Reflow oven - nitrogen hack

[IanJ]

Hi all,

Reflow ovens.

As I understand, to get nice shiny soldered joints reliably one must remove the oxygen from the air......and usually nitrogen gas is used for this.

Is it possible to hack I.e.  T962A and pump N2 through it when reflowing, possibly via a small manifold.

Just an idea!

Ian.

[Kjelt]

Just thinking out loud:
I would use vacuum or use Argon.
Vacuum is the best since it also eliminates bubbles forming in the paste.
Argon costs more than Nitrogen but is heavier than air so you only have to use a tiny bit if the pcb lays in a closed saucer or tray. However a bottom and sided closed tray might influence the reflow proces.
Flushing Nitrogen is going to cost you, I think the pro installations use some kind of chambers to preserve the Nitrogen or because they use huge quantities it just is less expensive.

[tautech]

Wouldn't a move to vapour phase be the best long term solution ?
https://www.eevblog.com/forum/manufacture/practical-diy-or-modest-cost-vapor-phase-does-it-exist-yet/
Links to other threads embedded^^^

[Kjelt]

Probably.
For costs, the Nitrogen costs €34 for 11 litres at 300 bar and the Argon €42.- the cylinder costs €180 and has a limited lifetime of ten years.

[mrpackethead]

Yes, we get awesomely nice shiney PB Free, with Vapour Phase.   

Not far from being ready to publish my project either!

[SWR]

Not far from being ready to publish my project either!

Looking forward to seeing that.

[SMTech]

Big commerical reflow ovens essentially come in two flavours, Air & Nitrogen, nitrogen can help with joint quality as at those higher temps there's then less O2 to to go around and react with things, however it is typically not a cheap option and a bright shiny joint isn't necessarily a sign of a good one. Nitrogen ovens oven have additional watercooling requirements and on the larger EMS front they probably have inhouse Nitrogen generators to feed them. However unless the industry they work in has some very specific requirements, most small to medium sized manufacturers will run Air based convection reflow and not see any issues. It would seem unlikely you would see a measurable improvement in small batch work.

If you are seeing poor wetting on your soldering its more likely to be the paste, your application technique or reflow profile. If its a bit yellow/brown, that's your paste and its flux composition it may also be the fact your little oven just isn't pulling away the flux resides and vapours and they are getting baked on. The brightest shiniest solder I have used is from Warton (and presumably this now also extends to SolderKing) but they don't have a paste that matches the flexibility of GC10 or BLT LFS-UFP. That said I wouldn't call the BLT joint dull and the residues are clear.

Vacuum is also used in reflow systems, however that is a process used to reduce voiding under devices with a large contact area, even with the best possible flux and profile, you will often find some of the flux can't escape from under such devices and so the inter metallic contacts between PCB & device is not what your sums might be expecting. Some industries are starting to specify very tight tolerances for how much voiding they consider acceptable.

It would seem fairly intuitive that by its very nature VP reflow would share the advantages of nitrogen reflow, and as discussed in numerous other threads if you can DIY it, should way outperform a S***y batch IR oven.

[Smallsmt]

Hi all,

Reflow ovens.

As I understand, to get nice shiny soldered joints reliably one must remove the oxygen from the air......and usually nitrogen gas is used for this.

Is it possible to hack I.e.  T962A and pump N2 through it when reflowing, possibly via a small manifold.

Just an idea!

Ian.

I used GC10 for many years and received always shiny nice solder joints in standard reflow oven processing.
In your oven, the heaters are too close to the circuit board.
This does not promote even heat distribution.
Circulating air would be a great improvement and more accurate process control.
I don't think N2 is the solution.

A disadvantage of vapor phase soldering is that the temperature cannot be easily adjusted and the layout must be optimized to avoid excessive gravestone effects.

[capt bullshot]

At a typical EMS site, you'd see one of these

[pisoiu]

This particular installation comes with a lot of logistical headaches and costs.
I would strongly advice against playing in diy environments with asphyxiating gases unless one knows very well what to do AND has the means to measure O2 partial pressure (scuba divers uses oxygen analyzers). In a poor ventilated area it can be very easy to spill nitrogen or argon outside an installation and to replace the breathable air inside, especially if it is available from a compressed gas source such as 200 bar tank.

[IanJ]

Hi all,

Ok....in the interests of safety, and with the tech info received here and on the web I think I'll look at my paste first so I'll try the GC10........and also the reflow profile, primarily the cooling phase which I read has to be right.

Thanks all.

Ian.

[mrpackethead]

A disadvantage of vapor phase soldering is that the temperature cannot be easily adjusted and the layout must be optimized to avoid excessive gravestone effects.

We make 100's of panels a week, using VP and our tombstone rate is very very low.   I  am able to get very good control of my temp profile, and i never have to worry about getting to low.   
Yes, VP is more suspectable to tombstones, because the Oxygen free environment increases the wetting of the solder paste.      This is overcome mostly by ensuring that your pasting process is good.

- Framed or pensioned stencils
- Electroplated.
- Considered and properly designed apertures.
- Ideally having vertical lift off..
- use a proper hard sharp edge stainless squeegee.
- Good paste ( GC10 really helps )
- Proper temp control of reflow.

