IPC-7095C-2013.pdf - 第126页

reﬂow . Non-coalesce of solder particles are seen, which indicates that solder paste does not have good oxidation resistance in the conditions it was reﬂowed. 7.8.5.4 How to Mitigate HoP Problem HoP is not an easy defect…

is called Liquidus Time Delay (LTD), and plays a critical

role in HoP (see Figure 7-52).

Looking at Figure 7-52, there are two TALs. However, the

TAL after collapse shown in the shaded area is the mini-

mum required TAL to achieve a good joint. In other words,

in order to minimize HoP, one must make sure that the true

TAL is long enough to form good joints.

7.8.5.3 Solder Paste The solder paste properties are

important to accommodate the soldering challenges of sol-

der ball excessive oxidation due to exposure to lead free

high temperatures and separation of the BGA ball from the

paste. Flux chemistry of these lead-free solders must have

adequate properties to prevent exhaustion of ﬂux activation

in high temperature. Three key paste properties which

affect the HoP defect are: solder paste stability with time

and temperature, solder paste wettability, and the oxidation

resistance of the solder paste. Low oxidation resistant paste

implies that ﬂux is not able to protect the solder powder’s

surface. There is high oxidation at outer surface also called

graping. Figure 7-53 shows paste printed on a board after

IPC-7095c-7-50

Figure 7-50 Head-on-Pillow Process Sequence Occurrences

Time

After BGA

placement

component

PCB

In Soak Zone

Above Solder

Melting Point

Head & Pillow

Defects

(a)

(b)

(c)

(d)

(e)

Temperature

Figure 7-51 HoP Due to High Package Warpage

Figure 7-52 Example of Liquidus Time Delay

220 ºC

Time [sec]

Outer ball

Inner ball

LTD

Figure 7-53 Solder Particles on Board Noncoalesced After

Reﬂow

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reﬂow. Non-coalesce of solder particles are seen, which

indicates that solder paste does not have good oxidation

resistance in the conditions it was reﬂowed.

7.8.5.4 How to Mitigate HoP Problem HoP is not an

easy defect to solve since there are many failure modes that

can cause this defect. In many cases, the HoP problem

consists of more than one failure mode at a time. The best

problem-solving approach would be to identify which are

the primary failure modes that need to be addressed to

eliminate HoP.

When a HoP problem exists, increasing solder paste vol-

ume will decrease the incidence of HoP. It may require

overprinting solder paste on some or all lands.

Some of the reﬂow parameters that have impact on HoP are

peak temperature and TAL. When dealing with HoP

defects, increasing TAL and peak temperature can decrease

the defect level by adding more time for the package to

contact the paste after full collapse and coalesce. One other

reﬂow parameter that could have an impact on HoP is soak

time. The impact depends on the type of paste used and its

behavior in high temperature. Follow the paste manufac-

ture recommendations and be sure not to stress and dry the

ﬂux before reﬂow.

Using nitrogen in the reﬂow oven has been used to reduce

oxidation and increase SMT yield for HoP.

7.8.6 Hanging Ball/Non-Wet Open (NWO) A common

failure signature that is similar to head on pillow is head

with no pillow, which is known as nonwet open or hanging

ball. Failure happens in packages with high warpage. The

solder paste on the land adheres to the ball instead of

remaining on the land where it is supposed to be during

reﬂow. Because of package warpage, a gap is formed

between the package ball and the solder paste and the sol-

der sticks to the ball and coalesces with the ball. Oxide

builds up on the PCB land since it is exposed to high tem-

perature of reﬂow without the protection of ﬂux resulting

in open joint known as hanging ball. Usually there is no

IMC formed on the land and it has the appearance of never

having any printed paste on it (see Figure 7-54).

Various factors affect hanging ball defects. They are type of

paste and ﬂux, board land surface ﬁnish (OSP), land design

(SMD vs. MD) and copper on inner layers (thermal/ground

planes). To reduce occurrence of hanging balls, one should

change to a different solder paste chemistry, or add more

paste volume, or use nitrogen; all the solutions common to

HoP. Increasing the BGA land size also reduces hanging

ball incidence since a larger paste area provides more sur-

face tension and holds on to the paste instead of letting the

balls take the paste with them.

