Electroless Plang Tutorial - storage.googleapis.com Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009 Why Use Electroless and Immersion

Electroless Pla,ng Tutorial

CMC LaboratoriesMarch 2009

Monday, April 20, 2009

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

2



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

3



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

4



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


Immersion Pla,ng



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



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



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

9



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



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



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

12


Electroless Pla,ng



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

14



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

15



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

16



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

17



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

18



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+



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

20



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


How can this type of balance be achieved?



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

23



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



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



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

26



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

27



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

28



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.

29



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

30



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.

31



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

32



NiP 10% Plated Film Structure

Hoon et. al., Journal of Materials Science 23 (1988) 1643



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



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

35

Heat

Catalyst



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



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

37



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

38



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

39



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



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.

41



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

42



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



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

44



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

45



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.

46


Electroless Plang Tutorial - storage.googleapis.com Laboratories 7755 S. Research Drive, Tempe, Arizona Plating Tutorial for Tong Hsing, March 2009 Why Use Electroless and Immersion

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