IPC-7095C-2013.pdf - 第124页

7.8.5 Head-on-Pillow BGA head-on-pillow (HoP) solder joint is deﬁned as a joint that is comprised of two metallur- gical distinct masses. One formed from the BGA ball and the other from the reﬂowed solder paste. They hav…

not signiﬁcant at the larger ball image sizes but becomes

more important when the land diameter shrinks to accom-

modate a smaller ball or smaller BGA pitch. This is shown

through the determination of the area connected to the land

after the void has been determined.

Image of a BGA ball 600 µm with a 25% void =

(Dvoid)

= 22500 µm

(Dball)

= 360000 µm

= 6.25% area taken up by the void.

Comparing area of ball image with area of void image

leaves 265073 µm

remaining (πr

Image of a BGA ball 300 µm with a 25% void =

(Dvoid)

= 5625 µm

(Dball)

= 90000 µm

= 6.25% area taken up by the void.

Comparing area of ball image with area of void image

leaves 66268 µm

remaining (πr

Appendix A provides recommendations for process

improvement regarding the occurrence of voids in the ball

at incoming (Type A and B voids) as well as those voids

that are discovered after the assembly has gone through the

attachment processes (Type C, D and E voids).

See Appendix A, Table A-1 to Table A-3 for corrective

action indicators.

7.7.3 Process Control Criteria The accept/reject criteria

for BGA assemblies is established by J-STD-001 and IPC-

A-610. Those documents provide the ﬁnal accept/reject cri-

teria used in contractual agreements. The process correc-

tive actions identiﬁed in the following sections are intended

to establish continuous process improvement based on the

size of the voids, their location, and method of determina-

tion of the void occurrences.

Void clariﬁcation is also deﬁned as to whether the voids

occur prior to attachment to the mounting structure, or after

the assembly has taken place. This useful information can

be correlated to reliability conditions based on the end-use

environment. Using the size limitation structure a process

can be established that helps to meet the customers’ deﬁned

acceptability conditions (see Appendix A).

7.8 Solder Defects

7.8.1 Solder Bridging

Solder bridging is unacceptable.

Electrical testing, optical inspection (endoscope) or X-ray

inspection is necessary to detect solder bridging. Poor sol-

der paste printing, inaccurate placement, manual ‘‘tweak-

ing’’ after placement, and solder splattering during reﬂow

are typical causes of solder bridging. Solder balls too large

for the gap between the two substrates can also cause

bridging.

7.8.2 Cold Solder The reﬂow proﬁle should reach tem-

peratures high enough to ensure that the solder melts com-

pletely and proper wetting of the land surface occurs. A

cold solder joint can reduce mechanical integrity and can

cause it to fail electrically or function intermittently. Opti-

cal inspection after cross-sectioning is the best way to

inspect for cold solder joints.

7.8.3 Opens Solder opens are also unacceptable. Electri-

cal testing, optical inspection (endoscope) or X-ray inspec-

tion is necessary to detect solder opens. Poor solder paste

printing, inaccurate placement and manual ‘tweaking’’ after

placement are typical assembly-related causes of solder

opens. Coplanarity and substrate solderability problems can

also cause opens. Excessive mechanical stress can also

cause solder joints to crack and create opens.

7.8.4 Insufficient/Uneven Heating A common process

problem is insufficient or uneven heating of the BGA. This

problem occurs more often during rework but can also be

seen in production when working with multilayer boards

with many ground or power planes. The problem can also

occur on double-sided boards when a shielded component

is on the backside near the location of the BGA. The prob-

lem results when a thermal conductor removes the heat

from the BGA before complete reﬂow can occur. The

X-ray image of this problem is characterized by a variation

in the size of the solder balls at different locations under

the package.

Insufficient heating is generally characterized in an X-ray

image by small partially reﬂowed solder balls in the center

or to one side of the package. Insufficient heating may also

be characterized by a jaggedness around the perimeter of

these solder balls; indicating that the solder partially

reﬂowed but not long enough to completely wet to the land

and collapse to a nice round ball. Misalignment of the sol-

der ball with respect to the land is also an indicator of

inadequate heating. The X-ray image of misalignment is

characterized by elongated solder balls which may or may

not have a consistent orientation.

