Lead Free: Predicting and Ensuring Reliability in Military Avionics
Introduction
Parts
Robustness
By now, everyone in military electronics, from the designer to the
manufacturer, from the engineer to the executive, and from the
lowliest sub-contractor to some of the highest reaches of the
Pentagon, knows about Lead (Pb) Free.
Popcorning
Be aware that a limited number of components
can become damaged after exposure to lead-free
reflow temperatures (240°C - 260°C peak). They
include
• Aluminum electrolytic capacitors
• Ceramic chip capacitors (wave soldering)
• Surface mount connectors (nylon housing)
• Specialty components (RF, optoelectronic, etc.)
Is lead-free inevitable? Unfortunately, yes. While certain aspects
may never be acceptable, such as tin plating in mission-critical
applications, there are strong indications that markets not required
to go lead-free, such as telecom, automotive, industrial controls,
even medical and avionics/military, are transitioning right now,
with timelines ranging from mid- 2007 to late 2009.
The simple answer here is to pay attention to
moisture sensitivity levels (MSL). MSLs are well
defined in J-STD-020D (see below). Two
additional words of warning
• Avoid any component with a MSL > 4
• Pay particular attention to plastic
encapsulated capacitors (tantalum, tantalum
polymer, aluminum organic)
What to do? Do not assume that lead-free/RoHS
compliant parts will survive lead-free reflow.
Identify the components at risk and measure their
temperature during reflow and rework. Compare
the results to manufacturers’ specifications.
So what is the reality of lead-free? The reality is that most
transitions to lead-free in the commercial world were “successful”.
Successful means that most companies are reporting similar to
lower levels of field returns for Pb-free products. But…
• Time required was often 50 to 100% longer than planned
• Use environment tends to be more benign (consumer / computer)
• Design life tends to be limited (3 yr - 5 yr)
• Some failure mechanisms of concern have been recently noted
PECs are often overlooked and are sometimes
not appropriately labeled (no MSL marking or
obsolete version of J-STD-020).
Qualcomm
Time Period
60C/93%RH
3000 hours
70 microns
-60C/60C
500 cycles
130 microns
Okada (Murata)[3]
-40C/85C
2200 cycles
85 microns
Gedney (iNEMI)[4]
60C/95%RH
3000 hours
60 microns
-55C/85C
3000 cycles
35 microns
-40C/90C
500 cycles
250 microns
Hashemzadeh
(Linköping)[2]
Brusse (NASA)[5]
Hilty (Tyco)[6]
60C/93%RH
5000 hours
450 microns
8500 hours
100 microns
-55/85C
3000 cycles
40 microns
Romm (TI)[7]
51C/85%RH
3634 hours
34 microns
Dittes (E4)[8]
30C/60%RH
450 days
275 microns
-55C/85C
3000 cycles
35 microns
≤60mil
Tg140 Dicy
All HF materials OK
60~73mil
Tg150 Dicy
NP150, TU622-5
All HF materials OK
≤ 60mil
60~73mil
Tg170 Dicy, NP150G-HF
HF –middle and high TG materials OK
73~93mil
B. Willis, SMART Group
IR-260℃
Tg150 Dicy
HF- middle and high Tg materials OK
Tg170 Dicy
HF –middle and high TG materials OK
73~93mil
Tg150 Phenolic + Filler
IS400, IT150M, TU722-5, GA150
HF –middle and high TG materials OK
93~130mil
Phenolic Tg170
IS410, IT180, PLC-FR-370 Turbo,
TU722-7
HF –middle and high TG materials OK
93~120mil
Tg150 Phenolic + Filler
IS400, IT150M, TU722-5
Tg 150
HF –middle and high TG materials OK
121~160mil
Phenolic Tg170
IS410, IT180, PLC-FR-370 Turbo
TU722-7
HF –high TG materials OK
≧131mil
Phenolic Tg170 + Filler
IS415, 370 HR, 370 MOD, N4000-11
HF –high TG materials OK
PhenolicTg170 + Filler
IS415, 370 HR, 370 MOD, N4000-11
HF material - TBD
≧161mil
TBD – Consult Engineering for specific
design review
≧161mil
1.Copper
thickness = 2OZ use material listed on column 260 ℃
thickness >= 3OZ use Phenolic base material or High Tg Halogen free materials only
lamination product use Phenolic material or High Tg Halogen free materials only (includes HDI)
4.Follow customer requirement if customer has his own material requirement
5.DE people have to confirm the IR reflow Temperature profile
2.Copper
3.Twice
IBM
The initial hope was that Sn3.0Ag0.5Cu
(SAC305) would be the accepted
replacement to SnPb. That has fallen apart in
the last 12-18 months, as portables have
asked for SAC105 (more shock resistant),
wave solder manufacturers have asked for
SNC, and every solder manufacturer is
adding their own secret ingredient (SNCX and
SACX).
