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Immersion Tin PCB

Immerstion Tin PCB is printed circuit board plating tin, is different process from hot air solder leveling ( HASL ), Some engineers like to say immersion tin as white tin. Rayming manufacture immersion tin thickness 0.8-1.5μm, If you need immersion tin pcb, Pls send pcb to sales@raypcb.com .

The surface treatment immersion tin in PCB is specially designed to facilitate SMT and chip packaging processing. The tin metal plating layer is chemically deposited on the copper surface. It is an environmentally friendly process that replaces the Pb-Sn alloy plating process. It is widely used in the circuit board surface treatment process, surface treatment of hardware, decorations, etc.

Immersion tin vs. Hot air solder level-lead free (HASL-LF)

HASL-LF and immersion tin are two more commonly used surface treatment processes for printed circuit boards. However, in the printed circuit board process, immersion tin is unknown to most people, so we should clarify the similarities and differences between HASL-LF and immersion tin.

The Similarity:

HASL-LF and immersion tin are surface treatment methods to meet the requirements of lead-free soldering.

The differences:

  1. Process flow

HASL-LF is pre-treatment – spraying tin – testing – molding – appearance inspection.

Immersion tin is testing – chemical treatment – tin sinking – molding – appearance inspection.

  • Process principle

HASL-LF is mainly to intrude the PCB board directly into the molten tin paste. After being leveled by hot air, a dense tin layer will be formed on the copper surface of the PCB.

Immersion tin mainly uses displacement reaction to form a very thin tin layer on the PCB surface.

  • Physical characteristic

The thickness of the tin layer of HASL-LF is about 1um-40um. The surface structure is dense, hard, and not easily scratched. The HASL-LF only has pure tin in the production process, so the surface is easy to clean and can be stored at normal temperature for one year. The surface discoloration problem does not easily occur during the soldering process.

The thickness of immersion tin is about 0.8um-1.2um. The surface structure is relatively loose, soft, and the surface scratches easily. Immersion tin undergoes a complex chemical reaction, so it is not easy to clean. The surface is easy residual syrup will cause discoloration during welding. The storage time is short. It can be stored for three months at normal temperature. If it stores for an extended period, the color will change.

  • Appearance characteristic

HASL-LF – The surface is brighter and beautiful.

Immersion tin – The surface is light white, dull, easy to change color.

Detailed introduction of PCB immersion tin

Chemical immersion tin is a widely used PCB surface treatment process. Its working principle is to change the chemical potential of copper ions to cause the stannous ions in the plating solution to undergo a chemical substitution reaction, which is an electrochemical reaction. The reduced tin metal is deposited on the surface of the copper substrate to form a tin plating layer. The metal complex adsorbed on the immersion tin plating layer catalyzes the reduction of tin ions to metallic tin, so that the tin ions continue to be reduced to tin. The reaction equation is 2Cu+4TU+Sn2→2Cu+(TU)2+Sn.

Chemical immersion tin is a PCB copper surface treatment technology. The products of this process will neither contaminate the solder (unlike chemical nickel-gold board) nor produce an additional polyester layer on the solder mask. The method can be evenly covered with a layer of tin in any size PCB hole and the position of the connection plate. To adapt to the global strategic development, gradually realize the needs of lead-free and the development of the PCB market, use lead-free chemical precipitation. The replacement of the current hot air leveling process with tin technology has become a prerequisite for DPMC’s development plan.

The advantages immersion tin:

  • Good tin surface flatness
  • Has excellent electrical conductivity and solderability, can be soldered many times
  • Immersion tin layer does not contain lead, no pollution to the environment
  • Long storage period (one year)
  • Simple process and good working environment

Compared with other surface treatment processes:

  • Compared with HASL: Lead-free/smooth surface/plated layer
  • Compared with immersion gold (Ni/Au): Low cost
  • Compared with immersion silver: Higher oxidation resistance and better welding reliability
  • Compared with OSP: Can be tinned many times

Insufficiency of immersion tin:

  • Tin has low hardness and is easy to scratch
  • High requirements for incoming materials (requires uniform copper surface before tin sinking, no oxidation, fingerprints, glue stains, etc.)
  • Difficult to repair
  • High storage and transportation requirements

The surface of the PCB must be cleaned before tin sinking. CIMAPREP PR-514 acid cleaner is a concentrated agent used to remove oil and metal oxide particles on the board. It must be diluted to achieve its cleaning effect. CIMAPREP PR- 505 is a micro etching agent that can be used directly without dilution. After micro etching, the surface will be roughened to provide a homogeneous copper surface that is good for tin sinking.

