SMART Group Lead-Free Wave & Selective Soldering Workshop Dec 5th 2007

Lead-Free Machine Soldering:PCB Hole Outgassing

Nigel BurttProduction Engineering ManagerDolby 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 to direction 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)

Tear-out

Pin-holes in hole wall at

rear

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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

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