The search for the universal surface finish: an evaluation of the advantages--and limitations--of common surface finishes.There are over a dozen surface finishes on the market. All are useful for some applications, but none really fit the category of "universal surface finish." This article will highlight each of the common surface finishes with regard to advantages and shortcomings A shortcoming is a character flaw. Shortcomings may also be:
Frequently used in tax accounting, an itemized account or claim separately lists amounts that add up to the final sum of the total account on claim. what is paramount to the PCB PCB: see polychlorinated biphenyl. PCB in full polychlorinated biphenyl Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. design community when choosing the correct finish for bare boards. My goal is to spark spark, in electricity: see arc. (language) SPARK - An annotated subset of Ada supported by tools supplied by Praxis Critical Systems (originally by PVL). http://sparkada.com. industry interest in resolving the technical issues surrounding sur·round tr.v. sur·round·ed, sur·round·ing, sur·rounds 1. To extend on all sides of simultaneously; encircle. 2. To enclose or confine on all sides so as to bar escape or outside communication. n. existing processes and to develop new products. My point of view is that of an OEM (Original Equipment Manufacturer) The rebranding of equipment and selling it. The term initially referred to the company that made the products (the "original" manufacturer), but eventually became widely used to refer to the organization that buys the products and , though I also share concerns of fabrication fabrication (fab´rikā´sh  n),n the construction or making of a restoration. and assembly. Finally, the paper will also discuss the industry needs going forward; especially regarding the tight pitch tendencies and higher speed requirements of future systems. The surface finishes profiled are: * HASL (language) HASL - SASL plus conditional unification. ["A Prological Definition of HASL, A Purely Functional Language with Unification Based Conditional Binding Expressions", H. Abramson in Logic Programming: Functions, Relations and Equations, D. DeGroot et al eds, P-H 1986]. . * OSP (Online Service Provider) See online service. OSP - Optical Signal Processor . * ENIG ENIG Electroless Nickel Immersion Gold (printed circuit board manufacturing process) . * Immersion immersion /im·mer·sion/ (i-mer´zhun) 1. the plunging of a body into a liquid. 2. the use of the microscope with the object and object glass both covered with a liquid. silver. * Immersion tin. * Electrolytic e·lec·tro·lyt·ic adj. 1. Of or relating to electrolysis. 2. Produced by electrolysis. 3. Of or relating to electrolytes. e·lec nickel-gold. * Electroless nickel/palladium/immersion gold. * Selective finishes. * Reflowed tin-lead. Certainly there are more products on the market than those listed above. However, they hold a relatively smaller market share and are not part of this article. Each finish has distinct limitations restrict its use in other, expanded applications. The electronics design and manufacturing industry really needs two things from material vendors: * A universal surface finish that satisfies all the requirements of the OEM. * A surface finish that can be used in applications that reach signal speeds in excess of 7 Gb/s. OEM requirements for a surface finish can be divided into two groups: Internal: * Signal integrity. * High-speed high-speed adj. 1. Operated or designed for operation at high speed: a high-speed food processor. 2. Taking place at high speed: a high-speed chase. 3. signal capacity. * EMI (ElectroMagnetic Interference) An electrical disturbance in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves (RFI) from chips and other electronic devices. Allowable limits are governed by the FCC. shielding. * RF capable. * High joint strength. * Contact resistance. External: * Cost, cost, cost. * Shelf life. * Whether it can be reapplied. * Wetting/solderability. * Good for 5-plus year end-of-life end-of-life Cardiac pacing noun The point at which a pacemaker signals need for replacement, as its battery is nearing depletion Medtalk adjective . Also, the surface coating Surface coating A substance applied to other materials to change the surface properties, such as color, gloss, resistance to wear or chemical attack, or permeability, without changing the bulk properties. must not slow the signal speed of boards that run in excess of 7 Gb/s. Immersion silver is the only surface finish that satisfies this requirement without compromising any of the other necessary characteristics for a final finish (TABLE 1). However, tarnish tarnish, n 1. surface discoloration or loss of luster by metals. Under oral conditions, it often results from hard and soft deposits. 