IPC-7095C-2013.pdf - 第105页

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 Techniq…

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

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console ﬂoor models. They are also available with a broad

range of X-ray source voltages. There is not a speciﬁc

voltage needed to inspect BGAs. The voltage required will,

in part, depend on the sensitivity of the particular X-ray

system employed, as well as the structure and characteris-

tics of the BGA under inspection. BGAs with copper heat

sinks for example, will require higher penetrating voltage

settings than PBGAs or CBGAs. BGAs with aluminum

heat sinks, on the other hand, do not require the higher

voltages since aluminum is a less dense material and is

therefore much more transparent to X-rays than copper.

7.3.3 Definition and Discussion of X-Ray System Termi-

nology

X-ray inspection systems are available in both

IPC-7095c-7-18

Figure 7-18 Fundamentals of X-Ray Technology

Figure 7-19 X-Ray Example of Missing Solder Balls

Figure 7-20 X-ray Example of Voiding in Solder Ball Con-

tacts

Figure 7-21 Manual X-Ray System Image Quality

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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