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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


X7R woes

Views: 4087

We are experiencing fracture problems with 0805 X7R capacito... - Dec 25, 2005 by Mariss  

#38684

X7R woes | 25 December, 2005

We are experiencing fracture problems with 0805 X7R capacitors. The capacitors seem to fail in tension; during hot-air rework they come apart in two pieces with the fracture near the metallization end.

Only X7R SMT components are affected, C0G and Y5V caps are fine as well as all other SMT parts. The X7R parts are 1nF and 10nF.

The SMT parts are placed by Zevatech FS-730 machines, the conveyor oven is an Essemtec. We use AIM WS-483 solder paste.

The cap mfg is AVX; we have also used Panasonic, Murata, Kemet, etc. with the same results.

The boards are a mix of SMT and thru-hole. After SMT assembly the panelized boards (12 / panel) are electrically tested via a pogo-pin test fixture here. The X7R caps are always good at this point.

We farm-out the thru-hole assembly to two seperate contract assembly shops and both return finished units with fractured X7R caps. In some cases every (8 1nF, 1 10nF) cap is fractured. Yeild is one bad board out of 3 (one or more fractured parts).

What the heck is going on here?

Mariss Freimanis Geckodrive Inc.

reply »

#38685

X7R woes | 26 December, 2005

First, when you see the temination on ceramic capacitors cracked or broken away, it is screaming 'mechanical stress' at you.

It's possible that your contractors' wave soldering process is destroying your X7R dielectric capacitors. X7R can be damaged by temperature >+85*C. * Wave solder preheats are >100*C just prior to entering the wave * Temperature changes between exiting the chip wave and the laminar wave can swing through >100*C * Do not exceed 3*C per second maximum rate of change in temperature.

Generally, we expect that X7R and BX dielectrics to be more sensitive to thermal shock than C0G dielectric ceramics. So, the money your purchasing saves by buying X7R dielectric capacitors is being compensated by your rework in the shop. [Read: They look good, you look bad.]

reply »

#38686

X7R woes | 26 December, 2005

That seems to be the case. We are replacing the X7Rs with C0Gs for future production as a work-around.

X7Rs are sold so I assume they can be used succesfully; is it just they shouldn't be used in mixed SMT / thru-hole designs?

Mariss

reply »

#38687

X7R woes | 26 December, 2005

Sure, you can use X7R dielectric ceramic capacitors on mixed technology boards. You need to be aware of the limitations and design and control your process appropriately.

Continuing: * BIG cracks show as hard failures * Small cracks show as a leakage problem

reply »

#38688

X7R woes | 26 December, 2005

As an alternative theory to the boards being damaged in the wave soldering process, it's possible that the panels are being handled in a way that flexes the panel and breaks the capacitors. In this possibilty, we'd expect capacitors in specific locations to be broken, regardless of dielectric type. So, this again points us toward the thermal limitations of the X7R dielectric.

reply »

#38690

X7R woes | 26 December, 2005

I'm assuming the fracture in tension; the fracture doesn't show even at X50. The broken edges are that clean. I would expect spalling at the fracture edges if it were a compression or torsional fracture. This makes me belive the board expands and pulls the caps apart during wave-soldering.

The broken caps reveal themselves during electrical test as an abrupt drop in capacitance when the board is gently flexed. Un-flexed and the capacitance is within tolerance (+/-10%).

Mariss

reply »

#38691

X7R woes | 26 December, 2005

Addendum:

The boards are 2" by 2.4" (50mm by 60mm) and have 158 SMT components and 20 thru-hole parts.

The nine X7R caps are distributed over the board; some are in the middle, others are near the V-grooved board edges. There is no discernable relationship between failed parts and their location making de-panelization an unlikely cause of failure.

