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Guidelines on How to Use W-CSP Packages and Create
Associated PCB Footprints
INTRODUCTION
The Wolfson Wafer level ChipScale Package (W-CSP) is a die-sized package, which obtains
electrical contact via solder bumps on the bottom surface of the device to a Printed Circuit Board
(PCB). But it differs from Ball Grid Array (BGA) package by using Redistribution Layer (RDL) instead
of bonding wire to connect the die pads.
This Application note sets out to explain some of the issues related to the design of a PCB footprint
for the W-CSP package and then goes on to describe practical considerations when soldering the
device to a PCB.
PACKAGE DIMENSIONS
The first thing to consider when creating a W-CSP footprint is the package drawing for the device.
Wolfson uses a number of different W-CSP packages based on JEDEC specifications as follows in
Table 1. Current W-CSP package I/O designs include from sixteen up to 42 solderable terminals but
the max terminal count is possibly increasing to 105. See Figures 1 – 4. Package dimensions are
shown in Figures 5 – 8. The dimensions of the packages are, at time of document release, as
follows. Please refer to the individual datasheets for the most current issue of the package
dimensions.
SIZE
PIN
BALL PITCH
1.640mm×1.640mmX0.7mm
16
0.4mm
2.552mmX2.602MMX0.7mm
25
0.5mm
2.590mm×2.500mmX0.7mm
28
0.4mm
3.226mm×3.440mmX0.7mm
42
0.5mm
Table 1 WLCSP Package Range
Figure 1 16 PIN WLCSP Package
WOLFSON MICROELECTRONICS plc
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July 2008, Rev 1.0
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Figure 2 25 PIN W-CSP Package
Figure 3 28 PIN W-CSP Package
Figure 4 42 PIN W-CSP Package
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B: 16 BALL W-CSP PACKAGE 1.640 X 1.640 X 0.7mm BODY, 0.40 mm BALL PITCH
DM052.A
3
D
A
DETAIL 1
2
G
3
4
A2
2
1
A1
CORNER
4
A
B
e E1
5
E
C
D
2X
e
D1
DETAIL 2
2X
0.10 Z
0.10 Z
TOP VIEW
BOTTOM VIEW
f
SOLDER BALL
f
bbb Z
h
1
Z
A1
DETAIL 2
Dimensions (mm)
Symbols
A
A1
A2
D
D1
E
E1
e
f
g
MIN
0.615
0.195
0.385
0.091
0.035
h
NOM
0.7
0.220
0.410
1.640 BSC
1.200 BSC
1.640 BSC
1.200 BSC
0.400 BSC
MAX
0.785
0.245
0.435
0.070
0.105
NOTE
5
0.260 BSC
NOTES:
1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS.
2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING.
3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE.
4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY.
5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH.
6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE.
7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C.
Figure 5 16 pin W-CSP Package
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B: 25 BALL W-CSP PACKAGE 2.552 X 2.602 X 0.7mm BODY, 0.50 mm BALL PITCH
DM056.A
6
D
DETAIL 1
A
2
g
5
A2
3
4
2
1
A
A1
CORNER
4
B
C
E1
e
E
5
D
E
2X
e
DETAIL 2
2X
D1
0.10 Z
0.10 Z
TOP VIEW
BOTTOM VIEW
f1
SOLDER BALL
f2
bbb Z
h
1
Z
A1
DETAIL 1
DETAIL 2
Symbols
A
A1
A2
D
D1
E
E1
e
f1
MIN
0.615
0.225
0.355
Dimensions (mm)
NOM
MAX
0.7
0.785
0.250
0.275
0.405
0.380
2.552 BSC
2.00 BSC
2.602 BSC
2.00 BSC
0.50 BSC
f2
0.266 BSC
0.291 BSC
g
0.035
h
0.070
NOTE
5
0.105
0.314 BSC
NOTES:
1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS.
2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING.
3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE.
4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY.
5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH.
6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE.
7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C.
Figure 6 25 pin W-CSP Package
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B: 28 BALL W-CSP PACKAGE 2.590 X 2.500 X 0.7mm BODY, 0.40 mm BALL PITCH
DM054.A
6
D
DETAIL 1
A
2
G
6
A2
5
3
4
2
1
A
A1
CORNER
4
B
e 5
C
E1
E
D
E
F
2X
e
DETAIL 2
2X
D1
0.10 Z
0.10 Z
TOP VIEW
BOTTOM VIEW
f1
f2
bbb Z
h
1
Z
A1
DETAIL 2
Dimensions (mm)
Symbols
A
A1
A2
D
D1
E
E1
e
f1
f2
g
MIN
0.615
0.195
0.385
NOM
0.7
0.220
0.410
2.590 BSC
2.000 BSC
2.500 BSC
2.000 BSC
0.400 BSC
MAX
0.785
0.245
0.435
0.070
0.105
NOTE
5
0.275
0.230
0.035
h
0.260 BSC
NOTES:
1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS.
