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CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Why Use Electroless and Immersion Pla,ng?
• Uniform thickness distribu,on
✴ Less sensi,ve to surface features than electroly,c
✴ Can plate inside tubes and occluded areas
• Can plate electrically isolated features
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Red‐Ox Reac,on Concept
• Red‐ short for “Reduc,on”
• Ox‐ short for “Oxida,on”
• Chemical “reduc,on” means to gain electrons in a chemical reac,on
• Chemical “oxida,on” means to give up electrons in a chemical reac,on
• In a red‐ox reac,on, one species gives up electrons (electron donor) and another species takes up these electrons (electron acceptor)
• An “electronega,ve” material wants to give up electrons in a chemical reac,on
• An “electroposi,ve” material wants to gain electrons in a chemical reac,on
• Redox behavior is used to understand all types of chemical reac,ons, including all types of pla,ng
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electropla,ng Example
8
e-
e- e-
e-
Anode Cathode
+
+ +
+ +
+
+
+
- -
-
-
- -
-
-
i
e-
e- e-
e-
Ionic Conductivity
V
(SO4)2- 2H+
H2SO4 in an aqueous
electrolyte
Cu2+
Metallic Ions Produced
Cu2+ Cu Metal Ions Plated Out
Anode Oxida,on Reac,on: Cu ‐>
Cu2+ + 2e‐
Cathode Reduc,on Reac,on: Cu2+ +
2e‐ ‐> Cu
For Electropla,ng, the external circuit transfers the electrons from the electron donor to the electron acceptor
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Oxidation- Reduction Reactions in Solution
Example: Fe Metal in AgNO3 SolutionAgNO3 in solution -> Ag+ + (NO3)-
When Fe is added, a redox reaction occurs:Fe -> Fe2+ + 2e-
2Ag+ + 2e- -> 2AgElectrons are donated from the Fe and accepted
by the Ag+ in solutionElectron transfer between the reactants occurs
in solution
The Fe was oxidized and the Ag was reduced in this reaction.
Fe Ag+e
Fe2+Ag
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Immersion Plating
Example: Fe Metal Rod in AgNO3 Solution
Same chemical Reaction as solution caseFe metal oxidizes (corrodes) and is dissolved in
solutionAg ions are reduced and plate out on the Fe rod
as Ag metal coatingElectrons are donated from the Fe to the Ag
Once the surface is coated with Ag, the reaction stops (no more Fe reactant)
Ag layer is about 5 microinches thick
Ag+
Fe2+
Fe
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
What Determines Whether Immersion Plating Will Occur?
B+
A2+
A
?
Reac%vity is based on order in electrochemical series
✦ Materials near the top of the series want to oxidize✦ Materials near the boVom want to reduce✦ If A is higher on the chart then B, then B will immersion plate on A✦ Pt will plate on everything except Pt
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Immersion Pla,ng Characteris,cs
• Very simple solu,on chemistry
• Extremely thin deposit (5‐10 µinches)‐ advantageous for coa,ng expensive noble metals (Au, Pd, Pt,Ag)
• Uniform surface coverage‐ used for pla,ng the inside of tubes, for example
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Immersion Pla,ng Formula,ons
Deposit Base Metal Bath Components
Gold
Copper
Nickel
Palladium
Pla,num
Silver
Copper Potassium Au cyanide, Potassium cyanide.
