PCB Surface Finishing
Comparison of several PCB surface finish types
As for the essential proceed in PCB production, the basic purpose of surface treatment is to ensure good solderability or electrical properties. Since copper in nature tends to exist in the form of oxides in the air, it is unlikely to remain as copper for a long time. Therefore, another treatment for copper is needed.
There are many kinds of surface treatments, they are HASL, OSP, ENIG, Immersion tin, and Immersion silver.
1. HASL or HASL LF
HASL should be the most widely used surface treatment technology. When the hot air is leveled, the solder and copper form a copper-tin metal compound at the junction, and the thickness thereof is approximately 1 to 2 mils. HASL provides a very reliable solder joint and shelf life. HASL makes component soldering very effective, but due to the thickness of HASL coating, the planarity of the surface may be unsuitable for fine-pitch components. The HASL deposit is made from a eutectic blend of tin and lead.
2. ENIG (Immersion gold)
ENIG is just one process that covers one thick layer of nickel-gold alloy with good electrical properties over copper, which processes long-term protection for PCBs to reach excellent mechanical properties. What’s more, the ENIG surface holds a special tolerance with the environment that other finishes fail to meet. Its capability to prevent the copper dissolution also brings a good performance to lead-free soldering.
OSP surface (Organic solderability preservative) is a process to generate a layer of organic film over clear bare copper in a chemical way. This layer of the organic film is resistant to oxidation, thermal shock, and wetting, which protect the copper surface from rust in a normal environment. Meanwhile, it could also be easily removed in the subsequent high-temperature welding for better soldering. Its simple process and low cost bring its wide usage in PCB production.
4. Immersion silver
The immersion silver process is relatively simple and fast. There is no need to apply a thick layer of armor to the PCB board, but also provides good electrical and solderability for the PCB board in a hot, wet, and polluted environment. The disadvantage is that it will lose its luster. And it also has good storage property.
5. Immersion Tin
Immersion Tin surface is adopted for its good flatness and lead-free nature. However, Cu/Sn intermetallic compounds with poor solderability are easily generated in the process. The biggest weakness of immersion tin surface is its short life, especially when stored in high temperature and high humidity environment, Cu/Sn intermetallic compounds will increase until the loss of solderability.
6 Characteristics of common surface treatment methods
|physical property||Sn-Pb HASL||Immersion silver||Immersion Tin||OSP||Immersion gold (ENIG)|
|Shelf life (month)||18||12||6||6||24|
|Thickness range (um)||1-25||0.05-0.20||0.8-1.2||0.2-0.5||0.05-0.2 Au
|Others||Thickness Nonuniformity||Easily scratched||Short storage||Sensitive to the environment||Easily-broken interface|
7. Comparison of cost and solderability
Cost: Electroplating nickel gold > ENIG > Immersion silver> Immersion tin > HASL > OSP.
Actual solderability: Electroplating nickel gold> HASL > OSP > ENIG >Immersion silver >Immersion tin
PCB surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: to ensure solderability, and to protect exposed copper circuitry. As there are many types of surface finishes, selecting the right one is no easy task, especially as surface mounts have become more complex and regulations such as RoHS and WEEE have changed industry standards.
a. HASL(Hot Air Solder Leveling) / Lead-Free HASL
HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using ‘air knives’, which blow hot air across the surface of the board.
Pros: Low-cost, Available, Repairable
Cons: Uneven Surfaces, Not good for fine pitch components, Thermal Shock, Not good for plated through-hole (PTH), Poor wetting
b. OSP (Organic Solderability Preservative)
OSP is a water-based, organic surface finish that is typically used for copper pads. It selectively bonds to copper and protects the copper pad before soldering. OSP is environmentally friendly, provides a coplanar surface, is lead-free, and requires low equipment maintenance. However, it’s not as robust as HASL and can be sensitive while handling.
Pros: Lead-free, Flat surface, Simple process, Repairable
Cons: Not good for PTH, Sensitive, Short Shelf life
c. ENIG(Electroless Nickel Immersion Gold )
ENIG is quickly becoming the most popular surface finish in the industry. It’s a double layer metallic coating, with nickel acting as both a barrier to the copper and a surface to which components are soldered. A layer of gold protects the nickel during storage. ENIG is an answer to major industry trends such as lead-free requirements and rise of complex surface components (especially BGAs and flip chips), which require flat surfaces. But ENIG can be expensive, and at times can result in what is commonly known as “black pad syndrome,” a buildup of phosphorous between the gold and nickel layers that can result in fractured surfaces and faulty connections.
Pros: Flat surfaces, Strong, Lead-free, Good for PTH
Cons: Black pad syndrome, Expensive, Not good for rework
The above types of PCB surface finish are commonly used. Besides, there are Immersion silver, Immersion Tin, Hard Gold, Nickel palladium (ENEPIG), and so on. Now, according to your own requirements, you can choose a suitable surface finish type.
