Reliability Issues for Printed Circuit Boards in Lead-Free ... - SMTA

Reliability Issues for
Printed Circuit Boards
in Lead-Free Soldering
Werner Engelmaier
Engelmaier Associates, L.C.
7 Jasmine Run
Ormond Beach, FL 32174
386-437-8747
engelmaier@aol.com
www.engelmaier.com
1

PCB RELIABILITY ISSUES
The electronic industry is forced to abandon
SnPb solders.
During soldering, the PCBs reach the highest
temperatures in the entire assembly—
225 to 245°C for SnPb solders,
260 to 280°C for SAC solders.
The damage done to PCBs by the much higher
lead-free soldering temperatures, reduces long-
term reliability and puts the very survival of
PCBs through the assembly process in question.
2

LIFE UNDER THE EU-RoHS
‘Serendipity’ no longer works under RoHS.
The survival of the PCBs, together with their
long-term reliability, needs to be assured upfront
and proactively.
The industry has experienced PCB failures during
LF-soldering with base materials advertised as
“Lead-Free Process Capable.”
3

Reliability Threats
Electronic System
Design for Reliability
&
Quality Manufacture
Components
Interconnects
4

Reliability Threats
Electronic System
Components
Design for Reliability
&
Quality Manufacture
PCB Interconnects
Barrel
Fracture
Shoulder
Fracture
Inner-Layer
Separation
Delamiation
5

Reliability Threats
Circuit Board Interconnect and
Structural Problems
• Plated-Through Holes [PTH]
• Plated-Through Vias [PTV]
• Base Material Delaminations
Solder Attachment Failures
Package Structural Problems
6

Reliability Threats to PCBs
PTH/PTV Barrel Fracture
PTH/PTV Barrel/Inner-Layer
Separation [ILS]
PCB Delamination
Problems with Micro-Vias
Problems with Traces
Inadequate Insulation Resistance
7

Added Reliability Concerns under RoHS
Due to ~35°C C Higher Soldering
Temperatures
The concerns are smaller for simple
PCBs, but increase with increasing
PCB complexity and thickness.
8

Important T’s s for SnPb vs. . SAC
Soldering T—SAC T
= 266°C
Large
CTE(z)
Soldering T—SnPb T
= 228°C
Solidus SAC = 217°C
Solidus SnPb = 183°C
T T = 67°C
T T = 78°C
T T = 116°C
T T = 33°C
Tg = 150°C
9

HOW TO SURVIVE ASSEMBLY
The Design-for-Reliability needs to begin with the
specifications for the PCBs submitted to your
PCB vendors for the bidding process.
When problems have arisen with PCBs, it almost
invariably is because the PCBs were not adequately
specified.
I have addressed these issues in a White Paper and
example FAB Notes specifying a PCB to survive the
RoHS-soldering processes and function reliably
afterwards.
10

PTH/PTV Interconnect Structure
z
y
x
11

Temperature Gradients
During Reflow Soldering
196°C 224°C 233°C 202°C
for Sn/Pb LF-solders
260°C 222°C
205°C 235°C
248°C 217°C
12

Pb-Free Soldering Processes
Hot-Air Solder Leveling (HASL)—270°C
Infrared (IR) Reflow —265°C
Forced Convection Reflow —260°C
Wave Soldering —265°C
Hand Soldering —260 to 300°C
13

ROOT CAUSES FOR
RELIABILITY THREATS
The threats to the reliability of PCBs include:
• thermal expansion/contraction mismatches
between resin and copper, causing barrel
fractures and inner-layer separations
—Glass Transition Temperature T g ,
—Thermal Expansion TE
• Thermal degradation of the resin system
—Decomposition Temperature T d ,
—Time-to-Delamination
T260/T288/T300
• Large internal water vapor pressures
—Moisture Bake-Out Required
16

Thermal Excursions (Operation)
Industry Standard IPC-SM-785
USE Tmin Tmax T Cycles Years Acc.Fails
CONSUMER 0°C +60°C 35°C 365 1-3 ~1%
COMPUTERS +15°C +60°C 20°C 1460 ~5 ~0.1%
TELECOMM 40°C +85°C 35°C 365 7-20 ~0.01% o.k.
AUTOMOTIVE-
UNDER HOOD 55°C +125°C 100°C 1300 ~5 ~0.1%
COMMERCIAL
AIRCRAFT
55°C +95°C 20°C 365 ~20 ~0.001%
INDUSTRIAL &
AUTOMOTIVE- 55°C +95°C 60°C 365 ~10 ~0.1%
PASSENGER
MILITARY
GROUND&SHIP
55°C +95°C 60°C 365 ~5 ~0.1%
SPACE 40°C +85°C 35°C 8800 5-20 ~0.001%
MILITARY
AVIONICS
55°C +95°C 80°C 365 ~10 ~0.01%
possible
wearout
17

