PCB Surface Treatment Crafts

OSP (Organic Solderability Preservatives)
OSP's have been around since the 1970's. It is widely believed that IBM was the first major corporation to give this finish credibility. The thickness of the OSP finish is almost unmeasurable (angstoms). The original formulas had a short shelf life of 3-6 months and could only withstand one or two heat cycles. By today's standards, this would be considered applicable only for lower technologies. The latest OSP formulas are far more robust and are designed for lead-free assembly. They can handle multiple heat cycles and have a one year shelf life.

HASL (Hot Air Solder Leveling)
The 63/37 tin lead solder has been the industry-standard since the inception of the original circuit board. If lead-free is not a concern, HASL is a very cost effective, reliable surface finish utilized in the manufacturing of lower technology PCBs. The HASL process can add stress to high layer circuit boards which can cause long-term reliability issues. This added stress, along with uneven solder height on dense SMT or BGA pads, are good reasons to replace HASL. Inevitably, tighter design criteria, advancing technologies, and/or environmental legislation will force the replacement of HASL. There are lead free alloys which can replace the conventional 63Sn / 37Pb solder in this process, but there are still capability limitations that exist within this process.

Immersion Tin
The immersion tin process has also been historically popular. It provides a consistently flat surface approximately 20-40 micro inches in thickness. This finish solders well, and is cost advantageous. However, finished PCBs have a limited shelf life and should be used within 3-6 months. Many PCB manufacturers around the world have this process in place.

Immersion Silver
With the pending lead-free legislation impacting PCB manufacturing on a global scale, the immersion silver process is rapidly gaining popularity as the lead-free surface finish of choice. While ENIG presently has a larger market share, over the past 12 months more immersion silver process lines have been installed in PCB facilities than any other finish. Immersion silver has a controlled thickness of 5-12 micro inches and a shelf life of at least 12 months. Silver is compatible with most assembly processes, is cost advantageous, and with its increased popularity, is becoming more commonly available.

ENIG (Electroless Nickel/Immersion Gold)
The ENIG finish has historically been the best fine pitch (flat) surface and lead-free option world-wide. Benefits to this surface finish are; long-term experience/knowledge of the product and excellent shelf life. The typical Nickel thickness is 75 micro inches and 3-5 micro inches of gold. Disadvantages include; limited availability, higher cost and this being the only surface finish that requires a two-part process. Also, if the process is not controlled, quality issues such as "Black Pad" may occur.

There are actually only two different types of surface finishes for PCB. Below is the most common on the market.
Organic • OSP
Metals • HASL (leaded and lead-free)• Immersion Sn• Immersion Ag• ENIG• ENEPIG• ASIG• ENEG

OSP is more like a lacquer, that prohibits oxygen attacking the copper underneath.
All other are coating are metals and they may be applied using one of two different methods = either electroless or immersion.
There is a big difference between these two processes for metallic coatings and it is important to be aware of these differences as we discuss finishes.

Electroless or Autocatalytic systems, both work in the same fashion in that they use an reducing agent inside the bath itself.
This means that the metal thickness increases during the whole period that the PCB is in contact with the solution.

• Nickel in ENIG
• Silver in ASIG or ESM100
• Gold in Electroless gold


General recommendation Handling / Storage condition / Time
A. Handling Recommendations: It is recommended that gloves are used for handling panels/circuits during all assembly processes. Or at the VERY least, handle the boards without touching the surfaces. Salts/acids from fingerprints will have a negative affect on the solderability.

B. Storage condition and time The storage environment should not to exceed 30℃ and 75% RH (except immersion tin). Boards should be stored in original vacuum packaging to limit air accessing the surface, and the board.

C. Baking Recommendations:
The purpose with baking is to reduce the risk of out-gassing, measling or delamination, by eliminating moisture or solvents within the laminate structure/board prior to soldering. Temperature and time of baking is to be determined on an individual basis. The time between baking and solderability testing should be kept to a minimum (not more than 24 hours) in order to prevent re-absorption of moisture into the laminate structure. Baking should be kept to a minimum, adhering to the production procedure, to prevent excessive oxidation and intermetallic growth.