2. a chemical process by which a metal surface is discolored or its luster destroyed. after assembly is still a primary issue and it prevents silver from being used as universally as once hoped. Extensive testing of tarnished silver PCBs for loss of functionality has shown the deposit remains usable USable is a special idea contest to transfer US American ideas into practice in Germany. USable is initiated by the German Körber-Stiftung (foundation Körber). It is doted with 150,000 Euro and awarded every two years. . EMI shielding, EMI leakage LEAKAGE. The waste which has taken place in liquids, by their escaping out of the casks or vessels in which they were kept. By the act of March 2, 1799, s. 59, 1 Story's L. U. S, 625, it is provided that there be an allowance of two per cent for leakage, on the quantity which shall appear , contact resistance, signal integrity, solderability Solderability defines whether a solder can perform as intended in-service under normal fabrication methods or processes. The electronic industry has a consensus specification which provides guidelines for classifying, testing, marking, and packaging accepted solder compositions. and joint strength have all been profiled in these studies. Despite the data for silver deposits, cosmetic cosmetic /cos·met·ic/ (koz-met´ik) 1. pertaining to cosmesis. 2. a beautifying substance or preparation. cos·met·ic n. issues remain the major cause for rejects. It is hoped that the new generation of tarnish-resistant silvers being tested will eliminate cosmetic tarnish on immersion silver plated PCBs. In our discussions on benefits and limitations of common surface finishes, it is important to note that the benefits and drawbacks of these deposits may vary according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. fabrication and assembly processing and OEM applications. Immersion silver is a good example. For most OEMs, tarnish is not an issue because there is no exposed silver on the PCB after assembly. However, some OEMs use exposed edge rails and/or and/or conj. Used to indicate that either or both of the items connected by it are involved. Usage Note: And/or is widely used in legal and business writing. uncoated internal pads for NPTHs (non plated through-holes), and these can exhibit tarnish after assembly or burn-in 1. (hardware) burn-in - screen saver. 2. (hardware, testing) burn-in - burn-in period. . Despite the many advantages and drawbacks of each surface finish, there are one or two major aspects that preclude pre·clude tr.v. pre·clud·ed, pre·clud·ing, pre·cludes 1. To make impossible, as by action taken in advance; prevent. See Synonyms at prevent. 2. its universal or widespread use: HASL: * Bridging of tight pitch outerlayer traces, and hole plugging on small through-vias. * The lack of coplanarity In geometry, a set of points in space is coplanar if the points all lie in the same geometric plane. For example, three points are always coplanar; but four points in space are usually not coplanar. is not the issue it was once thought. Reason: Newer, horizontal processors have minimized this effect, and assemblers' specifications usually permit a 0.003" standoff stand·off n. 1. A tie or draw, as in a contest. 2. A situation in which one force neutralizes or counterbalances the other. 3. A standoff insulator. adj. Standoffish. , making lack of coplanarity less likely to be a problem. OSP: * Lack of robustness of the deposit makes it difficult to rework re·work tr.v. re·worked, re·work·ing, re·works 1. To work over again; revise. 2. To subject to a repeated or new process. n. or clean. * Electrical test pins cut through the coating and expose potential corrosion sites. * To eliminate the latter issue, many fabricators apply OSP after electrical test. This exposes the test bed to defects--such as copper voids in the hole due to overly aggressive OSP preclean microetch--that will not be identified until after assembly. By this point the value of the scrap rises exponentially ex·po·nen·tial adj. 1. Of or relating to an exponent. 2. Mathematics a. Containing, involving, or expressed as an exponent. b. . ENIG: * The rise of "black pad" defects, which cause massive solderability failures. * Not suited for high-speed (>7 Gb/sec (GigaBytes per SECond, GigaBits per SECond) One billion bytes or one billion bits per second depending on whether the B is upper case or lower case. However, using "B" for byte and "b" for bit is not always followed and often misprinted. ) applications; there is evidence that the nickel nickel, metallic chemical element; symbol Ni; at. no. 28; at. wt. 58.69; m.p. about 1,453°C;; b.p. about 2,732°C;; sp. gr. 8.902 at 25°C;; valence 0, +1, +2, +3, or +4. in the deposit slows down the signal speed. Immersion silver: * Tarnish causes many cosmetic failures for boards with exposed silver after assembly. * Remaining UL limitations on its use. Immersion tin: * Lingering lin·ger v. lin·gered, lin·ger·ing, lin·gers v.intr. 1. To be slow in leaving, especially out of reluctance; tarry. See Synonyms at stay1. 