Thermal Excursions Mandated
for 20-Year Life of
F/A-22 Raptor Stealth Fighter Jet
Loading Cycles Tmin Tmax T
ESS 10 40°C +70°C 110°C
ATP 10 +25°C +70°C 45°C
Storage 550 +15°C +37°C 22°C
Flight 1 1 40°C +85°C 125°C
M M M M M
Flight 27 929 +20°C +85°C 65°C
M M M M M
Flight 36 1 +45°C +85°C 40°C
Tarmac 1 1 50°C 47°C 3°C
M M M M M
Tarmac 14 260 +20°C +33°C 13°C
M M M M M
Tarmac 20 1 +45°C +58°C 13°C
TOTAL: 59 10,500 -- -- --
18

T g ’s, CTE’s & Thermal Expansion
It is the combined effect of the glass transition temperature and
the above Tg coefficient of thermal expansion that determine the
thermal expansion at soldering temperatures.
Thermal Expansion [%]
4.0
3.5
3.0
2.5
2.0
1.5
TE(50=>260°C)
TE(50=>260°C)
TE(50=>260°C)
1.0
0.5
T gTg
0.0
T
260
g
0 50 100 150 200 250 300
19
Temperature [°C][

Thermal Stability/High Reliability
‘Standard’ FR-4
has improved
since the 1980’s.
With the advent
of the soldering
processes
necessary for
lead-free soldering,
further significant
improvement will
be needed.
Decomposition Temperature, Td T [°C]
350
340
330
320
310
‘80s
FR-4
Hi
Td
‘05
FR-4
LF
Hi
Tg/Td
Hi
Tg
300
100 125 150 175 200
Glass Transition Temperature, T g [°C]
20

Time to Delamination
Thermal stability as measured by Thermo-Mechanical Analysis
(TMA) on laminate.
Time to Delamination [min]
50
45
40
35
25
30
20
15
10
T260
Material T g T d
FR-4 140°C 320°C
High T d 140°C 350°C
High T g /T d 175°C 350°C
T288T300T300
05
225 250 275 300 325 350
21
Temperature [°C][
Source: Cookson Electronics/Polyclad, USA
T320

Decomposition Temperature, T d
Td is defined as the temperature at which 2% [Td(2%)] and
5% [Td(5%)] weight loss occurs at a heating rate of 10°C/min.
Even a 2% weight loss indicates significant degradation of the resin.
This weight loss signals an irreversible deterioration of the material.
Source: Cookson Electronics/Polyclad, USA
100
Weight [%]
95
98
200
Sn/Pb Soldering Range
LF Soldering Range
250
300
Temperature [°C][
Standard Standard FR-4 FR-4
350
High High Tg/T
/Td
400
22

Thermal Degradation/Decomposition
Massive delamination of conventional high-Tg FR-4 4-layer PCB after
lead-free assembly with 260°C peak reflow temperature;
shown in 2 views: left to emphasize the delamination, right to show
the structure details.
Source: Cookson Electronics/Polyclad, USA
23

Proposed
Soldering Temperature Impact Index
The problem with the current IPC slash sheets is that they are to
complex to be of practical use for the PCB designers. The STII is
designed to give the designer an immediate feed-back whether
or not to further consider this slash sheet product
Tg + Td
STII = [% thermal expansion 50 to 260°C]10
2
Examples: Tg =140°C, Td(5%) = 320°C, 4.3% TE 50260°C
STII = 187
Tg =175°C, Td(5%) = 350°C, 4.0% TE 50260°C
STII = 223
Possible range of STII ~150 to 250; specify STII 215
(for thin PCBs STII 205 is likely sufficient).
24

IPC-4101
IPC-4101 (December 1997)
—30 Slash Sheets
IPC-4101A (December 2001)—Amendment 1 (June 2002)
IPC-4101B (March 2006)
—55 Slash Sheets
includes six (6) “specification sheets describing FR-4 base
materials compatible with lead-free assembly”:
—IPC-4101 B/ 99: High Tg FR-4, inorganic fillers
—IPC-4101 B/101: Low Tg FR-4, inorganic fillers
—IPC-4101 B/121: Low Tg FR-4, no fillers
—IPC-4101 B/124: High Tg FR-4, no fillers
—IPC-4101 B/126: Very High Tg FR-4, inorganic fillers
—IPC-4101 B/129: Very High Tg FR-4, no fillers
25

