Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Poor solderability

Russ

#1983

Poor solderability | 9 February, 2000

Hello All! I have a problem that hopefully someone out there might have some ideas. I have an assembly that upon the "dental pick test" the entire solder fillet is coming up with the lead and leaving what appears to be a bare pad. I believe that these boards were reworked to a HASL finish from electroplate gold. They are processed through both SMT and wave. max temp at this location is 230 deg.C in the oven, and the preheat at wave is 200 deg. F HELP

Russ

reply »

#1984

Re: Poor solderability | 9 February, 2000

Russ: You've got stop soft-peddling this stuff. This doesn't sound like "poor solderability." It sounds like "no solderability." It also sounds like you�re dead meat. Three places to call:

1 Board supplier to get their suggestions on unsolderable boards. 2 Flux supplier and get the hottest stuff she has. 3 Priest and ask for forgiveness for whatever you did to deserve this.

So, what�s the extent of this problem? � Single, multiple boards? If multiple, how does this repeat? � Single, multiple components? If multiple, how does this repeat? � Single, multiple leads? If multiple, how does this repeat?

So, tell us the story again:

� Reworked fabs electroplated gold (over nickel?) converted to HASL, what else? Any thing about the components that you�re holding back? � Side 1: Paste, reflow profile etc � Side 2: Paste, reflow profile etc � PTH wave soldered: Flux, wave profile, etc � Problem appears where, when, to who, etc

My2�

Dave F

reply »

#1985

Re: Poor solderability | 10 February, 2000

Russ: If it�s true that they have been reworked to HASL I guess that something went awefully wrong at that stage. If you have some bareboards left have them checked thoroughly.

M2C

Wolfgang

reply »

Glenn Robertson

#1986

Re: Poor solderability | 10 February, 2000

Russ -

I agree with Dave and Wolfgang that the boards are the top suspect. The idea of "rework" from Gold to HASL sounds scary. Are you sure the Gold is electroplate and not immersion? Resolve that and then check the archives for "dark pad" or "black pad". If the boards turn out to be OK, one other possibility that was not yet mentioned is secondary reflow caused by the wave soldering. You can probably find something about that in the archives also. It has some definite characteristics that are fairly easy to identify, and it's also fairly easy to avoid. Hope this helps.

Glenn Robertson

reply »

Russ

#1987

Re: Poor solderability | 10 February, 2000

Okay, Okay, here is the rest of the story! These boards were definitely plated, reworked to HASL, The component that is the most succeptable is a 160 pin QFP .5mm pitch. This component has nickel/iron lead frame. Significantly reduced apertures (10mil wide)on a large (14 mil wide)pad with a 30% reduction in length. Reflow profile has max temp of 230 Deg. C with a TAL of 80 sec. Wave profile has preheat of 200 F. at 4 ft/min. I waved a very similar assembly (only minor cap and res. changes) and checked if reflow was occuring during wave process (when board was on wave I poked and prodded components to see if they were in reflow until assembly had completely exited). We are using WS609 OA paste with 857 OA flux, both of these from my understanding are very aggresive. That is the process story. An associate of mine mentioned excessive intermettalic formation from reflow profile. He did not know details and neither do I. Can anyone explain this? I have never seen a complete fillet come off of a pad before, only leads coming out of solder joints. I think that this is now long enough for you you Dave! As far as what I did to deserve this, it must have something to do with my last trip to Vegas.

P.S. Wolfgang did you receive that homeplate design?

Russ

reply »

#1988

Re: Poor solderability | 10 February, 2000

Hi Russ, yes, I did get the homeplate info, thanks. Thought that it looks that way, only didn�t know how to deal with that expression. Now your problem: it�s hard to tell without seeing it actually. All I wonder is this 80sec above liquidous, at what speed are you running and what does that mean for your preheat time and where on that board is that measured? Once activated the flux has its function abilities only over a fixed time ( about 2 to 21/2 minutes if my memory doesn�t fool me, better check that for your paste ). You probably regain oxides which mingle with your solder and form a somewhat weak compound. With the weakest (smallest) joints (typicall the finepitch leads) you will than notice the easiest peel off. That�s something I noticed a couple of years ago happening during rework of QFPs with hot air performed with really bad profiles and the powerful blowing of oxygen into the solderspots. For the wave: best way to judge on prehaet is to measure the temp. at the top of the PCB. It should be higher for no clean (about 120�C) than for others when the PCB enters the wave ( of course the fluids should be dried out ). For specific temb. better ask your Flux supplier.

sorry for not beeing of more help

Wolfgang

reply »

#1989

Re: Poor solderability | 10 February, 2000

Russ: Now that�s a good story. Nuff said on the Vegas trip, that explains everything. Especially when you say "it must have something to do with ..."

