Tips I Wish I Knew Before PCB Surface Finishes

2020-01-16 14:53Writer: qyadminReading:

What is pcb surface finish


       When you leave an order of PCBs (printed circuit boards), you should take items. including PCB substrate material. solder mask, silkscreen, surface finish, board size and thickness, copper thickness, blind. and buried vias, through-hole plating, SMT, panels, tolerances, etc. into consideration. before the real fabrication of your circuit boards. Among those items. the selection of surface finish belongs to the first class as surface finish. plays an significant role in contributing to the reliability of electronic products. As copper layer on PCBs can oxidized. the generated copper oxidation layer will reduce soldering quality. which will decrease reliability and validity of the end products. Surface finish is conductive to prevent pads from oxidation and guarantee excellent solderability. and electric performance.

      Surface finish, or surface coating, is the most important step in the process. between PCB manufacturing and PCB assembly with two main functions. one of which is to preserve the exposed copper circuitry and the other of. which is to provide solderable surface when soldering components to the PCB. As shown in Figure 1, surface finish located at the outermost layer of PCB. and above copper, playing a role as a "coat" for copper.

      The surface of printed circuit boards (PCB) made of. copper to ensure electric currents flow . These metals need appropriate protection from the elements to avoid oxidation. and other corrosion. A range of finish types provides different levels of protection against deterioration. They also aid in certain use, such as with soldering. Various circuit board finishes are available. from options that are affordable. and to expensive or difficult-to-manage finishes used in only the most advanced applications.


PCB surface finish specification

 
    Anyone involved within the printed circuit board (PCB) industry understand. that PCBs have copper finishes on their surface. If they left unprotected then the copper will oxidize. and deteriorate, making the circuit board unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry . and to provide a solderable surface. when assembling (soldering) the components to the printed circuit board.
 
      Hot Air Solder Leveling (HASL) was once the tried. and true method of deliver consistent assembly results. But, the ever-increasing circuit complexity and component density. has stretched the capabilities of even horizontal solder levelling systems to their limits.
 
    As component pitches became finer and a need for a thin coating became greater. HASL represented a process limitation for PCB manufacturers. As an alternative to HASL. alternative coatings have been around for several years now. both electrolytic and immersion process
 
      The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry. and to provide a solderable surface. when assembling (soldering) the components to the printed circuit board.

pcb surface finish determine factor

 
Oxidation protection of PCBs metal traces (usually copper).
Surface solderability for electrical and mechanical component attachment.
Surface bondability for chip mounted components using gold and aluminum wire.
Any combinations of the above.
Mechanical applications (e.g. stress, strains etc.).
Environmental conditions (e.g. temperature, relative humidity etc.).
Mechanical contacts requiring abrasion resistance and oxidation protection.
General Discussion of Available Surface finishes
 

pcb surface finish types


hasl pcb surface finish


      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.

      One of the unintended benefits of the HASL process. is that it will expose the PCB to temperatures up to 265°C. which. will identify any potential delamination issues. well before any expensive components attached to the board. it is 

Low Cost
Widely Available
Re-workable
Excellent Shelf Life
Disadvantages:
Uneven Surfaces
Not Good for Fine Pitch
Contains Lead (HASL)
Thermal Shock
Solder Bridging
Plugged or Reduced PTH's (Plated Through Holes)

 

pcb surface finish Immersion Tin 


      According to IPC, the Association Connecting Electronics Industry. Immersion Tin (ISn) is a metallic finish deposited by a chemical displacement reaction. that applied over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life.

      Copper and tin but have a strong affinity for one another. The diffusion of one metal into the other will occur. impacting the shelf life of the deposit and the performance of the finish. The negative effects of tin whiskers growth. are well described in industry related literature. and topics of several published papers.it is

:
Flat Surface
No Pb
Re-workable
Top Choice for Press Fit Pin Insertion
Disadvantages:
Easy to Cause Handling Damage
Process Uses a Carcinogen (Thiourea)
Exposed Tin on Final Assembly can Corrode
Tin Whiskers
Not Good for many Reflow/Assembly Processes
Difficult to Measure Thickness

 

osp surface finish pcb


OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface. from oxidation by applying a very thin protective layer of material. over the exposed copper. usually using a conveyorized process.

It uses a water-based organic compound that bonds to copper. and provides an organometallic layer that protects the copper before soldering. It's also green in comparison with the other common lead-free finishes. which suffer from either being more toxic or higher energy consumption.it is


Flat Surface
No Pb
Simple Process
Re-workable
Cost Effective
Disadvantages:
No Way to Measure Thickness
Not Good for PTH (Plated Through Holes)
Short Shelf Life
Can Cause ICT Issues
Exposed Cu on Final Assembly
Handling Sensitive

 

pcb surface finish - Electroless Nickel Immersion Gold (ENIG)


      ENIG is a two layer metallic coating of 2-8 μin Au over 120-240 μin Ni. The Nickel is the barrier to the copper and is the surface to which the components are actually soldered to. The gold protects the nickel during storage. and also provides the low contact resistance required for the thin gold deposits. ENIG is now the most used finish in the PCB industry. due the growth and implementation of the RoHs regulation. it is

Flat Surface
No Pb
Good for PTH (Plated Through Holes)
Long Shelf Life
Disadvantages:
Expensive
Not Re-workable
Black Pad / Black Nickel
Damage from ET
Signal Loss (RF)
Complicated Process

 

pcb surface finish Gold – Hard Gold


Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is durable. and is most applied to high-wear areas. such as edge connector fingers and keypads.

Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle. although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1. and Class 2, 50 μin gold over 100 μin nickel for Class 3.

Hard gold is not generally applied to solderable areas. because of its high cost and its poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin. so if this type of gold must used on surfaces to soldered. the recommended nominal thickness should be about 5-10 μin.it is

Hard, Durable Surface
No Pb
Long Shelf Life
Disadvantages:
Very Expensive
Extra Processing / Labor Intensive
Use of Resist / Tape
Plating / Bus Bars required
Demarcation
Difficulty with Other Surface Finishes
Etching Undercut can Lead to Slivering / Flaking
Not Solderable Above 17 μin
Finish Does Not Encapsulate Trace Sidewalls, Except in Finger Areas


Which type of PCB Finish should I use?


  The ultimate way to answer this question is most likely to compare the types of PCB surface coatings against a couple of well-defined characteristics that will likely affect your decision. An excellent list of features should include the next:

  1.Lead-free solder - Adheres to limitations of hazardous substances (ROHS)rules.

  2.Managing sensitivity - Vunerable to contaminants or breakage from managing.

  3.Wire bondable - With the capacity of developing good wire certain connections.

  4.Tight pitch - Could be used for limited pitch components, such as ball grid arrays (BGAs).

  5.Contact utilization - Cen the contact be utilized for contacts.

  6.Shelf life - Has good shelf life, can be stored for half a year are more.

  7.Additional expense - Typically provides cost to your PCB production.

  Now, with a couple of comparison qualities, we can better strategy the question which kind of PCB complete to use.

  Comparision of Types of PCB Surface Finishes


PCBSurfaceFinishes

 
    The characteristics above are essential and may be used to help you select the best kind of PCB surface complete. However, you should seek advice from your contract producer (CM) for specific cost variations and other factors that may impact your decision, like additional turnaround time, for example.

Tips for Surface manufacture

 
Organic Solderability Preservative (OSP)
       OSP has a limited shelf life. Its most frequent use is soldering when the protectant  dissipated during the process. thus no extra removal processes needed.
 
Caution: once removed, the bare copper exposed and subject to oxidation. When many finishes needed on the same PCB, OSP can applied over other types of surface finish. (e.g. wire bonding and soldering, mechanical contact surfaces and soldering, etc.).
 
Electrolysis Nickel Immersion Gold (ENIG)
      ENIG is a widely used surface finish for soldering, aluminum wire wedge bonding and mechanical contact points (connector pads, test points, etc.). The copper surface. has an electrolysis nickel layer deposited (150 micro inches smallest). to seal the copper. A layer of gold is. then deposited to protect the nickel from oxidation. and provide a solderable surface to the nickel. The gold absorbed and dispersed into the solder. The gold is an immersion process and the thickness is self-limiting (2 to 3 micro inches max). The nickel layer is very brittle and cannot subjected to stress. or strains in the Z axis without cracking. Flexible PCBs are especially susceptible. to this with all areas subject to potential bending. supported with rigidizing materials.
 
Caution: controlled ENIG processing can result in weak solder connections. which may not be visible and/or result in failure. A typical sign of failure is a flat black copper pad after the attached component has removed.
 
Electroplated Nickel Gold
       In today’s complex circuits. this surface finish is very limited because it requires that all surfaces to plated. have to connected (i.e. an electrical charge must be present for plating). These interconnections must then broken to make the circuit functional. The plated nickel is very solderable and not subject to the solderability issues of ENIG. The plated gold has no limits on thickness and can support wire-bonding processes. like Thermo Compression Bonding (i.e. ball bonding).
 
Caution: Thicker gold can result in solder joints. being too brittle when using lead based solders.
 
Tags:
surface finish. fine pitch. printed circuit. immersion silver. circuit board. immersion tin. hard gold. hot air. immersion gold. low cost. smaller components. leadfree hasl. electroless nickel. typical thickness. rohs compliant. solder mask. nickel immersion. excellent flatness. pitch bga. bga smaller. pcb manufacturing. organic solderability. lead free. long shelf. air solder. flat surface. finish type. immersion finish. finish excellent. solderability preservative. black pad. gold enig. cost effective. press fit. manufacturing process. 12 months. pcb assembly. simple process. 
 

pcb backup board
pcb back up board backup material