficult to justify total test cov-

erage within cost expectations based on current understand-

ing of electrical testing and a linear projection of present

testing concepts.

Regarding the I/O density of 200 to 1000 at various grid

segmentation, it becomes apparent that double and quad

density is capable for bare board continuity testing the

average I/O requirements. However, if components are

stacked ‘‘edge-to-edge’’ the testing becomes impossible

with this test approach. This is because a BGA on 1.0 mm

pitch contains 96 lands per cm

, and the quad density test

ﬁxture is only able to accommodate 62 probes per cm

Spreading components on the mounting structure relieves

some of that complexity but also consumes more space and

Figure 6-29 Pogo-Pin Type Electrical Contact Impressions

on the Bottom of a Solder Ball

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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