2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING.
3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE.
4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY.
5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH.
6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE.
7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C.
Figure 7 28 pin W-CSP Package
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B: 42 BALL W-CSP PACKAGE 3.226 X 3.440 X 0.7mm BODY, 0.50 mm BALL PITCH
DM049.C
6
D
DETAIL 1
A
2
G
6
A2
5
3
4
2
1
A
A1
CORNER
4
B
C
e
E1
D
E
5
E
F
G
2X
e
DETAIL 2
2X
D1
0.10 Z
0.10 Z
TOP VIEW
BOTTOM VIEW
f
SOLDER BALL
f
bbb Z
h
1
Z
A1
DETAIL 2
Symbols
A
A1
A2
D
D1
E
E1
e
f
g
MIN
0.615
0.225
0.355
Dimensions (mm)
NOM
MAX
0.7
0.785
0.250
0.275
0.405
0.380
3.226 BSC
2.500 BSC
3.440 BSC
3.00 BSC
0.50 BSC
0.060 BSC
0.035
h
0.070
NOTE
5
0.105
0.315 BSC
NOTES:
1. PRIMARY DATUM -Z- AND SEATING PLANE ARE DEFINED BY THE SPHERICAL CROWNS OF THE SOLDER BALLS.
2. THIS DIMENSION INCLUDES STAND-OFF HEIGHT ‘A1’ AND BACKSIDE COATING.
3. A1 CORNER IS IDENTIFIED BY INK/LASER MARK ON TOP PACKAGE.
4. BILATERAL TOLERANCE ZONE IS APPLIED TO EACH SIDE OF THE PACKAGE BODY.
5. ‘e’ REPRESENTS THE BASIC SOLDER BALL GRID PITCH.
6. THIS DRAWING IS SUBJECT TO CHANGE WITHOUT NOTICE.
7. FOLLOWS JEDEC DESIGN GUIDE MO-211-C.
Figure 8 42 pin W-CSP Package
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PACKAGE CONSTRUCTION
A cross-section of a typical RDL W-CSP is given in Figure 9.
RDL W-CSP construction consists of:
-
Die with bond pad and original passivation
-
Passivation 1
-
Metallization etch (RDL metal) to move solder bumps from peripheral bond pads to any bump
array pattern
-
Passivation 2
-
Under Bump Metal (UBM) contact
-
Ball drop 250/300µm solder sphere (solder composition: SnAg(3%)Cu(0.5%))
Solder ball
RDL metal
RDL Passivation 2
UBM
RDL Passivation 1
Bond pad
Si Wafer
Die Original
Passivation
Protective
surface coating
Figure 9 RDL W-CSP Cross-Section
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PCB DESIGN CONSIDERATIONS
The PCB design rules when using the CSP packages are fundamentally no different to those for
BGA devices of the same pitch. The most important considerations are those effects that occur
during the PCB assembly process, which do affect the PCB land layout.
It is recommended that the CSP device is not mounted in an area of the board that is subject to high
degrees of mechanical stress.
ROUTING OF TRACES
Routing strategy for CSP devices will depend upon the PCB technology being used and the volume
in which they are being manufactured. Best practice for high assembly yield and minimum
component footprint is a “via in pad” approach with a flat, filled microvia to an inner layer where the
traces can be fanned out. Where the device pitch and available technology permit a “dog-bone”
approach may also be used where the signal is tracked out to an adjacent via down to an inner layer
(or opposite side of PCB).
The trace-width for connecting to CSP pads (either for dog-bone or tracking directly out on top layer)
should be a maximum of half the pad diameter wherever possible. Exceptions may need to be made
for power and ground or other signals that are high current or are sensitive to voltage drop.
UNDERFILL
Underfill can affect the mechanical reliability of the die and substrate assembly, for example, board
level mechanical performance during bending, vibration, drop testing and thermal cycling. Underfill
material can affect the solder joints thermal cycle life. Wolfson performs die and board level CSP
qualification without the use of underfill.
CSP FOOTPRINTS DIMENSIONS
The actual PCB footprint used for a CSP device will depend on the intended assembly process. It is
recommended that footprint design be carried out with reference to IPC-7351 (latest revision) and the
assembler’s recommendations.
SOLDER MASK AND STENCIL DESIGN CONSIDERATIONS
Non-Solder Mask Defined (NSMD) pads are recommended for “via in pad” routing on CSP devices.