Aluminum Copper sulphate, Ethylenediamine
CopperNickel sulfate, ammonium nickel sulfate, sodium
thiosulfate
Copper, Nickel Palladium Chloride, HCL
Copper, Nickel, Gold or Palladium
Chloropla,nic Acid, HCL
Copper Silver Cyanide, Sodium CyanideSilver Nitrate, Ammonia
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Immersion Au Pla,ng Rate
0
0.038
0.075
0.113
0.15
0 5 10 15 20 25
Thi
ckne
ss µ
m
Immersion Time (Minutes)
80 85 90
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Ni‐Pd‐Au Example
• Developed at Texas Instruments in 1989
• Standard pla,ng for lead‐frames and other components
• Highly solderable
• Electroplated Nickel layer (160 ‐ 200 µinches)
• Thin electroplated Pd layer (10 µinches)
• Ultrathin immersion Au layer (2 µinches)
• Au layer reduces oxida,on of Pd and improves solder wecng
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Pla,ng Overview
• Immersion pla,ng is driven by oxida,on of the base metal ‐ pla,ng stops when base metal surface is completely covered
• Electroless pla,ng is driven by oxida,on reac,ons of pla,ng bath cons,tuents
• Electroless pla,ng is “autocataly,c” or con,nuous
• Pla,ng con,nues even when the surface of the base metal is covered
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Basic Components of an Electroless Pla,ng Bath
• Electroless bath chemistry is highly complex (compared to immersion or electroly,c pla,ng)
• Aqueous solu,on of metal ions (electron acceptors)
• Reducing agents (electron donors)
• Complexing agents
• Bath stabilizers
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Basic Electroless RedOx Reac,on
• Chemical reducing agents in solu,on oxidize and the metal ions in solu,on are reduced to form metal atoms
✴ (Chemical Reducing Agent) ‐> (Chemical Reducing Agent)+ + e‐
✴ (Metal Ion in Solu,on)+ + e‐ ‐‐> Metal atom
• The surface of the base metal to be plated acts as a “catalyst” for this reac,on
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
What is a Catalyst?
• “Catalyst” is a material that increases the rate of a chemical reac,on without directly par,cipa,ng in the reac,on
• Catalyst is not one of the reactants
• Metals, and par,cularly noble metals, are good catalysts for many reac,ons. Electrical insulators are not good catalysts
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Cataly,c Role of the Substrate Surface
• The electroless pla,ng RedOx reac,on occurs faster on the substrate surface than in the pla,ng solu,on away from the surface
• This results in electroless pla,ng on the substrate compared to metal precipita,on in solu,on
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Autocatalytic Reaction in Solution or On a Catalytic Surface
M+
A2+
✦Metal ion in solu,on M+
✦ Reducing agent in solu,on A✦ Pla,ng reac,on:
• A ‐> A+ + e• M+ + e ‐> M
✦ This reac,on can occur in solu,on or on the substrate surface✦ If substrate surface is a catalyst, the reac,on will occur faster on the substrate than in solu,on✦ If the substrate is not a catalyst, metal will precipitate out in solu,on✦ Pla,ng rate is a func,on of the specific Red/Ox reac,on, cataly,c surface, pH, temperature, metal ion/reducer concentra,on
M+
A2+
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Substrate Cataly,c Requirements
• Base substrate metal must be a catalyst for the autocataly,c RedOx reac,on in order for pla,ng to ini,ate
• Base metal surface prepara,on is cri,cal. Oxide and other contamina,on must be removed before pla,ng.
• If base metal cataly,c proper,es are poor, a Pd or Pt seed metal can be deposited in some cases (done on Ag)
• Plated surface must be a catalyst for the autocataly,c RedOx reac,on in order for pla,ng to con,nue once the surface is covered
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Chemistry Balance in Electroless Baths
Increa
sing Strengt
h of Re
dOx Reac
,on
Autocataly,c pla,ng will not occur or will occur very slowly
Metal will spontaneously
precipitate from the bath
Pla,ng will only occur on the catalyst surface and pla,ng rate will be high
enough for prac,cal applica,ons
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Role of Stabilizers
• Addi,ves that inhibit the rate of the RedOx Reac,on
• Used to prevent spontaneous metal precipita,on from the bath
• Heavy metal salts or organic compounds
• “Steric” inhibitors‐ physically prevent the reactants from coming in contact with each other
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
How Steric Inhibitors Work
Steric inhibi,on: the rate that these 2 people can find each other is inhibited by the other people surrounding them
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Role of Stabilizers
Increa
sing Strengt
h of Re
dOx Reac
,on
Autocataly,c pla,ng will not occur or will occur very slowly
Metal will spontaneously
precipitate from the bath
Pla,ng will only occur on the catalyst surface and pla,ng rate will be high
enough for prac,cal applica,ons
Stabili
zers U
sed to Ad
just RedO
x Reac
,on S
trengt
h
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Role of Complexers
• Complexers play a similar role to stabilizers
• Compounds that surround and bond to the free metal ions in solu,on (“Chelator”)
• Can be used to adjust the concentra,on of available free metal ions
• Adjusts the RedOx reac,on rate
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Build Up of Reac,on Products
• Electroless pla,ng tank is a closed chemical system
• By products from the reduc,on reac,on build up in the tank [electroless Ni, orthophosphite ions accumulate (HPO3
2‐)]
• Dissolved metal from the substrate builds up
• Decrease the pla,ng rate
• Eventually, the bath spontaneously precipitates
• New bath must be built
• Typical bath life,mes for rigorously maintained electroless baths are months (5 metal turns) compared to years for electroly,c baths
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Bath Management Requirements
• Reducer and metal ion concentra,on must be replenished and controlled
• pH and temperature must be carefully monitored to balance pla,ng rate with bath stability. Temperature control for some baths must be very ,ght (+/‐ 2C). It is very important that the temperature never exceed the bath vendor guideline.
