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January 2008

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 23 Jan 2008 12:50:03 -0600
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Why, thank you, Inge!
From your humble student.

That obstinate question does always pop up, how many is too many?

Again, it depends (yes, Doug, I know). 

The answer is simply if there is one, that is too many. I believe they are treated philosophically as a process indicator, and it is realized that if collectively they are not judged to be a reliability issue, no rework is required (not feasible) but you better not see them again. I have usually seen them documented on an MRB, whereupon quality engineering, design engineering, and manufacturing (and sometimes the customer) all review each case and disposition the hardware. This is especially important in the case of of high impedance/high frequency circuits, as these solder fines and balls can hinder the performance of the circuit by providing alternate paths for signal crosstalk. The cognizant design engineer must therefore be given a chance to review them as well.

Realize that if the above are not issues, and the board is conformal coated, then the chances of any of them becoming an issue is greatly reduced.

As to your next two questions, the answer is to use a good X-ray with at least a 5 micron feature detection capability,  acknowledging that you will not find them all due to shadowing issues. And no, they are not usually detectable in CSAM, never mind what the salesperson says.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: Wednesday, January 23, 2008 12:22 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] Solder fines

So spoke Master Stadem
And we all add:
Yeaaah, so said the Master.

Hmmmm...here is an obstinate one:
Isn't the AMOUNT of fines hard to decide?

The crowd said to the obstinate: shut up, The Master has spoken.

/Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Stadem, Richard D.
Skickat: den 23 januari 2008 18:29
Till: [log in to unmask]
Ämne: Re: [TN] Solder fines

Hi, Sue.
It depends (add one more Mountain Dew to the IOU list). Per J-STD-001 and IPC-610, greatly dispersed solder fines (reflowed and non-agglomerated paste particles) are acceptable if attached to the sides of pads, wedged in between the solder mask and the edge of the pads, trapped underneath of a component, are not easily dislodged and do not cause a violation of the minimum electrical clearance of the device (usually defined on the PWB fabrication drawing and typically .005").
Grossly smeared solder paste is shown as an unacceptable example.

If you are getting small entrapped fines, it is usually because the solder paste is a Type 4 or smaller particle size, rather than the more commonly used Type 3. If you are using Type 4, special consideration needs to be taken during design layout with the spacing between the mask and the edges of the pads. The printing process must be at or very near a 6 sigma level with appropriate aperture reductions to keep all of the paste on top of the pads, even after placement.
 
Solder balls are small groupings of agglomerated fines or else they are spattered from the larger agglomeration. They are usually seen in situations where "belly pad" components are used, and these types of components should have at least a 50% reduction in the stencil aperture as a rule of thumb. Other components that have a large flat surface soldered to larger flat areas on the board pads, such as larger chip capacitors, power transistors, etc, are prone to producing solder balls.
This is because unlike solder paste deposits on other leaded components, the solder has no place to flow to when it goes liquidus, and thus some spattering takes place. So again, a good stencil aperture reduction scheme must be used.

If the solder balls are present and are entrapped in no-clean flux, that may be acceptable for class 2 product. But do not go purely by the specifications, as some devices such as those intended for space, medical, traffic control, etc. may require much more stringent control......

Of course there are also other causes, one common one is printing solder paste immediately after the bare PWB was washed but may still have entrapped water within the vias. Another is contamination on the pads.
Another is microvias that literally outgas and explode through the solder during reflow due to the entrapped contamination because the board was not cleaned properly after ablation and prior to plating, or after plating. Another is poor solder paste handling (moisture absorption from being left out in fairly high humidity, or excessive shear on the printer, or because it was continually returned to the container and re-refrigerated (condensation). There can be lots of causes.

So have a good time, I hope this helps.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Sue Powers-hartman
Sent: Wednesday, January 23, 2008 9:58 AM
To: [log in to unmask]
Subject: [TN] Solder fines

We are having a problem with tiny solder fines that are clinging to fine pitch SOIC leads, not on the solder mask, that are not being washed off in the cleaning process. Are solder fines considered the same as a solder ball, which means they all have to be removed, or is it acceptable to leave them alone.

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