PCB Surface Treatments

Transcription

1 PCB Surface Treatments

2 Definition of surface finish A surface finish can be defined as a coating, either metallic or organic in nature, which is applied to a PCB in order to assure solderability of the metal underneath after various time of storage / conditions. Most of the surface treatment dissolves into the solder paste or wave solder during the soldering process and the solder joint is forming between the solder and the copper. One exception is ENIG / Immersion Gold where the solder dissolves the p thin layer of gold and forms a joint with the underlying nickel phosphorous alloy.

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

4 Definition of surface finish 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.

5 Definition of surface finish Electroless or Autocatalytic systems, both work in the same fashion in that they use an reducing agent inside the bath itself. This means that t 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 Electroless process Ni 2+ e - Ni 2+ e - Reducing Agent e - Ni 2+ Ni 2+ Ni 2+ Ni 2+ Cu Ni Cu Ni Cu Cu Ni Cu Ni Cu Cu Ni Cu Ni Cu Ni Cu Ni Cu Ni Cu Cu Cu Cu Reducing Agent e -

6 Definition of surface finish Immersion systems, This process uses a chemical displacement reaction to deposit a metal layer onto the exposed metal surface of the PCB. The base metal ENIG Gold bath donates the electrons that reduce the positively charged metal ions present in Au Ni 3+ + Au3+ Ni+ Au3+ Ni + Au Ni solution Au Ni Immersion layer will continue to grow, 3+ + however as the thickness of deposit Au Ni Au Ni Au Ni Au Ni Au Ni increases, the rate of deposition falls. Ni Ni Ni Ni Ni Therefore the process is self-limiting. Cu Ni Cu Ni Cu Ni Cu Ni Cu Ni TininImmersionTin Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Silver in Immersion Ag Gold in ENIG

7 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 C and 75% RH (except immersion tin). Boards should be stored in original vacuum packaging to limit air accessing the surface, and the board.

8 General recommendation Baking 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.

9 General recommendation Baking D. Baking Recommendations given by NCAB Baking can be advantageous and may be employed before any type of soldering operation. The recommended conditions are 120 o C 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.

10 HASL (Hot Air Solder Level) Lead-Free Typical Thickness: 1-40µm ADVANTAGES DISADVANTAGES + Nothing Solders Like Solder + Easily Applied + Long Industry Experience + Easily Reworked + Multiple Thermal Excursions + Good Bond Strength + Long Shelf Life + Easy Visual Inspection + Low Cost - 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 Assemblies - Inconsistent Coating Thicknesses - High Process Temperature deg C - Not Suitable For HDI Products

11 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 strong air blowing hot air drying

12 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+)

13 Baking g/ Temporary masking HASL (Leaded and Lead-Free) 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.

14 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 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 - SAC ±5 C

15 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. 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 h 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.

16 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.

17 Quality aspects HASL (Leaded and Lead-Free) With lead-free ee solder,quality control o is more critical ca that with traditional a 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

18 OSP (Organic Solderability Preservative) Typical Thickness: µm ADVANTAGES DISADVANTAGES + Flat, Coplanar pads + Reworkable (at PCB Fabricator) +D Doesn t taffect tfinished i dhole Size + Short, Easy Process + Low Cost + Good Soldermask Integrity + Environmental friendly + Clean process - 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 Cleaning - Limited Shelf life - Easy to scratch

19 Short process description OSP (Organic Solderability Preservative) OSP is thin layer of either Benzimidazole or imidazole lacquer 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

20 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 i and time General recommendation needs to be followed. If nothing else is specified, IPC6012 sets the storage time according to JST-STD-003 STD 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+)

21 Baking / Temporary masking / Others OSP (Organic Solderability Preservative) 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 i 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.

22 Available standards and test methods OSP (Organic Solderability Preservative) We have same lack of a available standards as for HASL 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 to 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 - SAC ±5 C

23 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.

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

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

26 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 t that t 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.

27 Baking g/ Temporary masking Immersion Tin Immersion Tin is excellent surface treatment, but also rather sensible so extra concern need to be addressed at the handling 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.