Copper dissolution is the reduction or
elimination of surface copper conductors due
to repeated exposure to Sn-based solders. It
is a significant concern for industries that
perform rework. Research has determined
that contact time is the major driver, with
some indications of a 25 second limit on
contact with molten solder. This creates a
limit of 1X rework.
Gold Circuits
400
300
200
10 0
0
0 Months Shelf HASL
- 10 0
6 m onths Shelf HASL
0 m onths Shelf Im m Ag
-200
0 m onths Field Im m Ag
6 m onths Shelf Im m Ag
-300
-400
0
The three biggest issues in lead-free
assembly are which lead-free solder to chose,
whether to go mixed (lead-free BGA with
SnPb solder) and figuring out how to rework.
For mixed assemblies, keep peak
temperature above 220C. A defect-free mixed
assembly does well under thermal cycling, but
less is known about vibration and mechanical
shock.
18%
2
4
6
8
10
12
Tim e (sec)
P. Biocca, Kester
Dunn
Hashemzadeh
Ando
Type
Sinusoidal
Random
Sinusoidal
Frequency (Hz)
10 – 2000
10 – 4500
10 – 2000
20 G
3.5 Grms
20 G
N/A
Maximum
Acceleration
Duration
Frequency (Hz)
N/A
5 minutes
Sinusoidal
Random
50, 100, 200,
250
10 - 2000
Maximum
Acceleration
Copper dissolution is already having a
detrimental effect as some major OEM unable
to repair ball grid arrays (BGAs)
If you are concerned, request mitigation
at the part manufacturer (nickel
underplate, annealing, minimum tin
thickness, palladium). Of the four
options, palladium is the only guarantee
(and is increasingly popular).
Mechanical
Shock
SAC405
SAC305
SNC
SNCX
SAC105
SACX
22 hours
2000 G
500 G
1000 G
3000 G
1
6
0.3
Events
50
100
18
Company
Package
Plating
Intel
QFP / TSOP
Sn[1]
Samsung
QFP / TSOP
NiPdAu
Texas Instruments
QFP / TSOP
NiPdAu
TSOP (Discretes)
NiPdAu
TSOP (Memory)
SnAg or SnCu
TSOP (LSI)
NiPdAu or SnAg or SnBi
QFP / TSOP
NiPdAu[2]
QFP
Sn or SnPb
TSOP
NiPdAu
QFP
Mostly Sn-Cu, Sn-Bi; some NiPdAu[3]
TSOP
Mostly NiPdAu, with some Sn-Cu, Sn-Bi[4]
QFP / TSOP
Pd or SnPb
QFP
100% Sn
TSOP
NiPdAu
Hynix
TSOP
SnBi
Freescale
QFP / TSOP
Sn[5]
NEC
QFP / TSOP
Sn, SnBi, or NiPdAu
Micron
TSOP
Sn
QFP
Pd
TSOP
SnBi
AMD
QFP
Sn, SnCu, or SnPb
IBM
QFP
N/A[6]
STMicroelectronics
Infineon
Renesas Technology
Sony
Philips/NXP
Matsushita/Panasonic
Solder
Long-Term Reliability
Environment: Cold Temperature
Failure Mechanism: Cold Pest
Risk: Extremely Low
Alloy
Temp
Time
Pest?
Sn0.2Ag (Murphy)
-78C
1.5 months
No
SAC305 (RIM, Christian, IPC/JEDEC 2005
-42C
3 months
No
SAC305 (Nihon, Sweatman, JEDEX 2005
-45C
6 months
No
SAC (IBM, Kang, ECTC 2003
-40C
24 months
No
Environment: Temperature Cycling
Failure Mechanism: Creep, Elastic/Plastic Fatigue
Risk: Finite element modeling based on material
behaviors (creep equation) and an epidemiological study
of results from accelerated life testing suggest that will
be a minimal statistical difference in time to failure
between SnPb and SAC305.
10
-2.7427
y = 301550x
1
0.1
10
100
Change in Temperature (oC)
These findings assume realistic worst-case
environments (Tmax < 90C) and take into account the
influence of long dwell times.
Environment: Vibration
Failure Mechanism: Elastic/Plastic Fatigue
Risk: Additional data required, but initial results imply
similar performance to SnPb when subjected to loads
similar to field conditions.