X-ray inspection at a 45° angle is also a useful technique

to locate signatures associated with insufficient heating or

nonwetting. The solder ball should contact and completely

wet to the land forming a smooth pillar. Signatures associ-

ated with insufficient heating include incomplete wetting to

the land, or an image elongation in the solder connection

indicating that the solder ball and solder paste did not ﬂow

together to form a single solder joint (see Figures 7-47 and

7-48).

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7.8.5 Head-on-Pillow BGA head-on-pillow (HoP) solder

joint is deﬁned as a joint that is comprised of two metallur-

gical distinct masses. One formed from the BGA ball and

the other from the reﬂowed solder paste. They have incom-

plete or no coalescence. This defect is known by many

other names such as head and pillow, head in pillow, ball

in cup, ball in socket, and hidden pillow (see Figure 7-49).

The sequence of Head-on-Pillow defect formation is

explained in Figure 7-50. First, the BGA ball is placed on

the solder paste which has been printed on the printed cir-

cuit board (PCB) lands (Figure 7-50 (a)). As the BGA on

the PCB enters the reﬂow soldering oven and its tempera-

ture increases, a gap develops between the ball and the sol-

der paste due to some factor, such as dynamic warpage of

the package and/or PCB (Figure 7-50 (b)). The solder on

the PCB land melts and ﬂux covers its surface. The solder

ball also melts and its surface, which typically has little or

no ﬂux covering it, starts to oxidize (Figure 7-50 (c)),

When the BGA package collapses, the ball once again

makes contact with the molten solder paste mass. At this

time, if the ball and the molten solder paste mass coalesce

together, then a good solder joint is formed. But, if there is

insufficient ﬂux activity and too much oxide coating on the

surface of the ball, it results in head-on-pillow.

7.8.5.1 Dynamic Warpage If the package warps during

reﬂow due to Coefficient of Thermal Expansion (CTE)

mismatch between the substrate and silicon as the package

temperature rises in the oven, the warping effect will cause

some solder balls to be lifted up from the solder paste on

the board. This action increases the oxide growth on the

surface of the ball as ﬂux is left behind on the pad, leaving

the ball devoid of ﬂux. Usually when package warpage is a

primary cause, the HoP defect will occur at the highest

warpage area of the package and adjacent joints will be

elongated. Figure 7-51 shows a high warpage BGA were

the corners are lifted resulting in HoP.

Boards may warp or sag during reﬂow resulting in

increased gap between the paste and the balls on the pack-

age. When the board is too thin and not supported during

reﬂow, board warpage can be a primary cause for HoP.

Usually when either board or package warpage is a primary

cause, more than one ball will show sign of HoP defect.

Also the adjacent joints will be elongated.

7.8.5.2 Reflow profile The reﬂow proﬁle parameters

have strong impact on HoP because of temperature differ-

ential (dT). The dT can be seen within a single component

due to board design (copper distribution), laminate or pack-

age material, and the type and size of the package. There is

also dT between the outer and inner rows of BGA balls

because of airﬂow.

Generally, the temperature of the outer row of BGA balls

are higher than the temperature of inner row balls, result-

ing in time delay between outer and inner row balls melt-

ing. However, even though they melt at different times, the

package collapses only after the inner ball melt. This time

delay also exposes the outer two balls to high temperatures

for longer time without the protection of ﬂux. This results

in their oxidation and, hence, contributes to HoP. This time

difference between inner and outer ball becoming liquidus

Figure 7-47 X-Ray Image Showing Uneven Heating

Note the solder balls are larger at the bottom than the top.

Figure 7-48 X-Ray Image at 45° Showing Insufficient Heat-

ing in One Corner of the BGA

Note the irregular shape of the solder bonds at the top of the image.

Figure 7-49 Example of Head-on-Pillow Showing Ball and

Solder Paste have not Coalesced

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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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