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Lead-Free Machine Soldering: PCB Hole Outgassing

Lead-Free Machine Soldering: PCB Hole Outgassing. Nigel Burtt Production Engineering Manager Dolby Laboratories, Inc. – European HQ. Lead-Free Wave and Selective Soldering Workshop. PCB laminate selection for Lead-Free. Laminate choice 1

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Lead-Free Machine Soldering: PCB Hole Outgassing

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  1. Lead-Free Machine Soldering:PCB Hole Outgassing Nigel Burtt Production Engineering Manager Dolby Laboratories, Inc. – European HQ Lead-Free Wave and Selective Soldering Workshop SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  2. PCB laminate selection for Lead-Free • Laminate choice1 • The common “standard FR4 material” (used pre-RoHS) uses “dicy” cured resins (epoxy cured with dicyandiamide) • Most studies (e.g. NPL project) have found that extra performance needed for lead-free soldering is best met by going to higher Tg laminates using “phenolic” cured resin systems, also often with additional filler material • Typical examples: Isola IS410, PCL-FR-370HR, FR408, Nelco N4000-11 • The improved thermal robustness has its drawbacks in terms of associated mechanical properties • Stiffer – multi-layer bonding force reduced which may lead to increased incidence of local delamination after soldering, also copper traces adhere less well which may give problems for rework • More brittle – greater risk of delamination at breakout edges • Harder to drill – can lead to outgassing problems as we shall see 1: see http://www.smartgroup.org/pcb2006/Amorgan.pdf SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  3. PCB finish selection for Lead-Free • Main choices2 • HASL • usually SnCu based alloy replacing SnPb – but flat enough for fine pitch SMT? • OSP • inexpensive, but short shelf-life • ENIG • Actually soldering to nickel – thin layer of gold just protects the nickel beneath it • High visual contrast between gold pad and solder joint aids inspection • Many possible hidden solderability problems (eg “black pad”) • Immersion Tin • problem with multiple hand solder rework? • Immersion Silver • Compatible metallurgy with SAC solder alloy family • Some storage care needed – avoid sulphides but silver oxides still conductive • Skin effect benefits for HF/RF designs? 2: see http://www.smartgroup.org/pcb2006/dprice.pdf SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  4. ENIG vs. Immersion Silver for Lead-Free at Dolby • ENIG • SMT and mixed technology • Pressfit OK • PCB keypad contacts • Visual contrast aids inspection of SMT • Problems with incomplete solder hole-fill with no-clean flux • Occasional solderability problems (e.g. “black pad”) - always found too late! • Immersion Ag • SMT, mixed and THT only (use in place of HASL) • Pressfit OK • No hole-fill problems • Careful storage needed • Reduced solder bridging defects at wave solder? • Hole outgassing!! SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  5. Drilling “tear-out” • Section usually needs to be taken at 450 todirection of glass fibres in the laminate in order to reveal this. • It seems that when the glass bundle is not aligned with the drill, it can be gouged and pulled out in chunks. The copper plating then has to try to fill the gaps left in the hole walls. • All laminates gouge this way but the gouging appears to be deeper and more severe with the phenolic-cured laminates used for RoHS assembly. • Lead-free is much less tolerant of copper wall defects due to higher temperatures and copper dissolution. SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  6. Wave and selective soldering “blow-holes” Outgassing from trapped moisture in the laminate or flux activity as the solder joint forms – large delta T as PCB contacts the wave, especially with lead-free alloys and even more so with selective soldering (higher processing temperatures mean increased vapour pressure.) Gases expand rapidly to produce “blow-hole” before the molten solder completely solidifies or remain trapped as a “void” after solidification. SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  7. Simple Check for Outgassing PCBs from Bob Willis http://www.bobwillis.co.uk/videoclips/outgassing02.wmv SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  8. Solder joint micro-section example (1) Drilling tear out leaving pinhole in copper resulting in outgassing and solder-joint void SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  9. Solder joint micro-section example (2) Shattered glass bundles causing copper wicking and leaving areas behind the copper wall that can trap moisture. SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  10. Solder joint micro-section example (3) Tear-out SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  11. Solder joint micro-section example (4) Pin-holes in hole wall at rear Tear-out SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  12. PCB ENIG vs. Immersion Ag Finish and Outgassing • ENIG • Nickel layer under the gold appears to “seal” any pores/pin-holes in the copper wall better than any immersion finish only, so blow-holes less common. • Preliminary findings from NPL study into laminate reliability suggests the nickel may add mechanical strength too. IST testing of PCB hole plating barrels shows that cracks develop around the glass fibre bundles but the nickel appears to help hold the underlying fractured copper together. • Immersion Ag • Thin silver layer does not seal the pin-holes (true also for Immersion Tin and OSP?) • Possible exacerbating effect of copper dissolution by lead-free alloys on copper hole plating after immersion finish dissolves? (Especially for selective soldering?) • Blowholes reduced but found even after pre-bake of PCBs - need to be careful not to cause tarnish (wrap in aluminium foil beforehand) • Study with one of our PCB suppliers showed that the outgassing problem was virtually eradicated if the PCB reverted to certain “lead-free capable” dicy-cured laminate instead of preferred phenolic-cured type. • Careful drill setups and even new drill bit designs may help cure “tear-out” SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  13. Weigh up the Pros and Cons • ENIG • Strong visual contrast • Nickel reliability benefits • Poor hole-fill • Invisible solderability problems ( “Black Pad” etc.) • ImmAg • Reduced bridging? • Better hole-fill • Compatible metallurgy • Store with care • Outgassing SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

  14. Any Questions? SMART Group Technical Committee info@smartgroup.org +44(0) 1494 465217 Nigel Burtt - Production Engineering Manager Dolby Laboratories, Inc. - European HQ njb@dolby.co.uk +44(0) 1793 842132 (c) SMART Group 2007 We accept no liability for the consequence of any action taken on the basis of information provided from us.   The information is provided in good faith. Any views or opinions presented to you are solely those of the author and reflect their understanding of current good practice and their interpretation of guidance, regulation and standards. SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

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