8. PCB Solder Mask
The solder mask means the green part on the printed circuit board. In fact, it uses the negative output, so when the shape of the solder mask layer is mapped to the PCB board, it is not the green oil, but the copper skin is exposed.
The functions of the solder mask layer on the PCB board are shown:
a. Preventing the physical breakage of a conductor circuit
b. Preventing short circuit from bridge connection in the soldering process
c. Preventing the copper layer from oxidation
d. Prevent short circuit between conductive lines and solder joints when proceeding with reflow soldering, wave soldering, and manual welding
e. Its high Insulating brings the possibility to the high density of PCB boards
The solder mask is mainly Liquid UV Photo-imageable Ink with green, red, blue, yellow, white, black, and purple colors, green should be the most commonly used one. And according to those colors available, there are some points in our products for your reference.
Solder Mask Cost
The cost for green, blue, red, blue, yellow, white, and black are the same without extra charge, and extra charge needed for the purple, matte green, matte black masks.
For solder bridge, the spacing between IC pins should be min 0.2 m for all colors except black who requires min 0.25 mm
l As for the normal case, the lead time for the green mask should be shorter than others
Since we just refer to the solder bridge above, now comes to a briefly knowing for it:
The solder bridge is the green oil between the two pads or the IC pins, For its narrow need, so it is called the solder resist bridge. If the distance between the pins is too small to meet the factory process, it will be recommended to cancel it and proceeded with full mask openings.
Mask opening: any area without mask printing can be called window opening, it includes areas for solder PAD, paste PAD, trenching, etc.
9. PCB Via Covering
Vias not covered
It means that the vias are exposed and the surface finish is applied to via barrel.
The solder paste might get into the vias and cause poor or non-existent solder joints.
It means that the vias are simply covering by solder mask ink.No additional process steps is required during fabrication.
Covering the annular ring and vias with solder mask ink in order to prevent exposure to the elements and reduce accidental shorting or contact with the circuit.
Plugged Vias with non-conductive media (eg. Epoxy/resin, solder mask ink)
The conductive vias in BGA require plugged vias. Because solder paste might wick away from the intended pad and flow down into the via, then creating poor or non-existent solder joints during assembly.
The diameter of the plugged vias requires to be smaller than 0.5mm.
10. PCB Silkscreen
The graphics and text on PCB a circuit board is usually printed with a permanent non-conductive epoxy ink. The color is typically WHITE, although other colors may be available. Requirements are addressed in IPC-4781 QUALIFICATION AND PERFORMANCE SPECIFICATION OF PERMANENT, SEMI-PERMANENT AND TEMPORARY LEGEND AND/OR MARKING INKS.
I’ve never heard of a problem with Silkscreen as a material, except maybe some discoloration after high PCB reflow temperatures for RoHS compliant soldering. Silkscreen problems are almost always caused by circuit board DESIGNERS, so I’m going to take this opportunity to mention a few guidelines (which I will probably repeat in a later section, but worth repeating because designers often do not give silkscreen the attention it deserves!)
If silkscreen epoxy is printed over lands PCB or pads that will be soldered, it will melt into the solder joint during reflow and weaken the attachment. Therefore, silkscreen should always be printed on top of the mask material and never over solderable surfaces. Many CAD systems can clip the silkscreen away from exposed metal, but when creating library components be sure to keep the component outlines and polarity marks away from the pins (0.25mm) by default, and pay attention when moving or rotating reference designators or when adding text to PCB the final board design.
The width of silkscreen graphics can also be a problem. If too thin, PCB the fabricator will have problems in the screening or printing process and skips can occur. If too thick, the text may be too blurry to read. As components get smaller in size and circuit density continues to increase, it is sometimes very difficult to print silkscreen at all, but try to use line widths not less than 0.15 mm (6 mils), 0.18 mm to 0.20 mm is preferred PCB.
Solder pads should never be overprinted, because this may lead to problems during soldering, assembly, and with the E-Test. If we should detect an overprinted solder pad (spacing < 150μm), then Multi-CB reserves the right to remove the marking print at this location. Relevant for the definition of the solder pads is the solder pad exemption. The marking print is usually automatically clipped with a spacing of 100μm to the solder-stop edge.
The spacing between the solder pad and marking print should be at least 150μm.
Always avoid overprinting solder pads!
|Silkscreen printing method|
|Item||productivity||For high/mass production|
|Coating thickness||It is relatively thick, but it gets scattered|
|Silk color||White, black, yellow|
It is a printing method using a plate material with chemical fibers in a mesh form. By rubbing the ink with a squeegee, ink is pushed out between the meshes and printed on the board. For silk printing of printed circuit boards, it is a general construction method.
|Coating thickness||It is thicker but different|
|Silk color||White / Black|
This is a printing method that absorbs ink by directly spraying it onto a substrate. This is an environmentally friendly method because no sheets or films are used. The print resolution is higher than “screen printing”. Inkjet white inks may have a slightly milky “milky” appearance.