IPC-7095C-2013.pdf - 第90页

7 ASSEMBLY OF BGAS ON PRINTED CIRCUIT BOARDS 7.1 SMT Assembly Processes The assembly processes for attaching BGA components are more forgiving than the processes for attaching ﬁne pitch peripheral components in some aspe…

the processes, and the shop-ﬂoor equipment. Applications

receiving messages from the equipment include:

• Work-in-process (WIP)

• Tracking, capacity, and throughput monitors

• Equipment utilization and line-balancing monitors

• Product quality monitors that incorporated data from test

and inspection

Linking all parts of a manufacturing operation through a

single hub has been the goal for many years. The CAMX

message-broker approach provides an extraordinary array

of tools to accomplish that task. The message broker can

connect any number of clients together. Clients can be fac-

tory equipment, a host computer, or the data hub at the

engineering manager’s desk. The system provides visual

representation of whatever the manager wants to know. The

manager can communicate with someone in Singapore, and

check the status of their work-in-process using a snapshot

of the data in any of a number of preset formats. He can

pick the format and instruct the software as to how fre-

quently the updates are required, and the broker does the

rest.

6.8.2.2 Information Exchange Benefits Major equip-

ment manufacturers have involved themselves in this effort

from the beginning, thus they support the CAMX effort to

extract information from their machines in the standard

format. Critical to the success of this new tool is its ‘‘plug-

and-play’’ construction. Any piece of equipment on the

ﬂoor can connect to the supporting architecture and data-

gathering software. Using XML as the standard ensures

that no one will misinterpret what the data means. Process

correction and product improvement require traceability of

faults, regardless of where a fault is identiﬁed. Knowing

where the fault came from and its root cause are critical.

In addition, increasing product quality and reducing costs

require understanding the actual fault spectrum. Histori-

cally, the same fault may be called by several names at dif-

ferent process points. Applying the CAMX standard, fault

names can be consistent from step to step within the manu-

facturing operation, thereby improving the overall level of

communication.

For electronics manufacturing service (EMS) providers, the

CAMX standards permit manufacturing ﬂexibility without

sacriﬁcing data coherence and comprehensive data analy-

sis, regardless of the process under evaluation. The soft-

ware is more streamlined and less complex than systems

that were developed for the semiconductor industry

because all of the equipment now conforms to the single

standard. The result is equipment that is both easier to use

and easier to support.

Standardization will open up opportunities to perform more

elaborate data analysis because the standards will reduce

the effort required to gather information. IPC-2501, as

implemented in the CAMX message broker, provides the

simpliﬁed web-based XML messaging activity for the other

IPC standard vocabularies. Equipment suppliers who need

a particular piece of information will merely ‘‘subscribe’’

to an application or system using the existing Web-based

and universal XML messaging SOAP protocol, regardless

of the architecture of the other interfaces, and download

responses in real time. SOAP is an XML messaging stan-

dard supported by all major computing and software

vendors. It forms the foundation of their Web-services

infrastructures.

The XML message type of machine communication was

less necessary when OEMs did their own manufacturing.

However, with the amount of outsourcing that kind of ver-

tical integration has become more the exception than the

rule. The new standards will make it possible for the OEM

customer to regain a measure of the insight into manufac-

turing activity that was lost due to outsourcing the work.

The use of a universal data format will also provide the

ability of suppliers to monitor inventory of their products

to prevent disruptions of the manufacturing line. Compo-

nent suppliers can monitor usage of their components on

the factory ﬂoor, scheduling purchases and taking over

similar functions that otherwise fall to the EMS company

or to the OEM.

6.8.3 Specifications In many instances, documentation

references other speciﬁcations. These should be clear and

should be provided in this same manner in which the origi-

nal package is provided (hard copy or electronic). Confor-

mance test circuitry shall be provided, thus the part can be

tested through destructive techniques. As a minimum, con-

formance test circuitry shall include:

• Board part number/revision letter

• Assembly part number and reference to Bill of Material

• Traceability identiﬁcation

• Date code

• Manufacturer’s identiﬁcation, e.g., commercial and gov-

ernment entity (CAGE), logo, etc.

• Special coding systems may be used provided they are

identiﬁed on the master drawing

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7 ASSEMBLY OF BGAS ON PRINTED CIRCUIT BOARDS

7.1 SMT Assembly Processes

The assembly processes

for attaching BGA components are more forgiving than the

processes for attaching ﬁne pitch peripheral components in

some aspects, but more demanding in others. Process

defect rates are signiﬁcantly less than ﬁne pitch peripheral

components; however, good process control is a necessity.

