AN2053

AN2053
PCB Mounting Practices to Obtain Optimum Performance
for High-Voltage BGA and LGA Packages
Author:
Rohit Tirumala
Microchip Technology Inc.
INTRODUCTION
The plastic, or over-molded, Ball Grid Array (BGA) is a
lead-less, low-profile surface-mount package. It offers
a high number of potential input and output
connections. Lead-less packages provide short, heavy
copper (Cu) paths with low inductance (L) and
resistance (R) connections from packages to Printed
Circuit Board (PCB), enabling high-speed operation
with low switching and conduction losses.
Furthermore, the short vertical structure provides very
low RJC (junction-to-case or die-to-ball thermal
impedance) for high-power dissipation. The BGA
package was developed for high density and
high-speed digital devices, but has been increasingly
used for packaging power devices, both single and
multi-chip. Figure 1 shows an example of a 36-terminal
BGA package outline.
Power packages typically require fewer connections,
but do require higher current connections. Therefore,
power BGA packages have larger copper land areas
with multiple solder balls for higher current connections
and lower RJC. Even lower package RJC, L and R are
achieved with Land Grid Array (LGA) packages. The
LGA replaces solder balls/pads with larger area solder
pads. BGA ball arrays are usually standard ball sizes
and pitch, whereas LGA pads are almost always
custom to maximize package-to-PCB contact areas for
each product. Figure 2 shows an example of a
3-terminal LGA package outline.
Side View
Side View
Bottom View
Bottom View
FIGURE 2:
3-Terminal LGA Package
Outline Drawing Example.
FIGURE 1:
36-Terminal BGA Package
Outline Drawing Example.
 2015 Microchip Technology Inc.
DS00002053A-page 1
AN2053
Plastic BGA and LGA devices are molded packages
with an internal PCB substrate for die/component
assembly. The PCB substrate is typically multilayer,
with copper trace layers and interconnecting copper
vias optimized for electrical and thermal performance
by the BGA and LGA device manufacturers. Figure 3
shows an example of a 36-terminal BGA Digital IC
package structure, and Figure 4 shows an example of
a 3-terminal LGA power transistor package.
Molding Compund
Die and Wire Bonds
BGA Substrate
BGA AND LGA ASSEMBLY
MATERIALS, TECHNIQUES AND
CONFIGURATIONS
The BGA package can accommodate many solder ball
materials, size and pitch, but in practice there are only
a few common/standard ball solders, sizes and pitches.
Solder balls are usually Lead-Free (Tin/Silver or
Tin/Silver/Copper) and Eutectic Tin/Lead. Pb-Free
solders are generally preferred, but Sn/Pb solders are
still common for compatibility with older processes and
components and for some lower temperature devices.
The same or similar type solder pastes are typically
used for BGA and LGA assembly to the PCB.
Standard ball pitch/diameters and associated ball
separations are given in Table 1.
TABLE 1:
Cross Section
FIGURE 3:
36-Terminal BGA Digital IC
Package Structure Example.
Molding Compund
Die and Wire Bonds
BGA Substrate
Cross Section
FIGURE 4:
3-Terminal LGA Power
Transistor Package Structure Example.
STANDARD BALL PITCH,
DIAMETERS AND
SEPARATIONS FOR BGA
PACKAGES
Ball Pitch
(mm)
Ball Diameter
(mm)
Ball Separation
(mm)
0.75
0.40
0.35
1.00
0.40
0.60
1.00
0.50
0.50
1.00
0.60
0.40
1.27
(50 mil)
0.51
(20 mil)
0.76
(30 mil)
1.27
(50 mil)
0.64
(25 mil)
0.63
(~25 mil)
The larger LGA pads are usually coated with the same
solders (Pb-free or Sn/Pb), but the lower profile solder
on the pads does not contain solder balls. LGA pads do
not have standard sizes, and are designed/optimized to
accommodate specific products and applications.
BGA and LGA users must build on these electrical and
thermal advantages with suitable Solder Mask Defined
(SMD) solder connections to the PCBs and optimum
electrical and thermal PCB design. This application
note will provide recommendations for BGA and LGA
PCB design and assembly techniques.
DS00002053A-page 2
 2015 Microchip Technology Inc.
AN2053
PCB Solder Pads for BGA and LGA
Solder and terminal pads located on BGA/LGA PCB
are either SMD or Non-Solder Mask Defined (NSMD),
and sometimes both SMD and NSMD on the same
PCB. NSMD are generally preferred and more
accurate/reliable, but SMD accommodate larger land
areas for multi-ball connections, and can
accommodate higher currents and power dissipation.
Figure 5 shows a PCB SMD Terminal Pad/Resist
layout and cross-section. Figure 6 shows a PCB NSMD
Terminal Pad/Resist layout and cross section.
