IPC-7095C-2013.pdf - 第92页

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 …

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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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

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nozzle must gasket against the part, not allowing vacuum

leakage. Tactile sensing, which helps control the Z axis

(vertical) stroke of the spindle, is desirable because it pre-

vents a component from being crushed between the

vacuum nozzle and the substrate.

7.1.4 Reflow Soldering and Profiling Reﬂow soldering

is a complex process with many variables. All mass reﬂow

systems incorporate convective, conductive and radiant

means of heat transfer, to what degree depends on the

design of the reﬂow system. All designs strive to achieve

the same fundamental results. Five different phases take

place during reﬂow. The ﬁve phases are: (1) evaporate sol-

vents from the solder paste, (2) activate the ﬂux and allow

ﬂuxing action to occur, (3) preheat the components and

printed circuit board, (4) melt the solder and allow proper

wetting to occur, and (5) cool the soldered assembly.

It needs to be understood that, regardless of the basic

reﬂow heat transfer concept used, the solder balls under-

neath a BGA component are primarily heated by conduc-

tive heating through the interconnect substrate. The reason

for this is illustrated in Figure 7-3. The optimal reﬂow pro-

ﬁle (temperature vs. time) including the peak temperature

varies with the speciﬁc package and the overall assembly.

However, as a guideline, when using SAC305 solder paste

for BGA packages containing SAC305 solder balls, the

minimum peak temperature for moderately complex

PCBAs may need to be not lower than approximately

240°C. When using tin/lead eutectic solder paste for BGA

packages containing SAC305 solder balls, the minimum

peak temperature in most cases can be as low as approxi-

mately 215°C. (See Table 7-3 for recommendations for

proper reﬂow temperatures.)

Over the years, reﬂow equipment has changed frequently.

Five design concepts have been used: vapor phase, lamp

infrared (IR), panel IR, and forced gas convection and

diode laser. Vapor phase reﬂow technology evolved ﬁrst

and was popular for a few years. Eventually, IR became the

preferred approach. Today, forced gas convection (95% of

total heat source) with IR assist (5% of total heat source)

technology is the method of choice with some applications

requiring an inert oven atmosphere. Diode laser soldering

is the newcomer to the industry and used mostly in high

end high reliability applications such as space, military and

medical industries.

7.1.4.1 Forced Gas Convection Convection heat is

today’s method of choice. Heat is transferred to the printed

circuit assembly (PCA) by low velocity heated gas. Forced

gas convection is a noncontact heating method, with some

heating accomplished by radiation. The rate at which heat

is transferred to the object is directly proportional to the

difference in temperature between the heated gas and the

PCA.

Inline forced gas convection systems have three main sec-

tions: (1) preheat, (2) reﬂow and (3) cooling. Each section

contains a number of top and bottom zones. The number of

zones has a direct effect on the transport speed and the

user’s ability to ﬁne tune the proﬁle. Adding more zones

allows the user to run a faster belt speed and it permits

more ﬂexibility in accurate proﬁling. Low volume reﬂow

systems have three or four zone sections (a zone section

includes top and bottom heaters). Medium volume reﬂow

systems have ﬁve or six zone sections, while the higher

volume systems use seven or more. Typically, a six zone

system will satisfy most reﬂow requirements, including

very large PCAs and reasonably high transport speeds (up

to 60 cm per minute). Proﬁle changes are accomplished by

adjusting the transport speed and top and bottom tempera-

ture settings.

7.1.4.2 Reflow Atmosphere The atmosphere of the

reﬂow furnace will affect solder wetting. Reﬂowing in a

nitrogen environment will result in improved wetting of the

solder joint. This will sometimes allow compensation for

marginally wettable surfaces on the boards as well as oxi-

dized solder balls. In order to obtain the maximum beneﬁt

of the solder paste in a nitrogen environment, it is recom-

mended to monitor the oxygen content and control it within

the limits you have established for your process.

With the migration of products into lead-free technology,

the solderability of the lead-free solder pastes is not as

good as that of tin lead. It is also possible to use up the

activators in the solder paste to clean the oxides off the

paste, land, and balls prior to reﬂow if paste selection is not

made correctly. This would result in a nonuniform shaped

solder joint and possibly nonwetting of the land. Some sol-

der paste formulations may also be affected by reﬂow

atmosphere in the formation of voids. When using lead-free

processes with OSP surface ﬁnishes, it may be desirable to

reﬂow the product using nitrogen, especially for hole ﬁll

problems in mixed assemblies. Hole ﬁll problems can be

encountered in lead-free process especially when using no

clean ﬂux and 1

or 2

generation OSPs. Use of water

soluble aggressive ﬂuxes and 3

generation OSP should

help minimize hole ﬁll problems.

IPC-7095c-7-3

Figure 7-3 Example of Peak Reﬂow Temperatures at Vari-

ous Locations at or Near a BGA

220°C

240°C

245°C

for LF-solders

230°C

235°C

260°C

225°C

for Sn/Pb

205°C 217°C

230°C

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