Mariss

reply »

#38694

X7R woes | 26 December, 2005

Putting aside that you want to: * Get this mess cleaned-up as soon as possible * Stop spending money on this issue * Get your volumes back to plan ... and all that other production meeting stuff

We'd be very nervous about shipping the X7R dielectric caps that passed test, because they could be have partial cracks that could become failures during customer use. So, we suggest that you section some acceptable X7R dielectic caps to assure your boss that he / she is not real likely to have to deal with screaming customers [hate that].

reply »

Cmiller

#38702

X7R woes | 27 December, 2005

I would assume you are using a depanelizer to cut the boards apart? If not you should be wheather its causing your current problem or not.

reply »

PWH

#38708

X7R woes | 27 December, 2005

Agree with Cmiller. We had a similar prob. with some caps. on a v-scored array. We bought a pizza cutter depaneliser and the problem reduced. It's a "FKN Systek" if you want to look at models.

reply »

John

#38711

X7R woes | 28 December, 2005

We found the DIP inserter was cracking SMT decoupling caps in the past.

reply »

Chris

#38719

X7R woes | 28 December, 2005

Your not using a jaw machine? Had a problem with jaw machine once. Could hardly see the cracks but give it a little heat and the crack open up. Adjusted jaws but ultimately had to move the cap to the chip shooter upstream. Moved the cap to the jaw machine in attempts to balance the line.

reply »

Cmiller

#38726

X7R woes | 29 December, 2005

John brings up a good point. We have seen that problem with an axial inserter.

Why someone would put SMT caps on bottom and TH resistors on top is beyond me but it but we do see it.

reply »

John S.

#38749

X7R woes | 30 December, 2005

Seen cap cracking before. It can be caused by thermal stress as noted above, but don't discount mechanical stress and handling. Your depanelization method is a strong candidate. We applied strain gages to baords and benchmarked various mechanical stresses in the process. Found a bad design in our depanel tool. We use X7R's a lot, but we control our designs. Move them away from cut points and monitor the mechanical and thermal stresses in your process. John S.

reply »

#38758

X7R woes | 2 January, 2006

Hi,

Just wondering while reading this thread, would 0402 caps be more immune to stress because they are smaller?

Regards,

Grant

reply »


Rob

#38761

X7R woes | 3 January, 2006

Yes - as they flex less due to their size. I do have the calculations somewhere from my field apps days but it may take some time to find. The problems occuring with the X7R's MUST be down to mechanical damage, possibly from:

a) Insertion (check your z axis height on this package versus the actual chip height

b) foot print issues

c) Auto insertion /depanalisation forces in local areas

d) Thermal expansion

Because it's affecting X7R & not Y5V, and as you've tried all the other manufacturers then it can't be a rogue material issue, and as X7R has improved deformation characteristics over Y5V, then it points to a local damage problem.

Tensile Strength:

COG/NPO: 103 N/mm2 X7R: 62 N/mm2 Y5V: 43 N/mm2

Compressive Strength:

COG/NPO: 588-637 N/mm2 X7R: 588-637 N/mm2 Y5V: 588-637 N/mm2

Bending Strength

COG/NPO: 137 N/mm2 X7R: 88 N/mm2 Y5V: 69 N/mm2

Coefficients of thermal expansion are identical for X7R & Y5V, & Poissons ratio is 0.3 for all materials.

(all data is from Murata technical data manual)

Hope that helps find the actual problem,

Rob.

reply »

Slaine

#38762

X7R woes | 3 January, 2006

Is this a new design or have you manufacted these in the past without a problem. we manufacture ceramic arrays from X7R and COG, X7R has a differnet dialectric strength so higher capacitor values can be achived. With the introduction of RoHS the termination ink we used was banned as it had Cadmium in it, we have found that the type of ink used has a relaition ship to the cracking created during the soldering process some are a lot worse than others. this could be a factors if its just started happening. Also if its a lead free board you have the higer temps of course. I agree with teh previous replys in the thread go with a smaller component if possible and its more than likely relaited to thermal expansion of the PCB. most cracking occurs during the cool down stage so this is where your sub contracter needs to concentrait his efforts, if we solder using a reflow oven (ceramic array for a connector with pins in it, not a PCB) we tend to reflow in zone 3 if possible and use the rest of the oven to control cool down, works for us.

reply »

#38783

X7R woes | 3 January, 2006

To recap:

1) The machines use laser centering, not mech jaw centering.