7.1.1 Solder Paste and Its Application The surface

mount assembly process uses solder paste to connect the

BGA balls to the lands on the board. Solder paste can be

applied to the lands using three methods: screening, sten-

ciling, and dispensing. BGAs are typically printed using

stenciling where solder paste is applied on the BGA lands

through the apertures on the stencil of similar size or

smaller to the BGA lands.

Stencil thickness and aperture size will determine the

designed paste volume, which can be critical for the solder

joint reliability of certain BGA types. Squeegee type and

process settings will impact the actual solder paste that is

deposited. The stencil thickness may need to be reduced

when using ﬁner pitch BGA components; however, care

needs to be taken when reducing stencil thickness so that

volume is not reduced for other components. Trapezoidal

stencil apertures (slightly larger opening on the bottom

than on the top) are generally preferred as they improve

paste release.

Solder paste consists of a homogeneous mixture of metal

powder particles and ﬂux. The metal content (typically

90% by weight) in the solder paste determines the amount

of solder in the solder joint. The most common solder paste

alloy is the eutectic Sn63Pb37, and SAC 305 for lead-free

(Sn96.5Ag3.0Cu0.5). Metal powder particles are generally

spherical in shape. A uniform shape aids the printing or

dispensing process and it decreases the surface area, which

minimizes oxidation.

Flux makes up the majority of the remaining substances in

the solder paste. The activators in the ﬂux remove oxides

from the solder particles, the land, and the BGA balls. They

promote good solderability during reﬂow process. Solder

balls, which form for a variety of reasons during reﬂow, are

a reliability concern, especially when ﬁne pitch devices are

involved. The solvents have an important role in control-

ling the tackiness of the paste and affecting the rheological

properties. The formation of voids in the BGA solder joint

may be related to solvents in the solder paste. Solvents

with low boiling points and/or improper reﬂow parameters

can increase the incidence of voids in BGA solder joints.

For successful ﬁne pitch BGA printing, the solder paste

must pass through very small apertures in the stencil. The

solder paste needs to remain printable and tacky for an

extended period of time, and it must maintain print deﬁni-

tion prior to and during reﬂow. Solder paste viscosity, par-

ticle size and stencil life are critical parameters for solder

paste application.

7.1.1.1 Particle Size and Paste Selection Many solder

pastes are available and one should be selected based on

(among other things) the print characteristics, ﬂux type,

and ﬁne pitch particle size. Print characteristics includes

that the solder paste print well, providing good print deﬁ-

nition without exhibiting solder paste slump. The ﬂux in

the paste should be active enough to exhibit good wetting

and reﬂow characteristics, yet it should be compatible with

the cleaning process or surface resistivity requirements if a

no-clean ﬂux is used. The diameter of the particle size

should not exceed the aperture width divided by 4.2. This

was determined through empirical experimentation. When

this rule is violated, the paste release and print deﬁnitions

are affected.

The solder particle size is classiﬁed by J-STD-005 (see

Table 7-1).

Particle size distribution affects the solder paste viscosity

and printability. Type 3 paste is the most commonly used,

and it is adequate for most printing applications. Ultra ﬁne

pitch CSP application may require Type 4 paste. Care

should be taken when moving to Type 4 powder as the sol-

derability is generally reduced due to the increase in the

surface area and the potential for increased oxidation of the

solder particles. It is often necessary to use nitrogen in the

reﬂow furnace when Type 4 powders are incorporated.

7.1.1.2 Stencil Thickness and Aperture Design As is

the case for all components, as the pitch of the part

decreases it often becomes necessary to decrease the sten-

cil thickness. For BGA components in the pitch range of

1.5 mm to 1.0 mm, a stencil can range from 0.15 mm

[0.006 in] to 0.18 mm [0.007 in] thick. For chip scale

packages (CSP) or ﬁne pitch BGAs with a pitch ≤0.80 mm,

a stencil thickness range of 0.1 mm [0.004 in] to 0.15

[0.006 in] mm is recommended. CSPs and other ﬁne pitch

area array packages are seldom the only components on a

PCA so a balance must be struck in the selection of stencil

thickness. Thin stencils, appropriate for optimal CSP print-

ing, will likely not provide sufficient solder paste for other,

larger pitch BGAs. This can lead to defects such as head-

in-pillow or reduce the thermal fatigue life of critical

ASICs.

