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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.
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).
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.
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.
Copper dissolution is already having a detrimental effect as some major OEM unable to repair ball grid arrays (BGAs)
Lead Free: Predicting and Ensuring Reliability in Military Avionics
-400
-300
-200
-100
0
100
200
300
400
0 2 4 6 8 10 12
Time (sec)
Wet
ting
Forc
e ( µ
N/m
m)
0 Months Shelf HASL6 months Shelf HASL
0 months Shelf ImmAg0 months Field ImmAg6 months Shelf ImmAg
1.Copper thickness = 2OZ use material listed on column 260 ℃2.Copper thickness >= 3OZ use Phenolic base material or High Tg Halogen free materials only3.Twice lamination product use Phenolic material or High Tg Halogen free materials only (includes HDI)4.Follow customer requirement if customer has his own material requirement5.DE people have to confirm the IR reflow Temperature profile
TBD – Consult Engineering for specific design review≧161mil
PhenolicTg170 + Filler IS415, 370 HR, 370 MOD, N4000-11HF material - TBD≧161mil
Phenolic Tg170 + Filler IS415, 370 HR, 370 MOD, N4000-11HF –high TG materials OK≧131mil
Phenolic Tg170IS410, IT180, PLC-FR-370 TurboTU722-7HF –high TG materials OK121~160mil
Phenolic Tg170IS410, IT180, PLC-FR-370 Turbo, TU722-7HF –middle and high TG materials OK93~130mil
Tg150 Phenolic + FillerIS400, IT150M, TU722-5Tg 150 HF –middle and high TG materials OK93~120mil
Tg150 Phenolic + FillerIS400, IT150M, TU722-5, GA150HF –middle and high TG materials OK73~93mil
Tg170 Dicy, NP150G-HFHF –middle and high TG materials OK73~93mil
Tg170 DicyHF –middle and high TG materials OK60~73mil
Tg150 DicyNP150, TU622-5All HF materials OK60~73mil
Tg150 Dicy HF- middle and high Tg materials OK≤ 60mil
Tg140 DicyAll HF materials OK≤60mil
IR-260℃Board thicknessIR-240~250℃Board thickness
No6 months-45CSAC305 (Nihon, Sweatman, JEDEX 2005
No24 months-40CSAC (IBM, Kang, ECTC 2003
No3 months-42CSAC305 (RIM, Christian, IPC/JEDEC 2005
No1.5 months-78CSn0.2Ag (Murphy)
Pest?TimeTempAlloy
y = 301550x-2.7427
0.1
1
10
10 100 1000
Change in Temperature (oC)
Nor
mal
ized
Cha
ract
eris
tic L
ife
Introduction
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.
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.
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
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.
What to do? Focus on your parts, focus on your boards, and focuson your solder.
SAC405
SAC305
SAC105
SACX
SNC
SnAg
SNCX
SnCuSnAgCu??SAC405
SAC305
SAC105
SACX
SNC
SnAg
SNCX
SnCuSnAgCu??
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.)
What to do? Do not assume that lead-free/RoHScompliant 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.
Parts
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)
PECs are often overlooked and are sometimes not appropriately labeled (no MSL marking or obsolete version of J-STD-020).
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).
What about whiskers breaking off? Prior research strongly indicate this only occurs during handling (table on right).
Finally, be aware of your options.
If you are slightly concerned, request that your suppliers follow JESD22A121 and JESD201. Require variable data (no pass/fail).
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).
Very concerned? Follow GEIA STD-0005-2 and consider OEM mitigation (solder dipping or conformal coating).
Robustness Popcorning Tin Whiskering
35 microns3000 cycles-55C/85C
275 microns450 days30C/60%RHDittes (E4)[8]
34 microns3634 hours51C/85%RHRomm (TI)[7]
40 microns3000 cycles-55/85C
100 microns8500 hoursRoom Conditions
450 microns5000 hours60C/93%RHHilty (Tyco)[6]
250 microns500 cycles-40C/90CBrusse (NASA)[5]
35 microns3000 cycles-55C/85C
60 microns3000 hours60C/95%RHGedney (iNEMI)[4]
85 microns2200 cycles-40C/85COkada (Murata)[3]
130 microns500 cycles-60C/60CHashemzadeh(Linköping)[2]
70 microns3000 hours60C/93%RHPeng (Freescale)[1]
Maximum LengthTime PeriodEnvironmentReference
50
1
2000 G
60 sec / frequency
6 G
50, 100, 200, 250
Sinusoidal
N/A
20 G
10 – 2000
Sinusoidal
Dunn
18100Events
0.36Pulse Width (ms)
3000 G500 G1000 G
MaximumAcceleration
Mechanical Shock
22 hoursDuration
18 GrmsMaximumAcceleration
10 - 2000Frequency (Hz)
RandomType
Extended Duration
N/A5 minutesDuration
20 G3.5 GrmsMaximumAcceleration
10 – 200010 – 4500Frequency (Hz)
SinusoidalRandomType
Resonance Sweep
AndoHashemzadehReferences’]
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.
N/A[6]QFPIBM
Sn, SnCu, or SnPbQFPAMD
SnBiTSOP
PdQFPMatsushita/Panasonic
SnTSOPMicron
Sn, SnBi, or NiPdAuQFP / TSOPNEC
Sn[5]QFP / TSOPFreescale
SnBiTSOPHynix
NiPdAuTSOP
100% SnQFPPhilips/NXP
Pd or SnPbQFP / TSOPSony
Mostly NiPdAu, with some Sn-Cu, Sn-Bi[4]TSOP
Mostly Sn-Cu, Sn-Bi; some NiPdAu[3]QFPRenesas Technology
NiPdAuTSOP
Sn or SnPbQFPInfineon
NiPdAu[2]QFP / TSOPSTMicroelectronics
NiPdAu or SnAg or SnBiTSOP (LSI)
SnAg or SnCuTSOP (Memory)
NiPdAuTSOP (Discretes)
Toshiba
NiPdAuQFP / TSOPTexas Instruments
NiPdAuQFP / TSOPSamsung
Sn[1]QFP / TSOPIntel
PlatingPackageCompany
Printed Circuit BoardsWhile typically overlooked by personnel preparing for lead-free, understanding lead-free printed circuit boards (PCBs) and their inherent risks is critical, as most issues experiencedby consumer/computer was related to their PCBs.
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).
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
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.
18%
SolderManufacturing
SolderLong-Term Reliability
Environment: Cold TemperatureFailure Mechanism: Cold PestRisk: Extremely Low
Environment: Temperature CyclingFailure Mechanism: Creep, Elastic/Plastic FatigueRisk: 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.
These findings assume realistic worst-case environments (Tmax < 90C) and take into account the influence of long dwell times.
Environment: VibrationFailure Mechanism: Elastic/Plastic FatigueRisk: Additional data required, but initial results imply similar performance to SnPb when subjected to loads similar to field conditions.
Environment: Mechanical ShockFailure Mechanism: Solder or Intermetallic FractureRisk: Lead-free does perform worse than SnPb, but an even bigger driver is the board plating (nickel-intermetallics vs. copper intermetallics).
Chai, ECTC 2005
Qualcomm
B. Willis, SMART Group
IBM
Gold Circuits
P. Biocca, Kester
S. Zweigart, Solectron
2512 SMT Resistor
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. 4-
5oC/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