I love WS609. It�s been berry, berry good to me.

Intermetalic Layer (IL). A compound formed at the interface of two different metals, whose atoms have an extremely high natural attraction for each other, so high, that they do not bond to other elements by any other means. Also, intermetalic compounds have fixed stoichiometric ratios. Whereas, alloys can have varying ratios of metals.

In soldering, a little IL is good. A lot of IL is bad. Yes, you could have the latter. There are 80 some messages on SMTnet on ILs. If you want more, I got it, but first ...

See, the whole electroplated gold to HASL conversion seems fairly ill advised. It�s a little bit tough to figure-out what happened, because you have such a goofy solder connection. So, tell me: * What do you see, when you look at the pad of one of these poorly soldered connections? * What's the story on reworking the boards? How thick was the electroplated gold? How thick was the nickel under it? What did they do those coatings before they added the HASL?

And you thought that story was enough for me ...

Dave F

reply »

#1990

Re: Poor solderability | 11 February, 2000

Hi Russ, I thought about it and what approach I would do. All that talk about causes and reasons doesn�t solve the problem. To determine wether it�s the PCB, the component or the process I would take a bare board, a component ( and maybe a different one ) and do normal hand soldering with no clean solder to see how that works. With good judgement abilities how that feels (just normal soldering or hard to solder ) and looks ( good joint, enough strenght ) you should be able to say it�s more likely the PCB, component or process you will have to look for closer. It shouldn�t be to hard to tell in which direction to go.

Hope that helps

Wolfgang

reply »

Dean

#1991

Re: Poor solderability | 11 February, 2000

Sounds like black pad to me. I just suffered this last week. Track your fab lot codes. Just in case. I processed 1000 fabs of 2 lot codes through 1 line (also WS609). 1 lot code failed with black pad the other did not. With one common mfg. process in place the failure mechanism pointed to the fabs.

Dean

reply »

Glenn Robertson

#1992

Re: Poor solderability | 11 February, 2000

Russ - I agree with the other guys that the problem could be black pad or some intermetallic problem, possibly related to the "conversion" to HASL finish. But my first guess is still partial reflow at wave soldering. You won't catch it by pushing on parts during soldering, because the problem involves only a partial melting of topside solder joints. Also, it's so transient that it's hard to pick up by standard thermal profiling. It's most common on fine pitch leads (and BGAs), and usually only on pads that have a short trace going straight to a via. The via provides a path for excessive heat to be transferred to the topside lead. The classic symptom is when the affected leads break loose easily, and most or all the solder stays with the lead. You can probably find more information on this (and black pad)in the archives. A quick test is to use Kapton tape to cover the vias on the back side. This almost always provides a "quick fix" unless your solder dwell time is really excessive. Good luck. Glenn Robertson

reply »

#1993

Re: Poor solderability | 14 February, 2000

Dean: I associate black plague as an immersion gold process problem (although I could easily be wrong): Everybody looks at the nickel, but the cause turns out to be the immersion gold is too aggressive. The immersion gold works by corroding the nickel. If it is too aggressive, it takes away the nickel and leave phosphorous behind, which makes it look like the phosphorous level is too high in the nickel bath.

My2�

Dave F

reply »

Russ

#1994

Re: Poor solderability | 14 February, 2000

It was the wave process, Just thought I'd let everyone know for future reference. Partial reflow at selected leads I am now aware that 63/37 does indeed have "a slushy zone" that I thought was not possible.

Russ

reply »

Reflow Ovens thermal process improvement

HeatShield Gel- thermal PCB shield during reflow