• Stabilizer and complexer concentra,on must be controlled. Stabilizer concentra,ons are very low, similar to grain refiners < 10 ppm
• Since pla,ng rates vary with bath life,me, coupons should be periodically plated and evaluated to determine the current pla,ng rate
• pH should be maintained +/‐ 0.2 of bath vendor guidelines
• During con,nuous use, analyze for metal every 2 hours and reducer every 4 hours
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Key Prac,ces to Enhance Bath Life,me
• Con,nuously filter all baths. This will filter out any small metal precipitates which can ini,ate bath instability
• Carefully inspect all polypropylene tanks for wear including scratches or other damage. Scatches can become nuclea,on sites for metal precipita,on. Use disposable tank liners if possible which can be discarded if scratched.
• Uniform solu,on agita,on is cri,cal to avoid local overhea,ng which results in plate out on tank heaters or other tank components.
• During periods of non‐use, turn off heat but con,nue filtra,on.
• At any sign of metal precipita,on, bath should be filtered out into a clean holding tank, allowed to cool and then analyzed.
• Inspect racks carefully for cracks in protec,ve coa,ng. If rack stripping agents are dragged into the electroless tank, it will cause instability.
• Frequently change the filter bags and filter chambers.
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Key AVributes of Electroless Pla,ng Compared to Electroly,c Pla,ng
• Electroless deposit has a more uniform thickness and is less dependent on surface features than electroly,c
• Pla,ng rate is controlled by chemical reac,on in solu,on
• Effected by concentra,on of reactants, temperature, pH, steric inhibitors and chela,ng agents
• Electrically isolated features can be plated
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Key AVributes of Electroless Pla,ng Compared to Electroly,c Pla,ng
• Electroless baths must be ,ghtly controlled so that the reac,on rate is not too slow or too fast
• Bath life,me is limited due to build up of reac,on products and impuri,es. Chemical costs are higher than electroly,c baths
• Cataly,c surface of substrate is key to ini,a,ng pla,ng. Elaborate surface prepara,on is some,mes required.
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Nickel
• Plate a NiP or NiB Alloy
• Nickel Phosphorus‐ Amorphous Metal✴ 1‐4% P (low phosphorus)
✴ 5‐9% P (mid phosphorus)
✴ 10‐13% P (high phosphorus)
• NiP has many outstanding proper,es✴ Excellent diffusion barrier (no grain boundary diffusion due to
amorphous structure)
✴ Good solder and braze wecng
✴ Very hard but also duc,le (unique combina,on of proper,es only found in amorphous metals)
✴ Excellent coverage in deep recesses and blind holes
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
NiP 10% Plated Film Structure
Hoon et. al., Journal of Materials Science 23 (1988) 1643
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
NiP Bath Composi,on
Bath Component Chemical Formula Concentra,on
Ni ion source nickel sulphate NiSO46H2O 20 kg/m3
reducing agentsodium
hypophosphiteNaH2PO2H2O 26 kg/m3
complexing agentssuccinic acid,
aminoacetic acid, malic acid, citric acid
300 ppm
inhibitor thiourea 0.5 ppm
pH adjustments sodium hydroxide NaOH pH from 4.6 to 5.3
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
NiP Pla,ng Reac,on and Condi,ons
• NiSO4 + NaH2PO2 + H2O ‐‐‐‐‐‐> Ni Pla,ng + NaHPO3 + H2SO4
• Temperature 87C +/‐ 1C
• pH will control phosphorus content (range 4.6 to 5.3 for 8‐10%)
• Pla,ng rate 5‐15 µm/ hour
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Heat
Catalyst
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Comparison of NiP and NiBParameter N‐B N‐P
pH 6.0‐6.5 (neutral) 4.5 ‐ 5.3 (acidic)
Bath temperature C 65 90
Cost factor 3‐8 x 1
Typical thickness µm 1‐2 3‐20
Composi,on Ni 99% B 1% Ni 90‐92% P 8‐10%
Structure Fine Crystals Amorphous
Melt Point C 1400C 890C
Stress tensile compressive
Resis,vity (μΩcm) 17 60
Solderability excellent good
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Ni Bath Maintenance Procedures