D. Baking Recommendations given by NCAB
Baking can be advantageous and may be employed before any type of soldering operation. The recommended conditions are 120oC for 2 hours.

Oven conditions
Baking needs to take place in clean oven to prevent any form of contamination during the baking process. The boards should also be placed in the oven in such a way that the air can circulate freely around the boards during the baking time. All baking can be considered as advanced ageing and therefore may affect the solderability negatively. As such the time and temperature referenced above must be seen only as recommendations – the customer must take responsibility to approve processes.

HASL (Hot Air Solder Level)
Lead-Free Typical Thickness: 1-40μm
ADVANTAGES:
+ “Nothing Solders Like Solder”
+ Easily Applied
+ Long Industry Experience
+ Easily Reworked
+ Multiple Thermal Excursions
+ Good Bond Strength Assemblies
+ Long Shelf Life
+ Easy Visual Inspection
+ Low Cost
DISADVANTAGES:
- Huge Co-Planarity Difference
- Not Suited for High Aspect Ratios
- Not Suited for < 20 Mil pitch SMT and BGA
- PWB Dimensional Stability Issues
- Bridging Problems on Fine Pitch
- Inconsistent Coating Thicknesses
- High Process Temperature 260-270 deg C
- Not Suitable For HDI Products

Short process description
√HASL (Leaded and Lead-Free)
micro-etch—overflow rinsing—clean water rinsing—blowing—flux coating—hot air solder leveling—cooling—hot water rinsing—soft brush scrubbing—DI water rinsing—strong air blowing—hot air drying

Storage condition / time / handling
HASL (Leaded and Lead-Free)
HASL is the most robust of all treatments if applied under correct conditions, however there are still reasons to be careful:
A. Handling Recommendations
Whilst the surface treatment is rather robust, general recommendations should be followed.

B. Storage condition and time
General recommendations must be followed.

If nothing else is specified, IPC6012 sets the storage time according to JST-STD-003 Category 2 in other words a coating durability up to 6 months, NCAB recommends the category is set to JST-STD-003 Category 3 (6 months+)

C. Baking.
HASL can withstand baking with maintains its solderability well as long as the deposit is not too thin. See general recommendations for baking recommendations.

D. Temporary masking / Peelable mask
Since the HASL treatment is a rather robust treatment and not so sensible to contamination, nearly all types of maskings (such as Peelable mask, Kapton tape etc.) are acceptable.

Available standards and test methods
HASL (Leaded and Lead-Free)

IPC-6012
“Coverage & solderable”. The coating durability category shall be specified on the master drawing according to J-STD-003, if the category is not specified the category shall be set to class 2.

J-STD-003
There are 5+5 difference methods described as t0 how to test the solderability of the PCB. The two listed below are the most common if nothing else is specified
A – Edge Dip Test (3sec dwell time)
C – Solder Float Test (5sec floating time)
Leaded solder - Sn60/Pb40 - 235±5° C
Lead-free solder - SAC305 - 255±5° C

Design aspects
HASL (Leaded and Lead-Free)
There are not so many design concerns for this surface treatment.

A. Small pitches QFP < 0.50mm & BGA < 0.80mm
On very small pitch the co-planarity will become an issue and it will be difficult to assemble due to surplus of solder on the tiny SMD features.

Storage condition / time / handling
HASL (Leaded and Lead-Free)
HASL is the most robust of all treatments if applied under correct conditions, however there are still reasons to be careful:

A. Handling Recommendations
Whilst the surface treatment is rather robust, general recommendations should be followed.

B. Storage condition and time
General recommendations must be followed.
If nothing else is specified, IPC6012 sets the storage time according to JST-STD-003 Category 2 in other words a coating durability up to 6 months, NCAB recommends the category is set to JST-STD-003 Category 3 (6 months+)

B. HFFR4 & High Tg FR4 coated with lead-free HASL.
We have, from NCAB point of view, seen a problem with increased cases of measling and copper peeling on high copper thicknesses (≥3Oz) and the possible root cause is low peel strength of the foil for such materials, also the CTE difference between copper and substrate and finally the temperature shock as a result of the HASL process itself.

