IPC-7095C-2013.pdf - 第77页

IPC-7095c-6-26 Figure 6-26 Heat Pathways to BGA Solder Joint During W ave Soldering A B Heat Pathways to the BGA Solder Joint During Wave Soldering BGA Wave Printed Circuit Board Heat Source C IPC-7095c-6-27 Figure 6-27 …

To avoid problems in BGA solder joints on the top side of

the board, their temperatures should not exceed 150°C dur-

ing tin/lead wave soldering and 190°C for lead-free alloy

wave soldering. This is less than the maximum temperature

allowed for the ﬁne pitch leaded components such as plas-

tic QFPs.

Figure 6-25 is an example of an acceptable temperature

proﬁle for the solder joints on a mixed technology board

during the wave solder process.

To determine the various ways of keeping the temperature

below 150°C (or 190°C for lead free), it is best to ﬁrst

identify the various ways in which the BGA solder joints

are heated during the wave solder process. Figure 6-26

illustrated three pathways. Pathway A is by conduction

through the thickness of the board from bottom to top.

Pathway B is by conduction through the barrel of the via,

along a conductor connecting the via to the BGA solder

joint land. Pathway C is by convection and radiation from

the preheaters on the top in the wave solder machine.

6.4.3 Methods of Avoiding Top Side Reflow The meth-

ods of avoiding top side reﬂow aim to reduce the heat

transfer to the BGA solder joints by one or more of the

three pathways described above. Figure 6-27 illustrates

these methods. A heat shield can be placed over the BGA

packages to avoid direct heating from the preheaters in the

wave solder machine. These shields can be mechanically

attached to the wave solder pallets.

Secondly, vias can be capped by solder mask on the bottom

side of the board. This via-capping process is very com-

monly employed in the industry, for a variety of reasons.

Via capping rules should be built into the design for manu-

facturability (DfM) process, during board design.

Vias that certainly need to be capped are those that are

connected to the BGA land with a short trace length or

those that are connected to a plane in the board.

Thirdly, a nonmetallic wave shield can be placed immedi-

ately below the BGA package locations on the bottom side

of the board to avoid the wave from touching these board

locations. The wave shield can also be attached to the wave

pallet by nonmetallic ﬁngers.

Selective wave pallets can utilize solid material under the

BGAs to prevent solder contact with the bottom of the

board and heat transfer up the vias to the lands. This will

prevent secondary reﬂow of the BGA joints.

The efficacy of each of these methods should be conﬁrmed

by measuring the temperature proﬁle of the BGA solder

joints during the wave solder process and ensuring that

they stay below 150°C.

Figure 6-24 Example of Top Side Reﬂow Joints

IPC-7095c-6-25

Figure 6-25 Example of Wave Solder Temperature Proﬁle of Topside of Mixed Component Assembly

150° C

160° C

183°C

80° C

Zone # 2

100° C

Zone # 1 Zone # 3 Chip Contour

40° C

60° C

Maximum peak

temperature for

BGA is 150°

Maximum peak

temperature for

Fine Pitch is 160°C

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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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

Not for Resale

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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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