  1. Immersion tin process sequence
Process No.Cylinder No.Processtime limitbest timetemperaturefiltration
11Remove oil stains2-4min3min30-40℃
22,3Two-stage washing1-2min2minRoom temperature 
34Microetching60-90sec60secRoom temperature
45,6Two-stage washing1-2min2minRoom temperature 
57Pickling60-90sec60secRoom temperature 
68,9DI washing60-90sec60secRoom temperature 
710Prepreg1-3min2minRoom temperature
811Tin sinking10-12min12min50-60℃
912Hot DI washing1-3min2min50-55℃ 
1013,14Two-stage DI washing1-3min2minRoom temperature 
1115,16Hot DI washing1-3min2min50-55℃ 
  • Immersion tin process characteristics
  • Bake at 155℃ for 4 hours (equivalent to storage for one year), or after 8 days of high temperature and high humidity test (45℃, relative humidity 93%), or after three reflow soldering, it still has excellent solderability.
  • The tin-immersion layer is smooth, flat, and dense. It is harder to form copper-tin intermetallic compounds than tin electroplating, and there is no tin whisker.
  • The thickness of the tin-immersion layer can reach 0.8-1.5μm, which can withstand the impact of multiple lead-free soldering.
  • The solution is stable, the process is simple, and it can be used continuously through analysis and replenishment without changing the cylinder.
  • Suitable for both vertical and horizontal processes.
  • The cost of immersion tin is much lower than that of immersion nickel and gold, which is equivalent to hot air leveling(HASL).
  • It has technical advantages for high-density boards that are easily short-circuited by spraying tin. It is suitable for rigid boards and flexible boards for thin-line high-density IC packaging.
  • Suitable for surface mount (SMT) or press-fit (Press-fit) installation process.
  • Lead-free and fluorine-free, no pollution to the environment, the waste liquid can be recycled.
  • Final Surface Cleaner
  • Composition:

M401 acid degreasing agent – 100ml/L

Concentrated H2SO4 – 50ml/L

DI water – the rest

Function: Remove oil stains, oxide layers, and fingerprints on the surface of the circuit board. This degreaser is compatible with all solder mask inks currently on the market.

  • Operating parameters:

Temperature: 30-40°C; Best value: 35°C

Analysis frequency:

Degreaser: Once a day

Copper content: Once a day

Control system:Degreasing agent: 80-120ml/L; Best value: 100ml/L

Copper content: Less than 1.5g/L

Supplement: M401; Add 10ml/L if the content is increased by 1%

Filtering: 20μ filter element is continuously filtered, and the filter element is changed when changing the cylinder.

Life: The copper content exceeds 1.5g/L, or the processing volume per liter reaches 500 feet.

  • Micro-etch
  • Composition: Na2S2O4 – 120g/L H2SO4 – 40ml/L DI water – the rest

Procedure:

  • Inject 85% DI water into the tank.
  • Add the calculated chemically pure H2SO4 and wait for it to cool to room temperature.
  • Add the calculated Na2S2O4 and stir until it is completely dissolved.
  • Make up DI water to the standard position.
  • Operating parameters: Temperature: Room temperature

Analysis frequency: H2S04: Once per shift

Copper content: Once a day  micro-eclipse rate: Once a day

Control system:

Copper content is less than 50g/L

  Micro-etch rate: 30-50μ; Best value: 40μ

Supplement: Na2S2O4: For every additional 10g/L, increase the content by 1%

H2SO4: For every additional 4ml/L, increase the content by 1%

Life: When the copper content exceeds 50g/L, dilute to 15g/L, and add Na2S2O4 and H2SO4.

  • Predip
  • Composition:

Prepreg M901 – 10% 

DI water – the rest

Purpose: Etch out the copper surface before tin sinking. This prepreg is not aggressive to any solder mask ink.

  • Operating parameters:

Temperature: Room temperature

Analysis frequency:

Acid equivalent: Once a day

  Copper content: Once a week

Refill: Acid equivalent Every time 100ml/LM901 is added, add 0.1 equivalent

Liquid position: Supplemented with DI water

Pass filtration: 20μ filter element continuous filtration

Life: Replace the sinking cylinder at the same time

  • Wastewater treatment: After neutralization with post-treatment waste liquid, the solid matter is filtered out.
  • Chemical Tin

Equipment: Both prepreg and chemical tin tanks are applicable.

Cylinder body: PP or PVC cylinder can be used.

Swing movement: The PCB frame swings in the cylinder to avoid gas agitation.

Filtration: 10μ filter element continuous filtration.