Design aspects
HASL (Leaded and Lead-Free)
There is not so much design concerns about this surface treatment.

C. Solder balls.
Holes plugged from only one side (partially plugged) with soldermask can entrap solder that the air-knives cannot remove from the surface (force it into the partially plugged hole). These solder balls may become dislodged and re-deposit onto the surface during the reflow operations

D. Edge plating on thick boards
We have experienced issues on thicker boards with edge plating, where the edge plating has been found to become loose after the lead-free HASL process.

Quality aspects
HASL (Leaded and Lead-Free)
With lead-free solder, quality control more critical that with traditional SnPb HASL.

A. Control the alloy.
It is critical to have good control over the alloy so all elements are within limits, the new Pb-F alloys dissolve more copper from the boards.

B. Use good flux.
Many of the low cost fluxes struggle with the heat and may perform badly.

C. Maintenance on machinery
The Pb-F HASL’s are more challenging to wet on the surface and are also much more easily to ‘blow off’ during processing so that thin layers provide insufficient pure tin and limits storage time and solder cycles.

OSP (Organic Solderability Preservative)
Typical Thickness: 0.15-0.30μm
ADVANTAGES
+ Flat, Coplanar pads
+ Reworkable (at PCB Fabricator)
+ Doesn’t Affect Finished Hole Size
+ Short, Easy Process
+ Low Cost
+ Environmental friendly
+ Cleaning
+ Clean process
DISADVANTAGES
- Difficult to Inspect
- Question Remains Over Reliability of Exposed Copper After Assembly
- Limited Thermal Cycles
- Can not be Reworked at EMS/OEM; Sensitive to Solvent Used for Misprint + Good Soldermask Integrity
- Limited Shelf life
- Easy to scratch

Short process description
√OSP (Organic Solderability Preservative)
OSP is thin layer of either Benzimidazole or imidazole lacquer.
Degrease—overflow rinsing—micro-etch—overflow DI rinsing--OSP—DI rinsing—strong air blowing--hot air drying

Storage condition / time / handling
OSP (Organic Solderability Preservative)
Since OSP is a rather thin organic and sensitive deposit, these recommendations should be followed.

A. Handling Recommendations:
Since the deposited layer is so thin and soft, it is important to handle the boards with care.

B. Storage condition and time
General recommendation needs to be followed. If nothing else is specified, IPC6012 sets the storage time according to JST-STD-003 Category 2 in other words a coating durability up to 6 months, NCAB recommends the category is set to JSTSTD-003 Category 3 (6 months+)

C. Baking.
Baking with OSP will have a negative effect on the solderability. So the customer must evaluate the baking process.

D. Temporary masking / Peelable mask
It is not recommended to use peelable mask on OSP treatment since the chemicals inside these masking material will/can a negative effect on the solderability.

E. Cleaning of Misprinted Solder Paste
The OSP coating is soluble to varying degrees in most solvents, acidic materials. Any solvent used to clean solder paste will dissolve some OSP

E. Process time
The time between first and last soldering should be as short as possible, preferable within 8-12 hours.

Design aspects
OSP (Organic Solderability Preservative)
There are some design concerns about OSP as a surface treatment.

A. ICT test points
It can become on issue that there is problems for test pins to penetrate the rather thin but hard layer of OSP and get connection with the test points. But there have been a lot of studies about this and the problem can be overcome with correct test pins and pressure.

B. Single sided plugged holes close to SMD pads.
There is a big risk the chemistry get trapped in these hole and contaminate the surface.

C. Multiple solder operations.
Even if OSP can handle multiple solder operations, the surface finish have it limitations and solderability can become on issue.

