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SMT process Blowhole/ Pinhole

iman

#23951

SMT process Blowhole/ Pinhole | 28 March, 2003

we have a issue of post-reflow oven (SMT) soldering defects of blowholes and pinholes. Any suggestions or knowledge on root cause is appreciated to be share?

BACKGROUND :

we DI water wash and oven bake the PCB bareboards at 90deg-C for 60mins, to remove possible dust, volatiles and also thermal condition the PCB (polymide).

The reflow oven profile plotting shows the profile to be within specs for the "supplier recommended specs".

Preheat between 150-180degC is average 60-75sec.

Reflow above 183degC is average 45sec.

Ramp is 1.7degC/sec.

Peak reflow temp averages 208degC max.

The component solder fillet affected is a MLF.

we control the environment to be 22-26degC at 40-60RH%.

post-reflow inspection shows blowholes at solder areas near to the base of the PCB pad. Although it also manifests as a violent explosion appearance the the top of the solder fillet region.

post-reflow inspection also shows pinhole in solder areas random in the solder fillets.

we tried to increase reflow time, it appears to reduce the occurence of blowhole, and marginal helps in the pinhole reduction. Any ideas how to further eliminate the pinholes?

we did try increasing the reflow peak temperature to average 215-218degC, but that just increased the rate of blowholes.

Appreciate any ideas or suggestions, on how to eliminiate this blowhole and pinhole issue. Thanks in advance!

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#23953

SMT process Blowhole/ Pinhole | 28 March, 2003

Your preheat temp seems excessive, what kind of solder paste do you use. can you try a max preheat temp of 140 degC? The reason for this is to let the solvents in the paste evaporate while keeping the activation of the flux for the very last moment before reflow takes place. With your preheat temp, your flux gets activated very early and is probably all spent by the time reflow is achieved.

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Kevin Facinelli

#23956

SMT process Blowhole/ Pinhole | 28 March, 2003

Could you be having outgasing from the melf?

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MA/NY DDave

#23959

SMT process Blowhole/ Pinhole | 28 March, 2003

Hi

Are all the other solder joints for the other components other than the MELF not having a problem. That is what you said, yet I wanted to check??

To say a less twisted way is this only with the MELFs and is it with all the MELFs??

If you have other Polimide Boards with the same laminate structure are any of them having problems??

YiEng, MA/NY DDave

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iman

#23963

SMT process Blowhole/ Pinhole | 29 March, 2003

Yep, only the MLF have this blowhole/pinhole issue. Other components like SOT(transistor), CAP, RES are hey-o-kie.

Btw, MLF = micro lead frame (IC/LGA) package. looks like a BGA without the solder balls. has direct attachment to the PCB ENIG pads with paste deposit in between the PCB and MLF solder pads. The solder paste volume was calculated and deposit in amounts to get a solder fillet fusion to the perimater exterior "walls" of the MLF package. Its in these "wall" solder fillets that we get the damm buggy.

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mk

#23964

SMT process Blowhole/ Pinhole | 29 March, 2003

http://www.sipad.com/Motorola/MotorolaFlipChip.htm

Check out this link. Sounds similar.

mk

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#23966

SMT process Blowhole/ Pinhole | 29 March, 2003

Iman

When you talk about your profile, can we assume you're measuring it on the pads of LGA? Profile the solder paste reflow by placing a thermal couple under the component pad, similar to a BGA.

On a slight tangent, as with soldering BGA, paste selection is an important driver of voiding in LGA.

On washing polyimide boards, quit doing this. If your fabricator is supplying dirty boards, you need a new supplier for all the money you're paying.

Property||Polyester||Polyimide||Aramid||Glass Epoxy Moisture Absorption||0.3%||2.9%||8-9%||0.05 - 3%

If you insist on baking the boards, baking at 90*C does very, very little to remove moisture. [Think about heating a pot of water.] You need to be above the boiling point of water [100*C] to even start outgassing moisture.

How does baking at 90*C "precondition" your boards? What is this "preconditioning"? If your intent is preheating, doesn't the much lower temperture of your plant get the old entropy thing going and cool the boards down to room temperature pretty fast?

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#23967

SMT process Blowhole/ Pinhole | 29 March, 2003

YiEng, MA/NY DDave

There are numerous package types that now fall under the rubric of land grid. Land grid devices [ie, BCC�, LGA, QFN, MicroLeadFrame�, etc] are essentially BGA devices with the balls removed and with no solder on the pads. Generally, any device that has bare pads under them that attach to corresponding pads on the board qualifies as a land grid.

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MA/NY DDave

#23969

SMT process Blowhole/ Pinhole | 29 March, 2003

Hi

Back later on this tread, yet I did want to slightly correct one thing DaveyF wrote. Again only slightly.

Even at 90C, moisture does start to leave the board. Now it doesn't do it so well as above 100C at sea level atmospheric pressure, yet it still percolates the moisture out of the substrate. I guess we would call this a gentle process.

A small disagreement.

YiEng, MA/NY DDave

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iman

#23970

SMT process Blowhole/ Pinhole | 30 March, 2003

MK,

this is a bad link, can help re-verify this link pls?

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iman

#23971

SMT process Blowhole/ Pinhole | 30 March, 2003

we are using a "recommend supplier profile", and we have gotten back to the talking table with them over this profile specs. Thanks for the pointer, have ask them on this issue. Have also asked for their phase-transistion diagram to understand their R&D determined liquidus and solidus temperature and timing characteristics.

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Bryan Sheh

#23975

SMT process Blowhole/ Pinhole | 31 March, 2003

I agree that we should bake the pcb at higher temp and for longer time.for example...95--115c for at least 4 hours. also,keep the temp from 150--170c ,60-90 seconds while preheating. completely keep the moisture and volitales out of pcb and solder paste.

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mk

#23976

SMT process Blowhole/ Pinhole | 31 March, 2003

http://www.sipad.com/Motorola/MotorolaFlipChip.htm Should work, I just tested it.

mk

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Neil

#24031

SMT process Blowhole/ Pinhole | 7 April, 2003

Iman,

How long after washing the boards do you process them? Are there any vias or Micro vias on any of the pads you see the blow holes on? A possible cause of a blow hole in a solder fillet or PCB is the escaping of Moisture under extreme temps. There is a possibility that your washing process is leaving moisture, which may be trapped in the PCB or finds its way in to the solder paste.

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iman

#24065

SMT process Blowhole/ Pinhole | 9 April, 2003

yes, we r profiling direct thermocouple attachment to the LGA pads.

based on direct experience of yourself or others, what paste (brand/type3?/type4?/alloy?/flux%?) gives the optimal repeatable characteristic-patterns to minimize this defect?

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iman

#24066

SMT process Blowhole/ Pinhole | 9 April, 2003

after oven bake dry the PCB, we process within 4-hrs, else need to rebake again under same baking parameter.

taking a look, the exact pads with the blowholes are full ENIG pads with no via holes under the solder-intend land areas. In event there is via hole in the pad, its far away to the edge of the land (pad) area, BUT this via hole is plug with solder mask and from the reverse side.

Interesting point, will look further into it. Thanks!

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