28 Available standards and test methods Immersion Tin There are available standards d that t define both the thickness, appearance and performance of immersion tin. A. IPC-6012 Solderable. IPC-6012 has not been updated yet with the reference to IPC B. IPC-4554 A complete spec for the finishes with clear demand on the thickness, performance and appearance. For solderability test it refers to J-STD-003 C. J-STD-003 Defines how the solderability test should be performed.

29 Available standards and test methods Immersion Tin IPC-4554 Key point from the standard. A. Thickness Minimum 1µm measured with XRF Min 0.4µm of usable tin for Category 1 boards Min 0.5µm of usable tin for Category 2 boards Min 0.6µm of usable tin for Category 3 boards B. Visual Uniform plating and complete coverage of surface to be plated.

30 Key ypoints 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. Topography of IMC after aging

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

32 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 µm as standard because some soldermask can not withstand any thicker deposit, due to undercut problem (see right hand image)

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

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

35 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.

36 Immersion Ag (Immersion Silver) Typical Thickness: μm 05-0 ADVANTAGES DISADVANTAGES + Good for Fine Pitch Product + Planar Surface + Inexpensive + Short, Easy Process Cycle + Eliminates Nickel + Doesn t taffect Hole Size + Medium Shelf-Life + Can be reworked/re-applied by the PCB Fabricator - Some systems cannot throw Into µvias with aspect ratios > 1:1 - Tarnishing must be controlled

37 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

38 Storage condition / time / handling Immersion Ag (Immersion Silver) Immersion silver, is an immersion finish and also rather 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

39 Storage condition / time / handling Immersion Ag (Immersion Silver) 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 i 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.

40 Storage condition / time / handling Immersion Ag (Immersion Silver) C. 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+)

41 Baking / Temporary masking Immersion Ag (Immersion Silver) Immersion Silver is sensitive treatment and it recommended to be careful when it comes to baking. 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. ti

42 Available standards and test methods Immersion Silver There are available standard that define both the thickness, appearance and performance of immersion silver A. IPC-6012 Solderable. The IPC-6012 amendment 1 is referring to spec IPC-4553 for the performance of immersion i silver B. IPC-4553 A complete specification with clear demands on thickness, performance and appearance. For solderability test it refers to J-STD-003 IMMERSION SILVER DEPOSIT The minimum thickness shall be012μm 0.12 with a maximum of 0.4um C. J-STD-003 Defines how the solderability test should be performed.

43 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.

44 ENIG (Electroless Nickel/Immersion Gold) Typical Thickness: 3-6µm Ni / μm Au ADVANTAGES DISADVANTAGES + Planar Surface + Consistent Thicknesses + Multiple Thermal Cycles + Long Shelf Life + Solders Easily + Good for Fine Pitch Product + Al Wire-Bondable - Expensive - Black Pad Issues on BGA - Waste Treatment of Nickel - Cannot be Reworked at PCB Fabricator - Not Optimal for Higher Speed Signals - Complex process requires good control - Not soldering direct onto surface copper

45 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

46 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+)

47 Baking g/ Temporary masking Electroless Nickel Immersion Gold 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 tibl to contamination, ti nearly all types of maskings are acceptable - such as Peelable soldermask, Kapton tape, etc.

48 Available standards and test methods Electroless Nickel Immersion Gold There are available standard that define both the thickness, appearance and performance of Electroless Nickel Immersion Gold A. IPC-6012 Refer to IPC-4552 for the demands. d 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. B. IPC 4552 A complete spec for the finishes with clear demand on the thickness, performance and appearance. For solderability test it refers to J-STD- 003 C. J-STD-003 Defines how the solderability test should be performed and judged.

49 Available standards and test methods Electroless Nickel Immersion Gold IPC-4552 Key point from the standard: d A. Thickness Minimum 2,54µm Nickel (100µ ) measured with XRF Minimum 0,05µm Gold (2µ ) measured with XRF B. Visual Uniform plating and complete coverage of surface to be plated.

50 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. PCB 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.

51 Quality aspects Electroless l 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.