IPC-4101/A/B
STATED INTENTION:
The stated intention for IPC-4101B “is to provide the
typical composition, application, performance
characteristics and market for each base material.”
Specific Keyword searches, e.g. “Lead-Free FR-4,” lead to
appropriate slash sheets.
These ‘Keywords’ are not requirements, “they are simply
information” and are intended “For Search Only.”
PROBLEM:
The ‘Keywords,’ while “For Search Only,” do imply
certain performance capabilities.
Lets look at an example
26

IPC-4101/A/B
For the six (6) “Lead-Free Compatible” slash sheets :
Slash
Keyword
Tg
Td
TE(50260
260°C)
Sheet LF
Hi Td
Lo zCTE
[°C]
[°C]
[%]
/ 99
/101
/121
/124
/126
/129
LF
Limited
NO
NO
Limited
NO
NO
Limited
NO
NO
Limited
150
110
110
150
170
170
160
325
310
310
325
340
340
335
3.5
4.0
4.0
3.5
3.0
3.0
3.2
STII
202
170
170
202
225
225
215
The current slash sheets need to either have the requirements
tightened up or the ‘Keywords’ changed to reflect their capabilities.
27

Impact of Copper Plating
Thickness on PTH/PTV Reliability
Thicker copper plating in PTH/PTV barrels clearly result in more
reliable PTH/PTV interconnect structures.
Note: The copper
plating thickness
requirement in
PTH/PTV barrels has
been reduced from
1.0 mils to 0.8 mils to
accommodate slower
plating speeds in
high-aspect ratio
PTH/PTVs.
Fluidized Sand -T-Shock 25°C260°C,
hole : 13.5 mils
Source: IPC-TR-579
28

Impact of Hole Diameter and PCB
Thickness on PTH/PTV Reliability
The reliability of PTH/PTV interconnect structures decreases with
decreasing drilled hole diameter and increasing PWB thickness.
Note: The testing for
the round robin
program reported on
in IPC-TR-579 was
arbitrarily, and
unfortunately,
terminated after 400
cycles.
T-Cycle;
-65°C+125°C
mils
PCB THICKNESS
Source: IPC-TR-579
29

Number of Cycles
1000
900
800
700
600
500
400
300
200
Impact of Hole Diameter on
PTH/PTV Reliability
FR-4 PCB, h=90 mils, T g =175°C
HATS: -40145
145°C, 0.25 min. dwells
ATC: -40145
145°C, 10 min. dwells
Source: Tim Estes, Conductor Analysis, Vicka White, Honeywell, USA
100
0
10 12 13.5 16
10
12 13.5 16
Drilled Hole Diameter [mils]
30

Reliability Assurance Test
Comparison: ATC, HATS & IST Results
100
Courtesy of Rick Snyder, Delphi Inc., USA
Failures,
[%]
80
60
40
Fab A
Fab B
20
Fab C
to
1250
0
0 100 200 300 400 500 600 700 800 900 1000
Cycles
Fabs: FR-4, T g =170°C, 31 mil-thick PCB, 8 & 10 mil-diameter holes
TS: -40°C+145°C/60 minute cycle
HATS: -40°C+145°C/14 minute cycle
IST: RT+170°C/10 minute cycle
31

Loss of Life
Due to Soldering Processes (1)
% Failed
90
75
50
25
10
5
2
1
10 2
30%
200 500 10 3 2,000 5,000
Cycles-to-Failure
FR-4 PCB-PTVs,
=13.5 mils [0.34 mm],
h =100 mils [2.54 mm],
Grid: 50 mils [1.3 mm]
10 layers, non-functional pads,
ATC: -55125
125°C
Assembly simulation:
5 x reflow @ 215°C,
= 4.1
No assembly simulation,
= 3.7
10 4
32

% Failed
90
75
50
25
10
5
2
1
10 2
33%
50%
Loss of Life
Due to Soldering Processes (2)
200 500 10 3 2,000 5,000
Cycles-to-Failure
Courtesy of Paul Reid, PWB Solutions, Canada
FR-4 PCB-PTVs,
=13.5 mils [0.34 mm],
h =102 mils [2.60 mm],
Grid: 50 mils [1.3 mm]
10 layers, non-functional pads,
ATC: +25150
150°C
Assembly simulation:
3 x reflow @ 230°C,
= 2.3
No assembly simulation,
= 5.1
10 4
33