All of those things however are things that you'd be doing for any 'reliable' process, vp or other.

[HHaase]

Yes, nitrogen can be a beast to integrate into a process.   I think we're up to 3 generators now at work.  Two for the select solder systems, and one for pressurizing the conformal coating tanks.   None of it is used in the reflow ovens, all 4 lines are air convection ovens.  We run thousands of boards a day and don't really see anything driving us to use nitrogen reflow right now.  Not running any 01005's yet, customers aren't asking for it, but we run 0201's and micro-BGA's all day long. 

If you're still at the level of using T962A ovens though, there's a lot of other paths to go down first which will pay you bigger dividends than nitrogen.  You'd be better served by looking into an oven that has hot-air convection,  plus the ability to monitor and control actual air temperature.   IR stuff just doesn't have the consistency you're going to need in the long term.   That and your stencil printing usually has the biggest effect on your final product of any part of an SMT process.  A good stencil is worth the extra cost, particularly from a stencil shop that will evaluate your gerbers and can make engineering suggestions.

Don't forget your solder-mask and pad design in your board layout too,  that causes us more grief than anything when customers don't have well defined pads or mismatches with the component size.

To be honest though, I wouldn't worry so much about chasing 'shiny' solder joints with lead-free. They're completely normal and acceptable as per IPC-A-610 section 5.1.  Your pad/component wetting, fillets, and complete reflow are what you should be focusing on.  GC10 tends to work well,  but you do need to make sure to mix it before using, otherwise the first few passes don't print very well. That's why they came out with GC-18 a couple years ago, to improve the initial print quality at the cost of 'only' a 6 month shelf life.    I find Kester NP545 to be a better performer, doesn't stink anywhere near as much as GC-10, but still gives a fairly long non-refrigerated shelf life. 

[Reckless]

I recently started using GC-10 and see it needs to be mixed well for initial prints.  Going to check NP545.  I'm thinking of upgrading to high reliability solder paste.   They are running $75 per 500 gram jar in qty.   

[mrpackethead]

I recently started using GC-10 and see it needs to be mixed well for initial prints.  Going to check NP545.  I'm thinking of upgrading to high reliability solder paste.   They are running $75 per 500 gram jar in qty.

I'm yet to find any solder paste that does not need to be well mixed before use. I find that GC10 needs to be 'kneeded a bit' before use..  I've tryed to figure out what physically happens when you 'work' it, but i dont' have any ideas.

[mrpackethead]

That and your stencil printing usually has the biggest effect on your final product of any part of an SMT process.  A good stencil is worth the extra cost, particularly from a stencil shop that will evaluate your gerbers and can make engineering suggestions.

This is very sage advice.  Its taken me a long time and lots of mistakes, but now I have got apature design to a point where i specify that my stencil should be made 100% and no changes.  Thats the ideal place to be, but it is like RF, and black magic.

To be honest though, I wouldn't worry so much about chasing 'shiny' solder joints with lead-free. They're completely normal and acceptable as per IPC-A-610 section 5.1.  Your pad/component wetting, fillets, and complete reflow are what you should be focusing on.  GC10 tends to work well,

Interesing observation, when i use GC10 with vapour phase i get veyr shiney joints.  Use it with hot air i don't...


Is the 'greyness' oxidation on the joint? and will the oxidation only occur at temp?  The joints remain shiny and dont' go dull over time.

[HHaase]

With the GC10,  the rep I spoke with from Henkel said it had to do with the higher solids content in the flux.   With GC-10 they pushed for the widest process windows, and most stable storage as possible.  But part of the compromise was the settling that causes the need to mix it.  Supposedly their GC-18 improves on the printing process but at the expense of a shorter storage time.

With 'High Reliability' fluxes there is also a compromise.  They do great with SIR testing and inerting of the flux residues,  but according to the sales reps I've spoken to there is increased voiding.  There's always compromises with any solder paste.

The non-shiny joints actually aren't oxidation, it's part of the shrinkage of the alloy during the cooling phase.  Vapor phase cools differently than either hot air or nitrogen.

[IconicPCB]

Some alloys have a touch of bismuth in order to achieve a grainy finish . A dull joint is easier on the inspectors eyes.

[HHaase]

Some alloys have a touch of bismuth in order to achieve a grainy finish . A dull joint is easier on the inspectors eyes.

Never thought of that aspect of a slightly dull solder joint, but it does make sense.
I'd wonder though if it's enough bismuth to create brittle joints,  or is it a very low bismuth content compared to a low-temp solder.

[pk7639]

I've recently come to own a few molecular sieve N2 generators. We're going to try one on the batch ovens. If anyone is interested....
https://photos.app.goo.gl/uFYY5Gg41YSA1pcY7

Looking at the specs, I think we may end up using half of a single tube......

[IconicPCB]

You should look to achieve 50ppm O2 and then bring the temperature up. Make sure You seal all the leaks and leave an escape path out of the oven through a one way valve.

Pack the oven with low thermal mass block of material to enhance purging of atmosphere in the oven.