Immersion Sn (Immersion Tin)
Typical Thickness: 1.00-1.20μm
ADVANTAGES
+ Soldering direct to copper
+ Good for Fine Pitch Product
+ Good Solderability
+ Planar Surface
+ Eliminates Nickel
+ Mid Expensive
+ Popular for press fit / backplanes
DISADVANTAGES
- Handling Concerns
- Contains Thiourea, Which Is Carcinogenic
- Difficult To Rework
- Growth of Intermetallic Concerns
- Whiskers concerns
- Aggressive against soldermask

Short process description
√Immersion Tin
degrease—overflow rinsing—micro-etch—overflow DI rinsing—pre-dip—immersion Sn—post-dip—alkali rinsing—double DI rinsing—hot air rinsing—drying board

Storage condition / time / handling
Immersion Tin
Immersion Tin is excellent surface treatment, but also rather sensitive so extra concern need to be addressed at the handling

A. Handling Recommendations:
It is important that gloves are used for all assembly steps.

B. Washing of boards.
No washing is recommended either prior to / between any soldering step.

C. Storage condition and time
The storage environment should not to exceed 25C and 50% RH.

D. Baking.
Baking before soldering will have a negative effect on the solderability and also ‘consumes’ storage time since baking will trigger the growth of the IMC between the tin and copper – reducing usable tin.

E. Temporary masking / Peel able mask
It is not recommended to use peelable mask on Immersion Tin, since it is an immersion surface with a porous structure, all masking can easily lead to contamination.

Key points with Immersion Tin
Immersion Tin
Thickness is the number one factor when it comes to being able to provide a good soldering result. The finish always requires at least 0.2μm fresh tin on top of the IMC layer.

Key points with Immersion Tin
Immersion Tin
The graph shows how much fresh tin remains after 3 reflow cycles and also after accelerated aging.

Quality aspects
Immersion Tin
As mentioned the thickness in number one to have a good working finish that can withstand multiple soldering operation.

A. Control the IMC layer.
It is important for the supplier, to have control over there process so the IMC layer is as thin as possibly when they dispatch the boards.

B. Use good chemistry.
There are many bad vendors of immersion tin chemistry, so it wise to approve famous brands (Atotech, Enthone etc)

C. Specify the thickness or refer to IPC-4554
Since the immersion tin is the most aggressive of all available finishes, many Asian factories only deposit around 0.7-0.8μm as standard because some soldermask can not withstand any thicker deposit, due to undercut problem (see right hand image)

Design aspects
Immersion Tin
There are some design concerns about this surface treatment.

A. Contamination
Since this treatment is very sensitive to contamination, holes plugged from one side (partially plugged) are not recommended. See right hand image. Also holes very close to SMD pads are not recommended, since the plating solution will be trapped inside and can contaminate and destroy the solderability.

B. Soldermask bridges between SMD pads.
Since this treatment is very aggressive to the soldermask, there is need for larger soldermask bridges. Normally we can produce 3-4mil bridges. But with immersion tin 5mil is the minimum required.

DFM Soldermask openings I Sn

There is an risk that via holes close to SMD pads will become partially exposed during production. This will lead to two issues:
1.Solder paste ‘escaping’ into the via hole during soldering.
2. Chemistry becomes trapped inside the via hole during production and it can lead to corrosion and contaminations problems.

Short process description
√Immersion Ag (Immersion Silver)
degrease—overflow rinsing—micro-etch—overflow DI rinsing—pre-dip—immersion Ag—DI rinsing—strong air blowing--hot air drying

Storage condition / time / handling
Immersion Ag (Immersion Silver)
Immersion silver, is an immersion finish and rather also sensitive to contamination. Silver is also sensitive to sulphur and chlorides

A. Handling Recommendations:
It is important that gloves is used for all assembly steps.

B. Process time
Immersion silver finish also contains OSP inside that works to prohibit tarnishing, and since this is consumed during the first reflow cycle, it is therefore important to keep the cycle time as short as possible, 8-12 hours. To prevent or limit tarnishing

C. Storage condition and time
1. Immersion silver boards should be packaged as soon as possible, to prevent exposure to chlorides and sulfides in the air.
2. Use sulfur free, pH neutral paper to wrap stacks and then plastic wrap. Storage should be in sealed bags to eliminate direct contact with air.
3. Adhesive tape / labels, stamps, markers and rubber bands are forbidden on silver boards.
4. If the original package is opened and not all of the boards consumed at the EMS/OEM during the build, they should be re-wrapped as soon as possible.