Ventilation: 15MPM ventilation is recommended.

Heater: Titanium Flon or Quartz heater.

Note: There should be no steel material in the cylinder.

  • Composition:

100% Sn9O2: This tin sinking liquid is not aggressive to any solder mask ink.

  • Operating parameters:

Tin concentration: 20-24g/L Best: 22g/L

Thiourea concentration: 90-110g/L Best: 100g/L

Sulfonic acid content: 90-110ml/L Best: 100ml/L

Copper ion concentration: It must be cooled and filtered when the maximum is 8g/L

Temperature: 70-75℃

Time: 10-15 minutes

Remark:

Acid Degreaser: SF          

Appearance: Colorless             

State: Liquid                 

Proportion: 1.02                  

Solubility: Completely soluble in water   

Odor: None                          

Main ingredients:                     

Methanesulfonic acid<10~25%           

Butoxyethanol<2.5~10%               

Water: The remaining part

Micro-etching agent: SF

Appearance: White

State: Crystal powder

Proportion: 1.35

Solubility: Completely soluble in water

Odor: None

Main ingredients:

Sodium persulfate<50~100%

Sodium bisulfate<10~25%

Basic agent for immersion tin: LP

Appearance: Colorless

State: Liquid

Specific gravity: 1.33g/l

Solubility: Completely soluble in water

Odor: Strong pungent

Main ingredients:

Methanesulfonic acid<10~25%

Water: Remaining part

Basic agent for immersion tin: 2000

Appearance: Colorless

State: Liquid

Specific gravity: 1.12g/l

Solubility: Completely soluble in water

Odor: Pungent smell

Main ingredients: 

Thiourea<10~25%

Water: Remaining part

Tin solution: SF-C

Appearance: Light yellow

State: Liquid

Specific gravity: 1.50g/l

Solubility: Completely soluble in water

Odor: Strong pungent

Main ingredients:

Tin methanesulfonate<25~50%

Water: Remaining part

Immersion tin additive

Appearance: Colorless

State: Liquid

Specific gravity: 1.02g/l

Solubility: Completely soluble in water

Odor: Pungent smell

Main ingredients:

Methanesulfonic acid<2.5~5%

Water: Remaining part

Immersion tin correction solution SN

Appearance: Colorless

State: Liquid

Specific gravity: 1.02g/l

Solubility: Completely soluble in water

Odor: Slightly irritating

main ingredient:

Methanesulfonic acid <5~10%

Thiourea <5~10%

Water: The remaining part

Immersion Tin Technical Capability

Board thickness: 20 ~ 400 mil

Board size: 3” X 6” (min) / 24” X 30” (max)

Aspect ratio: 15:1

Production capacity: 50,000 sqft/month

Tin thickness: 33.5 – 45.3 μ″

Immersion tin equipment

Vertical production line (with two cranes)

Equipment supplier: Universal (AEC)

All equipment of the liquid tank needs PP material

Swing range: 25~40MM

Swing frequency: 5~25 times/minute

Immersion Stin liquid

Manufacturer: Atotech (Germany)

Supplier: Atotech

Maintenance of tin sinking liquid:

  The tin sinking liquid is easy to maintain, and the main components can be supplemented through analysis to keep it within the best process range:

  • Each addition of 12ml/L tin sinking solution can increase the tin concentration by 1g/L, keeping the tin concentration between 20-24g/L.
  • Each addition of 10ml/L 10% thiourea solution can increase thiourea by 1g/L, keeping the concentration of thiourea between 90-110g/L.
  • According to the analysis value, supplement the content of organic sulfonic acid to keep it between 90-110ml/L.
  • The evaporation loss can be supplemented with deionized water.

Factors affecting the rate of tin sinking:

  • The influence of tin concentration: The speed of tin sinking will increase with the increase of tin concentration. The tin sink layer’s appearance does not change with the increase of tin concentration, so increasing the tin concentration effectively increases the tin sinking rate.
  • The influence of the concentration of organic sulfonic acid: The rate of tin precipitation increases with the concentration of organic sulfonic acid. When the content of organic sulfonic acid exceeds 110g/L, the rate is unchanged, but when the concentration of organic sulfonic acid is lower than 50ml/L, the tin layer formed will be foggy.
  • The influence of thiourea concentration: The tin deposition rate increases with the increase of thiourea concentration. When the thiourea concentration exceeds 250g/L, the appearance of the tin layer becomes rough and becomes foggy.
  • The influence of temperature: In the range of 40℃ to 80℃, the tin sinking rate increases with the increase of temperature.
  • Influence of time: The thickness of the tin layer increases with time, but the thickness becomes stable after 20 minutes at 60°C. Therefore, in production, choose to sink the tin at 60°C for 10-12 minutes, and a tin layer of 1.5μm thickness can be obtained.