7.8.7 Component Defects Component defects such as

popcorning and warpage are generally caused by improper

handling of the BGA component prior to reﬂow. Both of

these problems produce a characteristic signature in the

X-ray image. Popcorning causes the BGA package to

expand below the die; resulting in an increase in size (and

possibly bridging) of the solder balls in the center of the

package as they are squished between the package and the

board (see Figure 7-55).

BGA warpage is more subtle than popcorning and can be

more difficult to detect in an X-ray image (see Figure

7-56). Warpage tends to be the largest at the corners of the

package. The X-ray image of a warped BGA tends to have

large elongated solder connections at the corner of the

package where warpage has occurred. The illustration in

Figure 7-56a shows the X-ray image and Figure 7-56b

shows a video microscope image of the package. Note that

in Figure 7-56b the ripple in the substrate, which is char-

acteristic of a stress relief which likely occurred during

reﬂow.

Figure 7-54 Hanging Ball Examples

Figure 7-55 X-Ray Image of Popcorning

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7.8.8 Defect Correlation/Process Improvement It is

important to use inspection information to control the

manufacturing process to maximize quality and yield. The

manufacturing process leaves a signature (acceptable or

unacceptable) on each component; this signature can be

observed through inspection. The signature can be

observed using methods and tools discussed previously.

In many cases a visual inspection of a BGA is the ﬁrst clue

to any problems. An operator can look at the edge of the

BGA on all four sides. The distance between the BGA and

the circuit board should appear uniform and the solder balls

should appear consistent in shape.

To directly observe the solder connections under a BGA,

X-ray or optical inspection (endoscope) is necessary. These

methods can be used to inspect for obvious defects such as

bridges and missing balls. They are also useful for charac-

terizing the BGA reﬂow process. During inspection the

BGA solder balls should be examined for uniformity of

size and shape. In the absence of wetting indicators, the

solder balls should appear round and of equal size through-

out the package. A PBGA with a 0.75 mm diameter ball

prior to reﬂow will swell to a nominal 0.90 mm diameter

after reﬂow; a 36% increase. A 10-15% variation in solder

ball area from the center to the edge of the package is nor-

mal, but a larger variation can indicate a problem with the

reﬂow process.

X-ray inspection of the BGA from an angle is also useful

to examine the shape of the BGA solder ball in the area at

which it makes contact with the land. By changing the

angle of X-ray inspection, the land is shifted so that it does

not obscure the rest of the solder ball. This allows the

operator to inspect the shape of the solder connection as it

forms onto the land to verify that the land is in contact with

the solder ball and the solder is completely wetted to the

land.

Quantitative measurement of the solder bond X-ray image

can be performed using image analysis software. Such soft-

ware is useful, but not required for the inspection of BGAs.

The advantage of the software is in its ability to identify

and display subtle variations in the size and shape of the

solder bond image which is not easily observed by an

operator. These subtle variations are a signature of the

process used to manufacture the part and can be used to

monitor the process and to correct for deﬁciencies. A num-

ber of signatures can be correlated with known process

deﬁciencies.

7.9 Repair Processes

7.9.1 Rework/Repair Philosophy

Plastic ball grid arrays

are a forgiving component package. With their self align-

ment characteristics, a ball grid array can be placed as

much as 50% ball off land and when reﬂowed, the package

will properly align itself. With a controlled process and

appropriate equipment, rework should be kept to a mini-

mum.

There are many rework systems on the market, most of

which use a prism for placement, which allows the view-

ing of the land pattern on the board with the ball image

superimposed over it. Most systems also allow board pre-

heat and stored reﬂow proﬁles for many different compo-

nent sites. This section will focus on conditions that should

be met in order to successfully rework a plastic BGA.

There are four main rework functions for BGAs: removing

the component, redressing the site, placing the component,

and reﬂowing the component. These will be discussed in

the following paragraphs.

7.9.2 Removal of BGA When removing a BGA a deci-

sion has to be made regarding whether the component will

be reballed and used again or simply discarded. Special

considerations need to be made if the BGA will be reballed

and reused, i.e., maximum number of reﬂow cycles as rec-

ommended by the component vendor. Typically 3 reﬂows

IPC-7095c-7-56a.b

Figure 7-56 X-Ray Image Showing Warpage in a BGA

The x-ray image (a) shows a distortion of the ball bonds in the lower

right corner; characterized by large elongated bonds. The same

location as viewed through a visual microscope (b) shows warpage

and the solder ball peaking away from the package.

(a)

(b)

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