Table 7-1 Particle Size Comparisons

Solder Paste Type Mesh

Maximum Particle

Size [µm]

Type 2 -200/+325 75

Type 3 -325/+500 53

Type 4 -400/+500 38

Type 5 -500 25

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When making decisions about stencil thickness and volume

of paste to be deposited, it is safer to deposit more paste

than less paste with the goal of achieving six times more

bridges than opens (this is the ratio and not more defects).

The reasons are that bridges can rarely escape inspection

and they are sure to be caught by tests (ICT or functional).

Opens and intermittent opens, on the other hand, can easily

escape tests since they may make contact due to pressure

applied during testing but will result in ﬁeld failures. So

this simple guideline to target more bridges than opens

while designing stencil and land patterns is the most cost

effective method to prevent ﬁeld failures.

It is also very important to design a stencil aperture that

will provide good paste release. In order to ensure good

paste release, an aspect ratio of 1.5 is recommended.

Aspect ratio is the ratio between stencil aperture width and

stencil thickness. Another commonly used ratio is called

area ratio. An area ratio of greater than 0.66 is recom-

mended. The formula to calculate area aspect ratio is the

area of the stencil aperture divided by the area of the sten-

cil walls (see Figure 7-1). The idea behind designing sten-

cil with these strongly recommended aspect and area ratios

is to allow more paste release and minimize any paste

along the stencil walls which may partially clog apertures

and reduce paste volume on subsequent boards to be

printed next, resulting in insufficient solder joints or even

opens.

Area land contact/area stencil wall equals aperture/area of

aperture walls) = L*W/2*(L+W)*T >0.66

LxW

2(L+W)

Note: Aspect ratio is more common than area ratio in

aperture design since aspect ratio is one dimensional sim-

pliﬁcation of the area ratio when land length is much

greater than width i.e., a square aperture with a dimension

of 0.35 mm with a stencil thickness of 0.125 mm gives the

resulting aspect ratio:

0.35x0.35

2(0.35+0.35)x0.125

0.1225

0.175

= 0.70

Using an overprint or an aperture larger than the land may

be necessary to achieve this ratio, especially for ﬁner pitch

BGAs, Stencil apertures are generally one to one or slightly

smaller than the land in order to achieve gasketing. When

and if the stencil apertures need to be larger than the lands,

to achieve the desired area ratio for better paste release,

cleaning of the stencil after each print may be necessary.

Larger apertures will cause paste to leak under the stencil

resulting in paste smudging on boards to be printed next. It

should also be noted that stencil apertures are signiﬁcantly

larger than the lands when designing stencils for CBGAs,

CCGAs, and paste-in-hole processes.

A square or rectangular aperture with rounded corners will

also provide better paste release and volume deposition.

7.1.1.3 Importance of Paste Volume For plastic BGAs

much of their solder volume is supplied by the solder ball

on the part itself and the paste volume is not that critical.

For plastic BGAs (in both SnPb and lead-free conﬁgura-

tions) above 0.80 mm pitch, stencil thickness will be dic-

tated by the other component types used on the printed

board assembly. Solder volume and stencil thickness

become more critical for ceramic BGAs with noncollaps-

ible copper balls or high temp 90%Pb/10%Sn balls with

melting point of 302°C The solder balls used on ceramic

BGAs do not collapse during the normal reﬂow process

(see Figure 7-2).

For CBGA with noncollapsible balls (copper balls or 90Pb/

10Sn) where thicker solder volume is required, a stepped

stencil may be used. The step is typically 0.04 mm to

0.08 mm, and it can put two different paste thicknesses on

the board surface. A metal squeegee can be used success-

fully when the step is 0.04 mm. If a stepped stencil is used,

the step line should be at least 3.75 mm away from any

print aperture. Because the high lead content ball does not

collapse, having sufficient solder paste is critical. So in

addition to using stepped stencil, the stencil aperture may

also have to be much bigger than the land to achieve the

required paste volume for noncollapsible balls. The ﬁllet

between the land and ball depend upon the solder paste

volume. Ceramic BGAs and CGAs require a minimum vol-

ume of solder pastes to make a reliable solder joint. There

are various suppliers of this type of technology who can

provide the speciﬁc solder paste volume for their package.

Table 7-2 contains an example of the solder paste volume

requirements for some of the ceramic packages using either

tin/lead or lead free (SAC) solder paste (see Table 7.2).

The paste volume requirements should be the same for

both tin/lead and lead free paste. Also the paste volume

IPC-7095c-7-1

Figure 7-1 Aspect and Area Ratios for Complete Paste

Release

Solder Stencil Feature Dimensions

Aperture length (L)

Stencil thickness (T)

Aperture width (W)

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