IPC-7095C-2013.pdf - 第91页

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…

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

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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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)

January 2013 IPC-7095C

Provided by IHS under license with IPC

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requirements are the same for both copper balls or high

temperature 90Pb/10Sn balls since neither ball collapses

during reﬂow and all the solder ﬁllet requirement have to

be met by the paste.

There are ceramic BGAs that do not use noncollapsible

balls and use lead free (SAC) solder balls that do collapse

during reﬂow and contribute towards the formation solder

ﬁllet just as tin/lead balls do in plastic BGAs. The numbers

shown in Table 7-2 do not apply to either plastic or ceramic

BGAs that use collapsible balls.

In order to achieve the correct solder volume of solder

paste for a ceramic package with noncollapsible balls (cop-

per or Pb90/Sn10), it is necessary to overprint the land with

a larger aperture or use a thicker stencil. For other compo-

nents on a circuit board with CBGAs, the aperture size may

need to be adjusted to compensate for the thicker stencil or

it may be necessary to step the stencil.

7.1.2 Component Placement Impact Getting into BGA

technology also requires some new assembly capability.

Depending upon the type of pick and place system, a

change in package carrier mechanism may also be required

to transfer packages from matrix tray to the pick position.

Fiducials may help vision systems to recognize the exact

location of the land pattern for the BGA, similar to what is

used for ﬁne-pitch peripheral leaded parts. Large BGA

parts on tape-and-reel will require 44 mm and 56 mm feed-

ers depending on the body size. However, most common

feeders for BGAs are JEDEC trays.

7.1.3 Vision Systems for Placement Placement accu-

racy is a very critical part of the BGA process. It is

strongly recommended to not move a BGA after machine

placement to correct placement problems as this may cause

solder bridging in adjacent solder joints, since the connec-

tions cannot be seen visually. The placement machine’s

accuracy is largely dependent on the vision system and the

ability of the nozzle to hold the component. Matching the

vision system to the application is also important. The

vision system determines the X, Y and theta offset of each

component prior to placement. In addition to determining

the component offset, the vision system can also inspect the

component for dimensional integrity and missing solder

balls. CCD (charge-coupled device) camera-based systems

employ two lighting methods, referred to as binary and

gray scale. Both methods can be sensitive to contrast and

lighting changes.

Gray scale systems use front lighting, which illuminates

the component from below. Surface features are reﬂected

into the CCD camera for processing. Binary systems use

back lighting, which illuminates the component from

above. The outline of the component is projected into the

CCD camera for processing. Binary imaging, which is the

older of the two methods, locates a feature using the con-

trast between black and white images. Gray scale systems

can usually interpret 256 levels of contrast. Both systems

use an algorithm to determine the center of the component.

Binary imaging requires less computing capability than

gray scale imaging.

Gray scale imaging places BGA components based on ball

location while binary imaging places BGA components

based on the component outline. In some cases the toler-

ance between the BGA outline and the balls is signiﬁcant.

Gray scale imaging is more desirable for placing BGA

components because it eliminates placement error due to

variations in the component outline.

Placement machine nozzle designs vary from supplier to

supplier. It is important that the correct nozzle is chosen

which will have sufficient surface area to hold the part

without any shifting during the placement process. The

IPC-7095c-7-2

Figure 7-2 High Lead and Eutectic Solder Ball and Joint Comparison

PBGA Substrate

PWB

62Sn/36Pb/2Ag

CBGA Substrate

PWB

Eutectic Sn/Pb

90/10 Pb/Sn Ball

Table 7-2 Example of Solder Paste Volume

Requirements for Ceramic Array Packages

Component

Pitch

(mm)

Solder

Paste Alloy

Volume in µm

Low (min) High (max)

CBGA/

CuBGA

1.27

Tin/Lead or

Lead-free

78,660,000 163,870,000

CBGA/

CuBGA

1.00

Tin/Lead or

Lead-free

40,968,000 75,380,000

CCGA/

CuCGA

1.27

Tin/Lead or

Lead-free

60,632,000 124,542,000

CCGA/

CuCCGA

1.00

Tin/Lead or

Lead-free

32,321,000 81,935,000

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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