2) The X7Rs are scattered on the board, center and edges. Improper de-panelization would favor damaging the parts near the edges of the board; this is not the case.

3) Both ROHS and leaded X7R parts have been used. We still use lead solder at this point. Both kinds fail.

4) The boards are functionally tested after SMT reflow before thru-hole assembly and de-panelization. Not a single instance of a failed X7R has ever been logged. The problems occur after thru-hole / depanelization.

5) The fracture is always at the metallization to cap body junction.

----------------------

A different source mentioned prompt washing of the panels may be an issue with X7R caps. As much as a week may pass between reflow and thru-hole assembly and wash. The solder paste used is a water-soluable flux variety.

Any credence for this time delay as a cause?

I have microscope jpeg photos of a typical fracture if anyone wants to see them. Thanks again to everyone that has taken the time to offer suggestions on this problem.

Mariss

reply »

Slaine

#38792

X7R woes | 4 January, 2006

Hi, Id like to see the photos if possible, you can tell a lot from cross sections and the shape of the crack if you have any.

timlivesey@netscape.net

reply »

dougs

#38794

X7R woes | 4 January, 2006

I've seen cracked caps due to de-panelling on caps 40mm in from a board edge, if the cap lies on a stress line it can be cracked quite far in, i'd go with the other replies and make sure you use a "pizza cutter" type de-panelling machine for v-scored boards.

reply »


Rob

#38795

X7R woes | 4 January, 2006

I can't see how prompt washing of the boards could cause a problem in X7R but not in Y5V or NPO. Yes, if you dropped a board into the wash straight out of the oven without letting it cool down then it's not going to like it, but surely no-one does that?

Are you using different footprints for the X7R's, or are they identical?

If you can't find the answer, or cant solve it easily:

Syfer do a range of Flexi Caps http://www.syfer.com/category_docs/flexicap.pdf that are designed to withstand bending (www.Mouser.com are local to you)but I'm guessing on the 1nf 0805 you'd be better off going for a COG at around $0.009.

reply »

#38813

X7R woes | 4 January, 2006

It was suggested that a delay between reflow and washing the boards (hours?) would allow the water soluable flux residue to somehow damage the X7R capacitors.

I'm wondering if that makes sense at all?

Mariss

reply »


RDR

#38815

X7R woes | 4 January, 2006

To answer your question it was mentioned that allowing the boards to cool slightly between reflow and wash may help this case to eliminate thermal shock from immediately going into colder water straight out of the oven with no cooling.

In looking at this thread I would be confident that it is either ICT test fixture busting these up or depaneling / handling. Are you confident that ICT will catch these cracked capacitors? If they are for filtering or decoupling they may not be individually tested and could pass test even if defective.

I read in this thread about a 2 week delay between washings with OA flux? Don't know about this being able to cause this type of damage. but I would be leary of corrosion at all of the solder joint locations. OA flux left on assemblies can be catastrophic if subjected to certain environmental conditions like high humidity, temp, etc...

reply »


Rob

#38826

X7R woes | 5 January, 2006

Hi Mariss,

"water soluable flux residue to somehow damage the X7R capacitors"

I think you are looking into the wrong area. Yes flux is aggressive and can lead to problems, however it would not single out X7R capacitors above capacitors of other dielectric material.

My money is on physical damage, either from insertion force (SMT or the later PTH insertions) or ICT/depanelisation as previously mentioned.

reply »

Lead Free Wave Solder - 1 Click SMT

PCB 3D AOI