IPC-7095C-2013.pdf - 第75页

Unless the via is ﬁlled or plated shut, then air is likely to remain under the solder paste forming a void in the solder ball when reﬂowed. For metal-deﬁned lands with microvias, crack-propagation will start from the voi…

Vias capped on one side can pose a variety of reliability

issues. Each issue should be evaluated for relevancy to the

speciﬁc environmental design criteria:

• the hole may be partially ﬁlled creating uneven stresses

• the void may utilize a sizable part of the connection area

reducing the structural support

• the void may reduce the thermal path

When using via-in-pad technology, a void will occur in the

joint as shown in Figure 6-19, unless the via is capped on

the component placement side of the board. Most experts

agree that these void conditions, created due to entrapped

air, are acceptable and have no impact on the reliability of

the joint. There is no doubt that the conditions depend not

only on the process, but the size of the BGA land and the

diameter of the hole. In addition, there is a difference as to

whether the hole is a through-hole, blind via, or microvia.

Figure 6-20 shows the characteristics of the three hole con-

ﬁgurations and what takes place at incoming after the sol-

der paste has been printed and the BGA has been placed;

the conditions of the ball and hole during reﬂow soldering;

and, ﬁnally, the characteristics of the resulting solder joint.

One of the major reasons for the occurrence of the void

conditions is the entrapped gas that exists under the solder

paste during the original paste printing and BGA place-

ment. During the reﬂow operation, the entrapped gas and

the solder paste volatiles need to escape, and this creates

the minor occurrences of absence of solder at the center

part of the ball as shown in the illustration.

6.3.6 Fine Pitch BGA Microvia in Pad Strategies When

the pitch is less than 0.8 mm, there is not enough room to

place a via using current mechanical drill technology for

full array BGA components. For ﬁner pitch BGAs, in order

to achieve increased routing area, putting a microvia-in-pad

may be necessary. These are blind vias that connect to the

ﬁrst or second internal layer of the printed board. They are

typically laser drilled but, in some cases, a mechanical

drilling process is used (see Figure 6-21).

IPC-7095c-6-19

Figure 6-19 Cross Section of Via-in-Pad Design Showing

Via Cap and Solder Ball

IPC-7095c-6-20

Figure 6-20 Via-in-Pad Process Descriptions

BGA

Incoming - Before Assembly Process

During Reflow Soldering

Post Reflow Soldering

After Solder Paste

Printing & BGA Placement

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---

Unless the via is ﬁlled or plated shut, then air is likely to

remain under the solder paste forming a void in the solder

ball when reﬂowed. For metal-deﬁned lands with

microvias, crack-propagation will start from the void asso-

ciated with the via and move towards the outside of the ball

(see Figure 6-22).

These voids have been shown to decrease the amount of

stress that the solder joint can withstand when compared to

a comparable MD land without via in pad. For this reason

it is not recommended to use via in pad for high stress sol-

der balls such as the corner balls or those directly under the

edges of the die.

6.3.7 Power and Ground Connectivity When lands are

required within a ground or power plane, it is a common

practice to leave an opening in the solder mask covering

the plane to provide access to the land. When increased

fatigue life is needed for these locations or for thermal iso-

lation, an MD land can be created within the plane etching

crescent-shaped relief pattern around the land (see Figure

6-23). Two, three, or four spokes (SMD Segments may be

used. The MD Segment should be arranged so that the

SMD segment is facing towards the corner of the BGA to

provide maximum fatigue strength.

6.4 Impact of Wave Solder on Top Side BGAs

6.4.1 Top Side Reflow

Mixed Technology printed circuit

boards are typically assembled by ﬁrst reﬂow soldering the

surface mount packages on the top side of the board and

then wave soldering the through-hole packages (inserted

from the top side). For double-sided boards, the bottom

side SMT components are usually placed before top side

components and can be reﬂowed or adhesive used to hold

them in place. Bottom side components must be shielded

from the wave by a wave pallet if not secured with adhe-

sive. During the wave soldering process step, however, the

reﬂow soldered surface mount components on the top side

of the board are also heated. This heating can cause the

solder joints of these components to melt if the temperature

increases close to the liquidus point of the solder alloy.