Cu
Resist Opening
Top View
Resist
Resist
Cu
PCB
Cross Section
FIGURE 5:
PCB SMD Terminal
Pad/Resist Layout.
• Recommended SMD PCB resist opening diameters
are 0.075 – 0.10 mm less than ball diameters.
• Recommended NSMD PCB pad and resist
openings are given in Table 2.
• Recommended LGA solder resists opening are
0.05 mm larger than the LGA pads, but custom
pad sizes and shapes for LGA often require
different rules, which are usually provided by the
LGA device manufacturer.
TABLE 2:
RECOMMENDED NSMD PCB
PAD AND RESIST OPENINGS
Ball
Pitch/Diameter
(mm)
Solder
Resist
Opening
Diameter
(mm)
Solder Pad
Diameter
(mm)
0.75/0.40
0.45
0.30
1.00/0.40
0.45
0.30
1.00/0.50
0.50
0.35
1.00/0.60
0.60
0.40
1.27/0.51
0.75
0.60
1.27/0.64
0.80
0.65
Standard recommendations are generally suited for
lower voltages between pads, and larger spacing may
be required at higher voltages. Higher voltage (HV)
minimum pad spacing guidelines should be compliant
with UL Pollution degree 1 minimum creepage paths,
assuming a suitable HV underfill (see Table 3 for a
condensed list of creepage distances). The HV device
manufacturer will typically recommend optimum PCB
pads for attaching BGA and LGA products, but these
may vary for some user applications.
TABLE 3:
Operating Voltage
(Volts, AC RMS or DC)
Cu
Resist Opening
Top View
Resist
Cu
Resist
PCB
Cross Section
FIGURE 6:
Resist Layout.
PCB NSMD Terminal Pad
 2015 Microchip Technology Inc.
CREEPAGE DISTANCES FOR
POLLUTION LEVEL 1 FROM
UL840
Minimum Creepage
Distance (mm)
80
0.22
100
0.25
160
0.32
200
0.42
250
0.56
320
0.75
400
1.00
Example of underfill materials are:
• Loctite® 3153™ for BGA packages
• Hysol® E1926 for LGA packages
DS00002053A-page 3
AN2053
Printed Circuit Boards for BGA and LGA
STENCIL AND SOLDER PASTE
PCBs are typically multilayer to accommodate
interconnects for higher density balls and pads. The
copper pads and traces on different layers are
interconnected with copper vias. These vias create
both electrical and thermal connections between the
layers. Vias at pads and under balls must be filled or
tented to provide flat and solderable pad surfaces. This
is especially important for BGA pads, which must
ensure accurate ball locations.
Solder pastes are usually Pb-Free (Sn/Ag/Cu), are
Eutectic Sn/Pb, and it is recommended that Pb-Free
solder paste be used with Pb-Free solder balls and that
Sn/Pb solder paste be used with Sn/Pb solder balls.
Mixing paste and ball solder types is possible, but
should be done cautiously.
High glass transition (Tg) FR-4 laminate materials are
most common and economical, but other higher
performance, high-Tg laminates may be used to meet
special electrical, mechanical or thermal requirements.
In general, the laminate Tg should be greater than
+170°C.
The BGA and LGA solder pads are copper, and a
number of different finishes are available with
differences in cost, shelf life and long-term reliability.
Common
finishes
are
Organic
Solderability
Preservatives (OSP), Hot Air Leveling, Sn/Pb (HASL),
Electroless Sn, Electroless Ag and Electroless Ni/Au.
The choice of pad finish must be made based on
specific products, applications and the PCB fabricators
capabilities.
Soldering BGA and LGA Packages to
Printed Circuit Boards
The key processes are:
1.
2.
3.
4.
5.
Stencil solder paste onto PCB pads
Place BGA/LGA into solder paste
Reflow the solder
Clean and remove flux
Add underfill.
There are variations and options for specific materials,
equipment, devices and assemblers, but these
processes are used for almost all BGA and LGA
assembly.
DS00002053A-page 4
Solder pastes should be stencil grade with high
solids/solder (typically 90% by weight). No-clean solder
pastes are preferred, but flux removal/cleaning is
possible. There are many types of flux systems
including aqueous, semi-aqueous and solvent based.
New and complex cleaning chemistries are very
specialized, and the cleaning chemistry recommended
by the solder paste manufacturers is usually the best
option.
Stainless steel stencils are recommended, with the
stencil apertures and thicknesses shown in Table 4.
These may vary based on the solder paste and print
parameters.
TABLE 4:
STENCIL APERTURES AND
THICKNESSES FOR BGA
PACKAGES
Ball
Pitch/Diameter
(mm)
Stencil
Aperture
Diameter
(mm)
Stencil
Thickness
(mm)
0.75/0.40
0.35
0.125
1.00/0.40
0.35
0.125
1.00/0.50
0.40
0.150
1.00/0.60
0.45
0.150
1.27/0.51
0.55
0.150
1.27/0.64
0.60
0.150
PLACEMENT OF BGA/LGA INTO SOLDER
PASTE
The placement of BGA and LGA must be automatic to
ensure the typical requirements of ±0.10 mm, which is
within the ±0.05 mm accuracy of common pick and
place equipment.
