IPC-7095C-2013.pdf - 第104页

BGA package after reﬂow . Corner glue prior to reﬂow (pre- reﬂow) requires that the BGA package have suf f icient sub- strate available outside of the edge of the last solder ball in the outermost rows (see Figure 7-16).…

Proper ﬁllet height is desirable for maximum underﬁll per-

formance. The presence of a ﬁllet that extends between

25% and 100% of the way up the side of the package near

its midpoint is considered acceptable in most applications.

Keepout zones for other devices and open vias are required

around the periphery of the BGA being underﬁlled. A con-

servative rule for keep outs is 1.5X the height of the top of

the substrate of the BGA package from the PCB surface on

the nondispense sides of the package and 6.0 mm on the

dispense side of the BGA package.

Underﬁlls packages are cured in ovens. One desirable

method for curing these boards is using a single pass

through a standard SMT oven run at a lower than reﬂow

temperature. Many underﬁll chemistries can be cured in

5-20 minutes at 120-165°C which may be conducive to this

approach. Batch ovens may also be used. Underﬁll vendors

have been introducing new formulations that cure at lower

temperatures over less time.

Historically, underﬁll epoxies have not been practically

reworkable in an HVM environment. This was more

acceptable initially in devices such as early generation cell

phones where the cost of each board was relatively low and

scrapping a few boards was not a signiﬁcant cost penalty.

However, underﬁll is penetrating more and more high

value markets. As a result, underﬁll vendors are developing

chemistries that are more reworkable in HVM.

Partial or corner only underﬁll is done by dispensing

underﬁll in dot or ‘‘L’’ shaped patterns near the corner of

BGA packages. The underﬁll ﬂows into a roughly arc

shaped pattern and envelops several solder balls deep at

each corner (see Figure 7-14).

This method has the advantage over full underﬁll in that

much less underﬁll material can be used, and also that

underﬁll ﬂow time can be greatly reduced which can help

increase the production rate associated with the dispense

step. As expected, partial or corner only underﬁll does not

have quite the strength improvement of fully underﬁlled

packages; however, in many situations the performance

improvement gained with partial underﬁll is more than

enough to meet the market requirements for the package/

board being protected. (An experimental case showed a

1.5X increase in shock level where the onset of mechani-

cal damage was sustained for a partial corner underﬁll

BGA versus the same nonﬁlled BGA. This is very signiﬁ-

cant.)

Some manufacturers of mobile PC motherboards have used

partial underﬁll to increase the strength of the BGAs on

their boards.

7.2.2.2 Corner Applied Adhesive Corner applied adhe-

sive (sometimes called corner glue, corner bond or corner

tack) is a method for applying glue only to the corner

and/or outer edges of a BGA package. The theory is that by

re-enforcing the area of the package that is under the most

stress, the solder balls farthest from the center of the pack-

age, package performance can be improved. Corner applied

adhesives do not improve package performance as much as

conventional full capillary ﬂow underﬁll, but often the

advantage gained is signiﬁcant and enough to meet market

requirements. The corner glue approach has been widely

adopted by markets that require high mechanical require-

ments (shock, vibration, and bend) on large BGA packages

(20 x 20 mm to 45 x 45 mm). The mobile PC motherboard

market matches this description.

A photograph of a BGA package mounted with corner glue

is shown in Figure 7-15.

Corner glue can be applied either directly to the PCB prior

to BGA package placement and reﬂow or to the assembled

Figure 7-13 Examples of Underﬁll Voids - small, medium and large; upper left, lower left and left of solder balls, respectively

Figure 7-14 Example of Partial Underﬁll - package was

pulled from the PCB and dark underﬁll can be seen in the

corners

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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BGA package after reﬂow. Corner glue prior to reﬂow (pre-

reﬂow) requires that the BGA package have sufficient sub-

strate available outside of the edge of the last solder ball in

the outermost rows (see Figure 7-16).

The minimum width of available substrate where pre-ﬂow

corner glue can be applied is approximately 0.7 mm. With

packages with less than this amount of substrate available,

this process is not controllable in HVM. Current trends are

that package substrate sizes are shrinking and the use of

pre-reﬂow applied corner glue is expected to drop off.

The effectiveness of corner glue post assembly and reﬂow

methods depends on the type of glue chosen and the total

surface area contacted at each corner. Dispensed amounts

vary from essentially single glue dots at each corner to ‘‘L’’

shaped brackets of glue that extend down the side of the

package by up to six solder balls on each side of the corner.

Studies show that the longer ‘‘L’’ shaped dispensed brack-

ets can signiﬁcantly improve mechanical reliability (i.e.,

one study showed an improvement in shock performance

where the acceleration level caused the onset of mechani-

cal damage to occur, increasing from 180 G to 300 G.)

A good starting point for the amount of glue that should be

applied at each corner is that each leg of the ‘‘L’’ shaped

bracket should extend somewhere between 3 and 6 solder

balls deep. One pitfall of corner glue is using too little glue

in terms of the surface covered. Testing has shown that a

single dot of glue that covers no more than the width of

one solder ball down the side of the substrate does not sig-

niﬁcantly increase the shock or bend performance of a

BGA. This is because typically the strength of the solder

mask to underlying FR-4 or the strength of the BGA sub-

strate is low and these constructions crack very easily if the

surface area of the corner glue is too low (see Figure 7-17).

Other guidelines are that the glue should wet on average at

least 50% up the vertical side of the substrate throughout

the entire dispense line and that the epoxy material should

be forced to ﬂow to some degree underneath the BGA

package, even if the epoxy ﬂows in deeply enough to con-

tact some of the solder balls.