Solder Mask Defined (SMD) pads may be used as an alternative. The CSP footprint should
exclusively use either NSMD or SMD pads, the two types should never be mixed on the same
package.
For effective solder paste release it is recommended that the stencil aperture be square with rounded
corners (25µm radius) and trapezoidal sidewalls (larger opening to bottom side).
CSP DEVICE HANDLING
Due to CSP devices are silicon level packages rather than plastic encapsulated IC packages, CSP
devices are more easily damaged than other IC packages. It is recommended that a vacuum nozzle
be used to pick the device up in both manual and automated assembly. If tweezers need to be used
to lift or position a device they should always be plastic rather than metal.
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SOLDER PASTE RECOMMENDATIONS AND REFLOW PROFILE
Due to the size, pitch and depth of the stencil apertures for the W-CSP package it is recommended
that type 4 no-clean solder pastes be used for printing. For reflow it is recommended an IR or
Forced Convection system be used or a combination system of IR and Forced Convection. For
further information on soldering, please refer to WAN_0158.
INSPECTION OF SOLDER JOINTS AFTER REFLOW
Due to the pad layout of the W-CSP the solder joints are formed underneath the package and are not
visible. It is recommended that to ensure the joints are soldered sufficiently X-ray inspection be
utilized whenever possible. Visual inspection may be used for a cursory inspection to ensure that
there is no obvious solder bridging.
Shown below in Figure 10 is a typical x-ray inspection of the Wolfson 42-pin W-CSP Daisychain part
on test board.
Figure 10 X-Ray Inspection of 42 pin W-CSP Daisychain Part on Test Board
As can be seen the solder has reflowed to form acceptable joints and there is minimal voiding in the
solder joints, also there is no bridging visible between the joints. X-Ray inspection can also be useful
in highlighting possible process problems such as solder balling and voiding which are often
indications of poorly optimized reflow profiles.
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W-CSP REWORK
Wolfson does not recommend any rework on W-CSP part itself.
APPLICATION SUPPORT
If you require further information or require technical support, please contact Wolfson
Microelectronics Applications group through the following channels:
Email:
Telephone:
Fax:
Mail:
[email protected]
(+44) 131 272 7070
(+44) 131 272 7001
Applications at the address on the last page.
or contact your local Wolfson representative.
Additional information may be made available from time to time on our web site at:
http://www.wolfsonmicro.com
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IMPORTANT NOTICE
Wolfson Microelectronics plc (“Wolfson”) products and services are sold subject to Wolfson’s terms and conditions of sale, delivery
and payment supplied at the time of order acknowledgement.
Wolfson warrants performance of its products to the specifications in effect at the date of shipment. Wolfson reserves the right to
make changes to its products and specifications or to discontinue any product or service without notice. Customers should
therefore obtain the latest version of relevant information from Wolfson to verify that the information is current.
Testing and other quality control techniques are utilised to the extent Wolfson deems necessary to support its warranty. Specific
testing of all parameters of each device is not necessarily performed unless required by law or regulation.
In order to minimise risks associated with customer applications, the customer must use adequate design and operating
safeguards to minimise inherent or procedural hazards. Wolfson is not liable for applications assistance or customer product
design. The customer is solely responsible for its selection and use of Wolfson products. Wolfson is not liable for such selection
or use nor for use of any circuitry other than circuitry entirely embodied in a Wolfson product.
Wolfson’s products are not intended for use in life support systems, appliances, nuclear systems or systems where malfunction can
reasonably be expected to result in personal injury, death or severe property or environmental damage. Any use of products by the
customer for such purposes is at the customer’s own risk.
Wolfson does not grant any licence (express or implied) under any patent right, copyright, mask work right or other intellectual
property right of Wolfson covering or relating to any combination, machine, or process in which its products or services might be or
are used. Any provision or publication of any third party’s products or services does not constitute Wolfson’s approval, licence,
warranty or endorsement thereof. Any third party trade marks contained in this document belong to the respective third party
owner.
Reproduction of information from Wolfson datasheets is permissible only if reproduction is without alteration and is accompanied by
all associated copyright, proprietary and other notices (including this notice) and conditions.
Wolfson is not liable for any
unauthorised alteration of such information or for any reliance placed thereon.
Any representations made, warranties given, and/or liabilities accepted by any person which differ from those contained in this
datasheet or in Wolfson’s standard terms and conditions of sale, delivery and payment are made, given and/or accepted at that
person’s own risk. Wolfson is not liable for any such representations, warranties or liabilities or for any reliance placed thereon by
any person.
ADDRESS:
Wolfson Microelectronics plc
26 Westfield Road
Edinburgh
EH11 2QB
United Kingdom
Tel :: +44 (0)131 272 7000
Fax :: +44 (0)131 272 7001
Email :: [email protected]
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