2512 SMT Resistor
S. Zweigart, Solectron
SnAg
?? SnAgCu SnCu
18 Grms
Pulse Width
(ms)
Toshiba
Very concerned? Follow GEIA STD0005-2 and consider OEM mitigation
(solder dipping or conformal coating).
6G
60 sec /
frequency
Maximum
Acceleration
Normalized Characteristic Life
Board thickness
IR-240~250℃
Resonance
Sweep
Extended
Duration
If you are slightly concerned, request that
your suppliers follow JESD22A121 and
JESD201. Require variable data (no
pass/fail).
Maximum Length
Room Conditions
Duration
Manufacturing
Board thickness
Wetting Force ( µN/mm)
Reduce your risk, and wasted resources,
by realizing that matte tin over copper
tends to have a finite length (< 0.5 mm).
This will tend to limit your focus to
components with 0.8 mm pitch or less
(some companies focus on less than
0.65 mm pitch).
Finally, be aware of your options.
Printed Circuit Boards
Finally, do not forget to qualify. This includes performing tests
to assess conductive anodic filament (CAF) formation (IPC
TM-650 2.6.25) and PTH fatigue (most commonly with
interconnect stress testing) and construction analysis.
Environment
Type
Avoiding thermal shock cracks in ceramic capacitors
• Orient terminations parallel to wave solder
• Reduce maximum case size for wave soldering
from 1210 to 0805
• Maintain a maximum thickness for wave soldering
of 1.2 mm
• C0G, X7R preferred for wave soldering wave
• Use manufacturer’s recommended bond pad
dimensions or smaller for soldering
• Adequate spacing from hand soldering operations
• Room temperature to preheat (max. 2-3oC/sec.)
• Preheat to at least 150oC
• Preheat to maximum temperature (max. 45oC/sec.)
• Cooling (max. 2-3oC/sec.)
• Make sure assembly is less than 60oC before
cleaning
• Maintain belt speeds to a maximum of 1.2 to 1.5
meters/minute
• Eliminate touchup or rework with solder irons
Solder
Second, chose your solderability plating. No one plating is
universal, as each plating has its risks and its benefits.
• Electroless nickel/immersion gold (ENIG) can provide long
storage life and prevents copper dissolution, but comes at
risk of black pad, dewetting, crevice corrosion, poor shock
performance, and poor adhesion with large BGAs
• Immersion silver (ImAg) can also provide long storage life,
but is susceptible to planar voiding, can cause degradation
of plated through holes, and is very sensitive to sulfur
gases (both in storage and in the field)
• Immersion tin (ImSn) and organic solderability plating
(OSP) are lower cost options, but have limited storage life
(6-12 months) and OSP can cause poor hole fill during
wave soldering
• Lead-free hot air solder level (HASL) has seen increasing
market share, primarily because of long-term storage and
resistance to copper dissolution, but may not be compatible
with some BGAs and thick (>90 mil) PCBs
Reference
Peng (Freescale)[1]
What about whiskers breaking off? Prior
research strongly indicate this only
occurs during handling (table on right).
What to do? Focus on your parts, focus on your boards, and focus
on your solder.
First, chose your laminate wisely. The glass transition
temperature (Tg) should be appropriate. Too low, and you’ll
experience delamination, warpage, and plated through hole
(PTH) cracking. Too high and you’ll experience drilling issues
and pad cratering (and pay higher costs). And don’t forget
thermal stability (either time to delamination, T-260 or T-288,
or temperature to decomposition, Td).
Tin whisker are probably the number one concern
of military and avionic companies. Why? Because
the current state of knowledge is relatively limited,
with uncertainty as to root-cause (plating
chemistry, contaminants, etc.) and how to
accelerate this mechanism.
References’]
In addition to these words of warning, there has been a recent
divergence in lead-free solders (see below). These variations, from
changes in tin-silver-copper (SAC) composition to widespread
acceptance of tin-nickel-copper (SNC), often come with little
reliability information and can lead to confusion and consternation.
While typically overlooked by personnel preparing for leadfree, understanding lead-free printed circuit boards (PCBs)
and their inherent risks is critical, as most issues experienced
by consumer/computer was related to their PCBs.
Tin Whiskering
Environment: Mechanical Shock
Failure Mechanism: Solder or Intermetallic Fracture
Risk: Lead-free does perform worse than SnPb, but an
even bigger driver is the board plating (nickelintermetallics vs. copper intermetallics).
Chai, ECTC 2005
1000