D. Baking.
Baking always is recommended before soldering, but on silver boards this can have a negative effect on the solderability. As stated, within the immersion silver formulation there is also a mix of OSP to prohibit tarnishing. So the EMS/OEM customer must approve any baking process so solderability is not destroyed. NCAB have carried out practical tests and can achieve good solderability after baking – but care must be taken here.

E. Temporary masking / Peel able mask
It is not recommended to use peelable mask on Immersion silver product as all masking can easily lead to contamination.

Design aspects
Immersion Silver
There are some design aspects that should be considered for silver.

A. Contamination
As with immersion tin, the treatment is very sensitive / susceptible to contamination. Via holes plugged from one side (partially plugged) are not recommended. Also via holes very close to SMD pads are not recommended, since the plating solution may become ‘trapped’ inside the hole and potentially contaminate or destroy the solderability.

Short process description
√Electroless Nickel Immersion Gold
degrease—double rinsing—micro etching—overflow rinsing—DI rinsing—pre-dip—Pd activation—double DI rinsing—immersion Nickel—double DI rinsing—immersion Au—double DI rinsing—drying board

Storage condition / time / handling
Electroless Nickel Immersion Gold
Electroless Nickel Immersion Gold is an excellent surface treatment and also rather robust against treatment/handling

A. Handling Recommendations:
The surface treatment is rather robust, but general recommendations should always be followed.

B. Storage condition and time
General recommendation needs to be followed.

If nothing else is specified, IPC6012 sets the storage time according to JST-STD-003 Category 2 in other words a coating durability up to 6 months, NCAB recommends the category is set to JST-STD-003 Category 3 (6 months+)

C. Baking.
ENIG can withstand baking whilst maintaining solderability. See general recommendation for details.

D. Temporary masking / Peelable mask
Since the ENIG treatment is a rather robust treatment and not so sensitive/susceptible to contamination, nearly all types of maskings are acceptable - such as Peelable soldermask, Kapton tape, etc.

Design aspects
Electroless Nickel Immersion Gold
There are rather few design concerns associated wit ENIG, however:

A. Soldermask defined BGA pad
Soldermask defined BGA pads should be avoided, due to the risk of brittle joint and also the risk of black pads since the chemistry has less possibility to be rinsed, especially on smaller BGA pads. See graphic.

B. Single sided plugged via holes.
As with most finishes, via holes plugged from one side (partially plugged) are not recommended. Also holes very close to SMD pads are not recommended, since the plating solution can become ‘trapped’ inside and may contaminate or reduce the solderability.

C. Soldermask bridges between SMD pads.
As with immersion tin, this treatment is aggressive towards the soldermask, therefore larger soldermask bridges may be necessary with some factories.

Quality aspects
Electroless Nickel Immersion Gold
There are two quality aspects that are worth highlighting:

A. Black Pad.
This is the result of a lack of balance within the ENIG plating chemistry.

In principle it is either caused by a too aggressive immersion gold deposition process OR an overly active nickel surface. Whatever the cause, the result is overetching of the nickel (uncontrolled gold immersion reaction) which leads to an enrichment of phosphorus in the upper most Ni layer. Since the gold immersion reaction makes the Ni atoms go into solution it leaves the P atoms on the surface.

B. Brittle fractures.
Recent studies have shown that brittle solder-joints may form on an ENIG surface even if there is no black pad defect. The brittle fractures occurs in the Inter Metallic Compound (IMC) that is formed when soldering against Ni.

About PCBway

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