2. questions surrounding the growth of whiskers See metal whiskers. and dendrites over time, especially at room temperature. * Some vendors' processes need a more vigorous deposit; some do not with stand multiple thermal cycles. Electrolytic nickel-gold: * Difficult to get the minimum 70 pin of nickel plated in the small thru-vias. * Nickel slivers cause shorts across traces. * Excess gold in high current density areas (HCDAs) causes gold embritdement of the solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i. fillets. Electroless nickel-palladium-immersion gold: * For most OEMs the cost is excessive. Selective solder strip: * Expense for process too great. Most often used because there is no single surface finish that can satisfy all the requirements on the PCB. At best, an expensive compromise. Reflowed tin-lead: * Solder slumping Slumping is a categorical description of an area of techniques for the forming of glass through the use of heating glass to the point where it becomes plastic. It is generally formed by the force of gravity. causes hole size violations on thick backpanels. Surface Finish Selection TABLES 2 and 3 are used by some OEMs to identify the proper surface finish according to the technology of the boards. As can be seen, no single finish satisfies all the applications for assembly, even if we disregard the (odd) need for wire bonding Wire bonding is a method of making interconnections between a microchip and other electronics as part of semiconductor device fabrication. The wire is generally made up of one of the following:
Although the information contained herein breaks no new ground, we strongly believe that it needs periodic review to focus the need for a universal surface finish and improve existing processes, which in turn should ultimately lead to a paring of the number of surface finishes used. This would be welcomed by fabricators, which could achieve significant cost savings by reducing the amount of employees needed for final finishing, and reducing chemistry and materials acquisition, floor space and utilities consumption. Assemblers This is a list of assemblers. Hundreds of assemblers have been written; some notable examples are:
OEMs would benefit from the reduced number of surface finishes by not having to track which finish is applied to which part number (the same part number can have different finishes on multiple lots or date codes, in some cases). Logistically, much time and money is spent to organize, disposition and classify clas·si·fy tr.v. clas·si·fied, clas·si·fy·ing, clas·si·fies 1. To arrange or organize according to class or category. 2. To designate (a document, for example) as confidential, secret, or top secret. when each finish should be used. Funds can also be preserved by eliminating the expense of testing, studying and qualifying each surface finish on the market. Finally, customers would benefit from having uniformity in the PCB coatings which make swaps, replacements and even reliability matches for the boards in their systems easier to track and predict.
TABLE 1. Advantages and Disadvantages of Common Surface Finishes
SURFACE FINISH THICKNESS
HASL 50-1500 [micro]in
OSP 0.2-0.6 [micro]m
(Benzimidazoles)
ENIG 125-250 [micro]in Ni
2-8 [micro]in Au
Immersion Ag [greater than or
equal to] 5 [micro]in
Immersion Sn 26-60 [micro]in
Electrolytic (hard) 50-200 [micro]in Ni
Ni-Au 5-50 [micro]in Au
Electroless 120-240 [micro]in Ni
Ni/Pd/Immersion 10-30 [micro]in Pd
Ag 3-8 [micro]in Au
SSS See applicable
(Selective surface finishes
Solder Strip)
Reflowed 50-1500 [micro]in
Sn-Pb
SURFACE FINISH ADVANTAGES
HASL "Nothing solders like solder."
Easily applied.
Lots of industry experience.
Easily reworked.
Good bond strength.
Withstands multiple thermal cycles.
OSP Flat, coplanar pads.
(Benzimidazoles) Reworkable (by fabricator).
Doesn't affect final hole size.
Short, easy process.
Cu-Sn IMC formed has been reported to be
stronger and more robust than Cu-Sn from
HASL and Ni-Sn from Ni-Au.
ENIG Planar surface.
Consistent thicknesses.
Withstands multiple thermal cycles.
Long shelf life.
Solders easily.
Good for fine-pitch product.
Immersion Ag Good for fine-pitch product.
Planar, surface.
No black pad concerns.
Short, easy process cycle.
Eliminates nickel.
Doesn't affect final hole size.
Long shelf life.
Can be reworked/reapplied by the fabricator.
OK for multiple Insertions.
Inexpensive.
Drop-in process for the assembler.
Good for ultra-high speed signals.
Immersion Sn Good for fine-pitch product.
Planar surface.
Eliminates nickel.
Can substitute for reflowed solder in
selective strip.
Inexpensive.
Electrolytic (hard) Plated Ni-Au can be used as etch resist.
Ni-Au Available for "mixed technology" products.
Au wire bondable.
Electroless Pd keeps Ni from passivating in presence of
Ni/Pd/Immersion "porous" gold coating.
Ag AI wire bondable.
Planar surface.
Good for fine pitch product.
SSS Hot bar reflow for TAB devices.
(Selective Viable alternative to HASL on thick product.
Solder Strip)
Reflowed Fast process.
Sn-Pb Inexpensive.
Time tested.
Good for solderability.
SURFACE FINISH DISADVANTAGES
HASL Huge coplanarity differences resulting
in off-contact printing and
assembly defects.
Contains lead.
Not suited for high aspect ratios.
Not suited for < 0.020" pitch.
PCB dimensional stability issues.
Bridging problems on fine pitch assemblies.
Inconsistent coating thicknesses
(on varying pad sizes).
OSP Assembly line changes may be required;
(Benzimidazoles) not a drop-in.
Question remains over reliability of
exposed Cu after assembly.