 2015 Microchip Technology Inc.
AN2053
REFLOW SOLDERING
The recommended solder reflow equipment is Forced
Air Convection (FAC), or a combination of FAC and
Infrared (IR). Other types of equipment can achieve the
required profile, but are less common and require
special care to control.
There are two standard profiles solder reflow profiles:
one for Pb-Free (Sn/Ag and Sn/Ag/Cu) solder, and the
other for Eutectic Sn/Pb solder. The recommended
profiles are shown in Figure 7 and Figure 8,
respectively. In general, it is recommended to begin
with the solder paste manufacturers’ recommended
profile, which addresses their solder composition,
solder particle size/shape, and organic system. Any
recommended profiles are a starting point, and must be
adjusted to match the requirements of specific products
and assembly processes. Factors to consider include
the product size and mass, the type of assembly
equipment, carriers and more. The nominal peak
temperatures are +245°C for Pb-Free solder and
+225°C for Eutectic Sn/Pb solder.
300
Temperature (ƒC)
250
Typical
Minimum
Maximum
200
150
100
50
0
Time (s)
FIGURE 7:
Typical Pb-Free (Sn/Ag and
Sn/Ag/Cu) Solder Reflow Profile.
300
Temperature (ƒC)
250
Typical
Minimum
Maximum
200
150
100
50
0
Time (s)
FIGURE 8:
Typical Eutectic Sn/Pb
Solder Reflow Profile.
 2015 Microchip Technology Inc.
DS00002053A-page 5
AN2053
CLEAN AND REMOVE FLUX
BGA AND LGA SOLDERING REWORK
Cleaning and baking are recommended, and may be
advantageous even for no-clean solder pastes. Solder
pastes containing fluxes require more aggressive
cleaning materials and processes. The specific cleaning chemistry and equipment required is dependent on
the solder paste flux and organic chemistry, and the
solder paste manufacturers’ recommendations are the
best starting point for determining cleaning chemistry
and associated cleaning equipment. Again, any cleaning process will need to be optimized for specific solder
reflow processes and products.
Although rework should not be required in a
well-controlled assembly process, it is possible to
remove and replace BGA and LGA packages.
However, it is not possible to rework individual solder
joints. Removal is usually accomplished by heating
locally below the device to be removed and removing
the device after the solder melts. Before replacing the
device with a new BGA/LGA, the pads must be clean
and flat. The replacement device will typically need
added flux to hold during automatic placement, and
should also be heated locally from the back of the PCB.
Any rework should be performed before underfill.
UNDERFILL FOR BGA AND LGA
Underfill is recommended for all BGA and LGA
assemblies, and it should be used automatically unless
there is a good reason not to use it. It is even more
important with larger packages, higher power
dissipation and higher voltages. It can:
• Provide mechanical strength against vibration,
coefficient of thermal expansion (CTE) mismatch
and other stresses
• Improve heat transfer to the PCB
• Eliminate dirt, moisture and other contaminates
that can collect under the BGA and LGA
packages initially and over the life of the product
• Eliminate high-voltage arcing or leakage between
pads, and allow reduced spacing between pads.
The minimum spacing with underfill should meet
UL Pollution degree 1 minimum creepage, but
specific products and application conditions may
allow tighter spacing or require larger spacing.
BGA AND LGA RECOMMENDATIONS
DISCLAIMER
The BGA and LGA recommendations are based on
typical products, materials, applications and assembly
processes/equipment. Although these recommendations will often be sufficient, they should always be validated for the specific products and product
requirements. This is especially important for
high-power and high-voltage products, which can have
unique requirements.
Underfills are very specialized with specific
formulations for BGA and LGA. Their viscosity is
adjusted for specific space height to make underfill
application easy and virtually void free. It is generally
applied along one or two adjacent edges of the BGA
and LGA package, where the surface tension pulls the
underfill under the package while pushing air out the
opposite edges.
INSPECTION OF BGA/LGA PACKAGES
The BGA and LGA solder pads are hidden under the
packages and cannot be visually inspected. However,
current solders pastes, stencils and placement
equipment are well controlled and can provide very
high assembly yields. The yields are generally verified
with 100% testing, and sometimes supported by X-ray
inspection. X-rays are well suited for sample checking
and analysis of solder defects.
DS00002053A-page 6
 2015 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
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ISBN: 978-1-5224-0128-5
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 2015 Microchip Technology Inc.
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DS00002053A-page 7
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DS00002053A-page 8
 2015 Microchip Technology Inc.
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