Typical dispense equipment for post-reﬂow corner glue

includes a pneumatic source that supplies air to a syringe

and needle set up. This equipment is low cost and condu-

cive to being set up in manufacturing environments where

labor rates are fairly low compared to available capital.

Corner glues are epoxies similar to underﬁlls. Typical cure

cycles include 5-60 minutes at 60-180°C. Some UV light

curable versions of these materials are also being intro-

duced.

7.2.3 Depaneling of Boards and Modules The method

for depanelization can be achieved using a number of

different techniques. These include simple scoring, a com-

bination of routing and scoring, and a combination of rout-

ing plus breakaway tabs (see IPC-2222).

Scoring is the machining of a shallow, precise V-groove

into the top and bottom surfaces of the laminate. As scor-

ing allows the removal of rails and individual parts from a

pallet, positional accuracy is critical. Routing deﬁnes the

ﬁnal assembly periphery. The routing channels are pro-

duced using router bits of different diameters, leaving

breakaway tabs to hold the board in place during assembly.

Extreme care should be taken when breakaway tabs are

removed. Avoid bending the board, especially near BGA

components. Bending may cause BGA solder joints to

crack, typically starting with the corner balls. Custom tools

Figure 7-15 Corner Applied Adhesive

IPC-7095c-7-16

Figure 7-16 Critical Dimension for Application of Prere-

ﬂow Corner Glue

Figure 7-17 Typical Corner Glue Failure Mode in Shock if

Glue Area is Too Low - Solder Mask Rips Off Board and

Does Not Protect the Solder Joints

IPC-7095C January 2013

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should be fabricated, or machines designed for breakaway

tab removal should be purchased. These tools or machines

should reduce or eliminate stress near BGA components

during breakaway tab removal.

7.3 Inspection Techniques The following paragraphs

are inspection techniques that may be applied at different

times during the development of the BGA assembly pro-

cess or as an auditing mechanism during production. Table

7-4 provides some recommendations as to applicability of

the inspection method.

7.3.1 X-Ray Usage X-ray inspection is generally used

when there is a high proportion of hidden solder joints that

are not visually accessible, and when there are a signiﬁcant

number of untestable solder joints. Examples of untestable

solder joints are redundant connections, and back-to-back

BGAs where the fanout vias are inaccessible and space

does not allow for additional test points. X-ray methods

can complement the test process chosen, and can provide

faster feedback to the manufacturing line. Depending on

the capability of the X-ray system being used, X-ray is

capable of detecting solder related defects such as bridging,

open solder joints, insufficient solder, and excessive solder

volume. Other defect types such as missing balls, misreg-

istration, and package popcorning can also be identiﬁed. In

addition to defect detection, X-ray can be used to provide

trend analysis for solder volume and solder joint shape.

X-ray is the only nondestructive method of ﬁnding voids in

BGA joints.

Figure 7-18 shows the principles of X-ray equipment being

used with the X-ray to be above the sample; some equip-

ments have the tube below or at an angle to the specimen.

The general characteristics provided in the ﬁgure apply to

most X-ray systems. X-ray inspection has become a gener-

ally accepted tool for solder joint evaluation and analysis,

and as a monitor for the reﬂow process. X-ray inspection

techniques can be employed most effectively through the

understanding of principles of X-ray image acquisition.

X-ray can be effective in conﬁrming solder bond integrity

of BGAs and as a monitor for the reﬂow process. X-ray

inspection techniques can be employed most effectively

through the understanding of:

• Principles of X-ray image acquisition

• Analysis of the X-ray image (in the light of the reﬂow

process)

Use of X-ray requires some caution regarding overexpo-

sure on vulnerable materials or components. Figures 7-19

and 7-20 show the characteristics of X-ray images for both

voids at the interface, or missing solder balls in the BGA

attachment.

7.3.2 X-Ray Image Acquisition

7.3.2.1 Film Based X-Ray Inspection

Film based X-ray

inspection systems employ an industrial X-ray cabinet and

X-ray ﬁlm packets to record the X-ray image on ﬁlm. The

ﬁlm can then be viewed on video ﬁlm viewers for high

magniﬁcation examination of details. The process is slow

but can yield X-ray images of great detail and tonal accu-

racy.

7.3.2.2 Real Time X-Ray Systems Real Time X-ray

inspection systems utilize an X-ray source and a detector

system which converts the invisible X-ray image into a

video display signal. These systems provide immediate

imaging results of samples. The images produced from

these systems should not be distorted or include false arti-

facts induced by the X-ray system itself. Figure 7-21 illus-

trates a comparable level of image quality that should be

expected from a manual X-ray inspection system. Figure

7-22 illustrates examples of pin-cushion distortion and

voltage blooming. Real time systems are available in a

broad range of sizes from small desktop systems to large

Table 7-4 Inspection Usage Application Recommendations

Method

Process

Development

In-Line

Production

Failure

Analysis

Process

Auditing

NPI or Low

Volume

Production

Optical Inspection Excellent Good Excellent Good Good

Manual X-ray Excellent Good Excellent Good Excellent

Automated Transmission

X-Ray

Excellent Excellent Good Good Good

Automated Cross Section

X-Ray

Excellent Excellent Excellent Good Good

Scanning Acoustic Microscopy Excellent Fair Good Good Fair

Stand Off Measurement Fair Fair Good Good Fair

Automatic Optical Inspection

Solder paste

volume

Part

identiﬁcation,

solder paste

volume

Not Applicable

Part

identiﬁcation,

solder paste

volume

Part

identiﬁcation,

solder paste

volume

Destructive Analysis Good Poor Excellent Fair Fair

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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