Limited thermal cycles.
Cannot be reworked by assembler.
Sensitive to some solvents used for
misprint cleaning.
Limited shelf life.
Test pins cut coating, leaving
exposed copper.
ENIG Not wire bondable.
Expensive.
Should not he used on [less than or
equal to] 1.0 mm pitch; black pad issues.
Waste treatment of Ni.
Cannot be reworked by fabricator.
Ni is suspected carcinogen.
Not optimal for higher speed signals.
Immersion Ag High friction coefficient; not be suited
for compliant pin insertion (Ni-Au pins).
Some systems cannot throw into blind vias
with aspect ratios > 1:1.
Tarnishing must be controlled.
Immersion Sn Handling concerns.
Panels need to be routed and electrically
tested before coating.
Contains thiourea.
Limited rework cycles at assembler.
Horizontal process needs nitrogen blanket.
Electrolytic (hard) Exposed Cu sidewalls.
Ni-Au Ni slivers after SES.
Ni throwing power issues in small vias.
Costly process.
Excess Au easily plated on board edges
causing poisoning of solder joints.
Electroless Additional processing step for
Ni/Pd/Immersion fabricator (adds cost).
Ag Dip tank process.
Evidence that Pd poisons solder paste
after reflow.
Waste treatment.
SSS Multiple resist and photo cycles.
(Selective Difficulty in controlling plated
Solder Strip) Sn-Pb thickness.
Overlap (butt) line difficultio control.
Expensive.
Reflowed Solder Slumping may cause hole size
Sn-Pb reduction-violation.
Reflowing causes uneven deposit
thicknesses in the hole.
Heavy panels may require automated handling.
Not a planar deposit.
TABLE 2. Surface Finish Usability Based on Technology
SURFACE THROUGH- PRESS-FIT SMT FINE-PITCH
FINISH HOLES BGA
Selective Solder OK OK OK OK
ENIG OK OK OK No (1)
HASL OK OK OK No
Immersion Sn OK OK OK OK
Immersion Ag OK OK OK OK
OSP OK OK (2) OK OK
Electrolytic Ni-Au (4) No (5) OK OK OK
Reflow Sn-Pb OK OK No No
SURFACE EMI CARD EDGE FLIP
FINISH SHIELDS GUIDES CONNECTORS CHIPS
Selective Solder OK OK OK OK
ENIG OK OK OK OK
HASL OK OK OK No
Immersion Sn OK OK OK OK
Immersion Ag No (3) OK OK OK
OSP No No OK OK
Electrolytic Ni-Au (4) OK OK OK OK
Reflow Sn-Pb OK No OK No
SURFACE GOLD ALUMINUM
FINISH WIRE BOND WIRE BOND
Selective Solder No No
ENIG No No
HASL No No
Immersion Sn No No
Immersion Ag No No
OSP No OK
Electrolytic Ni-Au (4) OK OK
Reflow Sn-Pb No No
(1) Not recommended for 1.0 mm pitch or less. (2) Not the best or more
robust choice. (3) EMI shielding OK, but tarnish forms when left
unsoldered/exposed. (4) Electrolytic Ni Au will embrittle solder paste
due to excess gold plated in PCB high current density areas. (5) Not
for aspect ratios > 8:1 due to poor throwing power
TABLE 3. Attributes of Select Surface Finishes
AG ENIG
Coplanar deposit Yes Yes
Non-galvanic Yes No
Shelf life 12 months 12 months
Multiple rework cycles Yes Limited
Higher cost savings Yes No
CEM line changes needed No Yes
Doesn't reduce hole sizes Yes Yes
OK for [greater than or
equal to] 1.0 mm BGA pitch Yes No
HASL OSP
Coplanar deposit No Yes
Non-galvanic Yes Yes
Shelf life 9 months 6-9 months
Multiple rework cycles Limited no
Higher cost savings No Yes
CEM line changes needed Yes Yes
Doesn't reduce hole sizes No Yes
OK for [greater than or
equal to] 1.0 mm BGA pitch No Yes
MIKE BARBETTA is a consultant with 25 years experience in PCBs. He has worked for Cisco Systems “Cisco” redirects here. For other uses, see Cisco (disambiguation). Cisco System,Inc. (NASDAQ: CSCO, HKSE: 4333 ) is an American multinational corporation with 54,000 employees and annual revenue of US $28.48 billion as of 2006. , Sanmina/Hadco, Shipley Shipley, town (1991 pop. 28,815), West Yorkshire, N England, on the Aire River. Of its varied industries, light engineering and the manufacture of woolens and worsteds are the most important. and MacDermid, among others. He can be reached at mbarbetta@sbcglobal.net. |
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