Composition analysis:

  • Analysis of tin:
  • Reagents: 0.1N iodine solution, 30% sulfuric acid solution, starch solution.
  • Analysis steps:
  • Accurately put 2ml of solution into a 250ml flask.
  • Add 15ml of 30% sulfuric acid solution.
  • Add 100ml of deionized water.
  • Add 2ml starch solution.
  • Titrate with 0.1N standard iodine solution to the blue-purple endpoint, and record the number of milliliters V.

Calculation: Tin content Sn (Ⅱ) (g/L) = 2.69V.

  • Analysis of organic sulfonic acid:
  • Reagents:
  • 10% Mg EDTA solution: Add 122.76g Na2EDTA 2H2O and 39.6g MgSO4 to 800ml deionized water, adjust the pH to 7 with 1N NaOH solution and then add water to 1000ml.
  • Blue indicator solution or 0.1% ethanol solution.
  • 0.1N standard NaOH solution.
  • Analysis steps:
  • Accurately put 1.0ml tin sinking solution into a 250ml flask, and add 100ml deionized water.
  • Add 2ml Mg EDTA solution and 5 drops of bromophenol blue indicator solution.
  • Titrate with 0.1N standard NaOH solution until the solution turns from yellow to green at the endpoint (PH 6.7), and record the number of milliliters V.

Calculation: Organic sulfonic acid (g/L) = 9.61V

  • Analysis of thiourea
  • Analysis steps:
  • Cool the sample solution taken out of the tin sink to room temperature, and then filter to collect the filtrate.
  • Pipette 2ml of filtrate accurately into a 200ml volumetric flask, add deionized water to the mark and mix.
  • Accurately pipette 5ml diluent into a 1000ml volumetric flask, add deionized water to the mark and mix well (i.e., a total dilution of 20,000 times).
  • Measure the extinction value of the diluted solution with a UV photometer at 236nm, 10mm quartz cuvette, and deionized water as a reference.

Calculation: thiourea (g/L) = 128 × extinction value

The thickness of tin sink is about 0.8um-1.2um. The surface structure is relatively loose, soft, and the surface scratches easily. Sinking tin undergoes a complex chemical reaction with more chemicals, so it is not easy to clean, and the surface is rough. The residual syrup will cause the problem of discoloration during welding. The storage time is short and can be stored for three months at normal temperature. The color will change if it’s stored for an extended period.

Immersion tin’s structure

Some common defects in the production process of immersion tin PCB:

  1. Thin tin
  2. The tin surface is dark, and the tin surface is black
  3. PAD on green oil
  4. The edges of the holes are black
  5. Tin handprint
  6. Glue stains exposed copper, green oil on PAD
  7. Throwing oil and white characters
  8. Tin Whisk

Black strips
Black after PCB Drilling
Scratches on the tin surface
Fingerprints on the surface

Ink leakage in the hole

Tin whiskers in the hole

Immersion tin PCB usage and storage time

Two concerns:

  1. The tin whisker issue.
  2. Tin metal is an active metal. The tin surface is invaded by oxygen and other acidic gases after being in storage for an extended period, causing the tin surface to be oxidized or contaminated, thereby affecting the solderability.

Generally, double-sided & multi-layer immersion tin board: 6 months.

Inventory ≥ 3 months: Remove moisture from the baking sheet before going online. The temperature of the baking sheet is 120-135 ℃, time for 2-3 hours.

Inventory ≥ 6 months: Pay attention to the impact of PAD surface quality on solderability.

If it is a high TG board, it is recommended that the temperature of the baking board is about 10°C lower than the TG value.

Remarks: There will be certain risks when the oven is close to or more than one year after the oven is baked.

Each surface treatment process has its uniqueness, and the application range is not the same. Firstly, the surface of the tin sink is very flat, and the coplanarity is good. Secondly, the tin sink is lead-free, and many electronics manufacturers still prefer to choose Shen tin craft. However, Cu/Sn intermetallic compounds are easily generated during the immersion tin process, and the solderability of Cu/Sn intermetallic compounds is very poor. If the immersion tin process is used, particle size and the production of Cu/Sn intermetallic compounds must be controlled. The immersion tin particles must be small enough and non-porous. The deposition thickness of tin is not less than 40μin (1.0μm) is more reasonable. This is to provide a pure tin surface to meet the requirements of solderability.