Hence, care needs to be taken to prevent the solder joints

of these components from reaching the liquidus tempera-

ture.

6.4.2 Impact of Top Side Reflow BGA solder joints need

particular attention in this regard since the solder joints are

under stress during the wave soldering operation. If these

solder joints reach the liquidus temperature (183°C for the

eutectic tin/lead solder composition; 217°C for SAC alloy)

then there is a potential to de-wet or be pulled away from

the board or the package substrate due to the thermo-

mechanical strains induced in them at the elevated tem-

peratures. Since solder is extremely soft, even at tempera-

tures approaching the liquidus temperature, the potential

for cold solder, de-wetting or ball deformation exists when

their temperatures do not reach solidus.

Figure 6-24 depicts examples of ball deformation and

de-wetting for a BGA component located on the top side of

a motherboard. The solder joints of the BGA reached a

peak temperature of 180°C during wave solder. BGAs are

more prone to such defects than leaded surface mount sol-

der joints since there is less strain relief.

IPC-7095c-6-21

Figure 6-21 Microvia Example

PCB pad

Cu layer

Cross-section

Figure 6-22 Microvia-in-Pad Voiding

IPC-7095c-6-23

Figure 6-23 Ground or Power BGA Connection

Segment

Crescent Opening

Etched Into Plane

Copper Plane

Solder

Mask

SMD

Segment

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---

To avoid problems in BGA solder joints on the top side of

the board, their temperatures should not exceed 150°C dur-

ing tin/lead wave soldering and 190°C for lead-free alloy

wave soldering. This is less than the maximum temperature

allowed for the ﬁne pitch leaded components such as plas-

tic QFPs.

Figure 6-25 is an example of an acceptable temperature

proﬁle for the solder joints on a mixed technology board

during the wave solder process.

To determine the various ways of keeping the temperature

below 150°C (or 190°C for lead free), it is best to ﬁrst

identify the various ways in which the BGA solder joints

are heated during the wave solder process. Figure 6-26

illustrated three pathways. Pathway A is by conduction

through the thickness of the board from bottom to top.

Pathway B is by conduction through the barrel of the via,

along a conductor connecting the via to the BGA solder

joint land. Pathway C is by convection and radiation from

the preheaters on the top in the wave solder machine.

6.4.3 Methods of Avoiding Top Side Reflow The meth-

ods of avoiding top side reﬂow aim to reduce the heat

transfer to the BGA solder joints by one or more of the

three pathways described above. Figure 6-27 illustrates

these methods. A heat shield can be placed over the BGA

packages to avoid direct heating from the preheaters in the

wave solder machine. These shields can be mechanically

attached to the wave solder pallets.

Secondly, vias can be capped by solder mask on the bottom

side of the board. This via-capping process is very com-

monly employed in the industry, for a variety of reasons.

Via capping rules should be built into the design for manu-

facturability (DfM) process, during board design.

Vias that certainly need to be capped are those that are

connected to the BGA land with a short trace length or

those that are connected to a plane in the board.

Thirdly, a nonmetallic wave shield can be placed immedi-

ately below the BGA package locations on the bottom side

of the board to avoid the wave from touching these board

locations. The wave shield can also be attached to the wave

pallet by nonmetallic ﬁngers.

Selective wave pallets can utilize solid material under the

BGAs to prevent solder contact with the bottom of the

board and heat transfer up the vias to the lands. This will

prevent secondary reﬂow of the BGA joints.

The efficacy of each of these methods should be conﬁrmed

by measuring the temperature proﬁle of the BGA solder

joints during the wave solder process and ensuring that

they stay below 150°C.

Figure 6-24 Example of Top Side Reﬂow Joints

IPC-7095c-6-25

Figure 6-25 Example of Wave Solder Temperature Proﬁle of Topside of Mixed Component Assembly

150° C

160° C

183°C

80° C

Zone # 2

100° C

Zone # 1 Zone # 3 Chip Contour

40° C

60° C

Maximum peak

temperature for

BGA is 150°

Maximum peak

temperature for

Fine Pitch is 160°C

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---