Engelmaier Fatigue Life Prediction
N f
–0.6 D 0.75 f
+ 0.9 S u
E
exp D
f
0.36
( )
Plastic Strain
Elastic Strain
Low-Cycle
High-Cycle
Fatigue
Fatigue
Nf =mean cyclestofailure
D f =fatigue ductilityfrom IPCTM2.4.2.1
Effective Cyclic
Strain
Range
S u =ultimate tensilestrength
E =modulusof elasticity
This equation is based on empirical relationships between fatigue behavior
and tensile properties; it needs to be solved by iteration.
It is the basis for IPC-TM-2.4.2.1, ‘Flexural Fatigue and Ductility, Foil’ and ASTM-
E796 ‘Standard Method for Ductility Testing of Metallic Foil’
0.1785 log 105
N f = max eff
( )
34

Plated-Through Via
Barrel Crack
Barrel cracks occur near center of
PWB— highest load— at prepreg layer
— higher resin content.
Resin recession gives a clear
indication of the forces at work causing
the plastic deformation of the copper
barrel.
35

Inner-Layer (Post) Separation
Inner-layer adhesive failure
occurs on heating when resin
expands compressing barrel
and inner-layers cannot move
enough; on cooling the
receding resin produces the
separation seen.
36

Finite Element Model: Stress Results
These are the stress results for the copper barrel and innerlayers of the
printed wiring board.
Note: The largest
stresses occur at
the ‘shoulder’ of the
PTV barrel and the
surface pad and at
the center of the
barrel.
z
y
x
Courtesy: Shine Rominger, U.S.Navy-Crane, IN, USA
37

High Cu Dissolution in SAC Waves
• What we know about Sn·Ag·Cu (SAC) and Sn·Ag·Cu·Xy
solders:
Dissolve Cu and other base metals faster (about 2x) than Sn/Pb
can be a real problem at the mouth of plated-through holes
and vias
Double wave-single pass
35 μm [1.4 mils]
28 μm [1.1 mils]
Reflow
35 μm [1.4 mils]
22 μm [0.9 mils]
Source: Biglari, et al, IDEALS, Europe
Source: Th. Ahrens, Fraunhofer Institut, Germany
38

Vapor Pressure
Load Drivers
Another important load driver is absorbed
moisture within the PCB
• PCB resins absorb moisture; this moisture can be
removed prior to soldering processes by a moisture
removal baking step.
• The higher soldering temperatures for LF-solders
require more thorough moisture removal—the vapor
pressure roughly doubles from 220°C to 260°C.
Any remaining moisture will vaporize and create high
vapor pressure levels within the PCB.
These vapor pressures can rupture the PCB matrix and
separate PCB layers.
39

Water Vapor Pressure Induced
PCB Layer
Separation
Note: Top photo is
from SMT reflow
process; bottom
picture is from rework
procedure.
Courtesy of D. Mattix, Qualcomm Inc., USA
40

Micro-Via Resistance Change
After Assembly Simulation
Change in resistance of 86
daisy-chain nets consisting
of 146 μ-vias each after 6
reflow oven passes with
215°C C peak reflow
temperature.
Change in Via Net Resistance [%]
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
12 Points
Courtesy of Ron Rhodes, Conductor Analysis Technologies, Inc., USA
- 2.0
5 7 9 11
Hole Diameter [mils]
41

Micro-Via Resistance Change
After T-Cycling
Change in resistance of
110 daisy-chain nets
consisting of 146 μ-vias
each after 500 cycles of
-55+95°C.
Note: : 6 reflow cycles are
significantly more
damaging than 500
cycles with T=150°C
Change in Via Net Resistance [%]
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
- 2.0
4 Points
5 7 9 11
Hole Diameter [mils]
Courtesy of Ron Rhodes, Conductor Analysis Technologies, Inc., USA
42

SOLID
TECHNICAL
UNDERSTANDING
43

Thank You!
Are there any questions?
44

Workshops by Werner Engelmaier:
•Interconnect Failures and Design for Reliability for Plated-
Through Holes/Vias
•How to Specify PCBs to Reliably Survive the RoHS-Mandated
Lead-Free Soldering Processes
•Solder Joint Reliability—
•Part 1: Fundamentals in Solder Joint Reliability
•Part 2: Failure Mode and Root Cause Analyses (Fatigue,
Brittle Fracture, ENIG)
•Part 3: Acceleration Models, Accelerated Reliability Tests
and Screening Procedures
•Part 4: Reliability Issues for Lead-Free Soldering
•Reliability Concerns in Electronic Packaging
45