• See general bath maintenance procedures in slide 29
• Air or solu,on agita,on should be directed toward the work from below
• Air agita,on is OK for NIP but should not be used with NiB
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Au
• Very low pla,ng rates (1.5 ‐ 2 µm/ hour)
• Very Expensive✴ High bath cost
✴ High Au loss factor when baths precipitate metal and need to be replaced
• Can be difficult to ini,ate pla,ng
• Typically only used to plate isolated features
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Gold
• High pH Electroless Au✴ Used for HTCC and LTCC
✴ Highly alkaline with pH 13‐14
✴ Very low pla,ng rate: 2µm / hour
✴ 99.9% Au purity‐ wire bondable
• Neutral pH Electroless Au✴ Used for AlN and other materials not stable in alkaline solu,ons
✴ pH = 7.5
✴ Very low pla,ng rate: 1.5 µm/ hour
✴ 99.9% Au purity‐ wire bondable
✴ Difficult to keep autocataly,c reac,on from slowing down during pla,ng cycle
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Au Formula,on‐ High pH Bath
Bath Component Chemical
Au ion source Gold hydrochloride trihydrate
complexing agent Sodium potassium tartrate
reducing agent Dimethylamine borane
stabilizer Sodium cyanide
pH adjuster NaOH
pH value 13.0
temperature 60C
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Au Bath Maintenance Procedures
• Maintaining a Au bath as long as possible is very key because of the high cost of Au
• Very important to avoid localized hea,ng. This involves tank design, agita,on and heater placement
• A double walled tank is recommended for uniform hea,ng
• Rinse very thoroughly before Au pla,ng to avoid metal contamina,on dragged in from other tanks.
• Filtra,on of Au is extremely important.
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Copper
• Used for pla,ng PCB, EMF Shielding, and as a component in UBM
• Can be used to plate electrical insulators such as PCB or plas,cs if pretreated with SnCl2 and PdCl2 ac,vators to form cataly,c sites for pla,ng ini,a,on
• Most common bath has pH = 12 (alkaline) and uses formaldehyde as a reducer
• Pla,ng rates typically 1‐5 µm/ hour
• Newer baths use alkanol amines (quadrol) as reducers and have pla,ng rates 10 µm/ hour
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Copper Bath Composi,on
Bath Component Chemical
Cu ion source Cu2+ salt is used
complexing agent KNaC4H4O6 (Rochelle salt)
reducing agent H2CO (Formaldehyde)
stabilizer 2‐Mercaptobenzothiazole (MBT)
pH adjuster NaOH
pH value 12
temperature 25C
Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Summary
• Electroless pla,ng requires chemical oxida,on and reduc,on agents in the pla,ng solu,on. This chemical reac,on drives pla,ng
• This is very different from electroly,c pla,ng where reac,ons are driven by an external electrical circuit
• Electroless baths are more complex than immersion or electroly,c baths and much less stable
• When instability occurs in the bath, the metal plates out of solu,on and the bath must be recons,tuted
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Electroless Summary
• Bath instability can occur from build up of reac,on by‐products or from a bath control devia,ons in pH, temperature or chemical composi,on
• In general, electroless pla,ng rates are much slower than electroly,c rates
• Pla,ng cost are also higher
• Electroless pla,ng is generally only used for pla,ng isolated metalliza,on pads where electroly,c pla,ng is not prac,cal or for pla,ng difficult to reach features
• The lowest cost, most efficient and most used electroless bath is NiP
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Monday, April 20, 2009
CMC Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009
Immersion Pla,ng Summary
• Immersion pla,ng is driven by the oxida,on of the base metal (corrosion)
• More noble metals plate onto less noble base surfaces
• Immersion Au is owen used. It allows a very thin layer of Au to be plated on a base metal such as Ni. This prevents the Ni from oxida,on and improves solder wecng.
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Monday, April 20, 2009