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Solder balls

yukim

#29226

Solder balls | 24 June, 2004

Currently we are having a lot of solder balls, after a few hours of continous production.

This is how we work: load new solder paste at the beginning of the shift, less than 500grms. Then, the operator loads new solder paste periodically, every 3 to 4 hours usually, or more frequently when needed. We've beem doing so for years without any problem. We changed the solder paste in December last year and the problem started about 3 weeks ago. The solder paste's properties are: metal content 90.2~90.3% (spec 89.4~90.3%) Viscosity (Brookfield, KcPs) 760~780 (spec 580~780) Viscosity (Malcom, P) 1580~1750 (spec 1050~1750) We keep the solder paste at 5~10 Celcius, then leave at room temperature over 8 hours before using it. The solder paste expires in August or September this year (6 month shelf time).

A picture is posted in the following: http://nbbs.naver.com/nmulti/h_read.php?board_id=novayu_0&nid=341&page=1

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#29228

Solder balls | 24 June, 2004

What manufacturer of solder paste are you using?

Just curious.

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#29238

Solder balls | 24 June, 2004

There are many factors that can lead to solder balls. Since you have just recently seen this start to occur, it is obvious something has changed in the product or your process. Since we don't have a lot of details about your process, I would suggest looking at the following:

1. Monitor the temperature and humidity inside your facility. As summer hits us, generally the factor temperature and humidity increase. Both of these factors can contribute to solder balls. Higher temperatures will decrease the viscosity of the solder paste. A lower viscosity results in more slumping. If the solder paste slumps onto the solder mask, it is very easy for solder balls to be formed. Higher humidity can really affect water wash solder pastes. These pastes have the tendency to absorb moisture which will decrease the viscosity (and cause solder balls in the same way temperature does). Also, absorbed moisture can spatter during reflow and cause solder balls. 2. Optimize the reflow profile. A very slow ramp up to about 150 C will minimize solder paste slump during the reflow process and will reduce solder balling. Targeting a ramp rate of 0.5 - 1 C/sec would be ideal. 3. Look for other changes that occurred at the same time that the solder balling did. Did you begin using a new mfg. of boards or components? Have you tried changing lots of boards, components, or paste to see if there is a change in the solder balling? New stencils? New operators? New equipment?

-Tim

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yukim

#29246

Solder balls | 24 June, 2004

Tried with several lots of solder paste, manufactured in Feb and March this year. Work ambient: 25C, 60~70%. Is this humidity too much? I'll reduce the ramp in our profile.

One change: We changed the conveyor motor in our reflow oven just before the problem started to occur, but we did perform the speed calibration and verified the profile, and it was ok. Could this affect?

Another question: solder paste has its static viscosity very close to its upper limit (770KcP, being the limit 780). Could this affect?

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#29251

Solder balls | 25 June, 2004

60-70% RH will be a challenge if you are running a water wash solder paste (and could be the reason you are seeing the problems). If you are running a no-clean, humidity should not be an issue.

As long as the profile looks OK, I don't think the conveyor would be an issue.

High solder paste viscosity is actually better for reducing solder balls. However, usually solder paste manufacturers target around 900-1000 kcps for stencil printing. Although you should verify with your current manufacturer, it seems that the paste you are using has an awfully low viscosity spec.

Tim

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K

#29321

Solder balls | 30 June, 2004

We find keeping the work environment at about 22degC and humidity to about 40-50% is the best way to control the solder balls. Before we took measures we were out of control with solder balls especially in the summer months. We run aqueous solder thru a standard printer, but controlling the storage temp of the paste and warm up times is a sign youre on the right track. Turn up yur air con in the room (If you have it) and you will see an improvement... That is to say if the issue is not as a result of board design, inefficient masking or the oven ramp temps etc....

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blnorman

#29355

Solder balls | 2 July, 2004

We used to use Alpha paste. Every summer we had solder ball problems when the humidity rose. Time frame sounds like this is something to check in to.

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yukim

#29361

Solder balls | 3 July, 2004

The manufacturer says that since this solder paste has synthetic flux (not natural rosin) it is sensible to humidity. Worked with reflow profiles, but no remarkable improvements. We are changing solder paste every 2 hours.

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yukim

#29362

Solder balls | 3 July, 2004

So did you change the solder paste? And did it work? What solder paste is it?

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yukim

#29363

Solder balls | 3 July, 2004

What's the difference between static and dynamic viscosities? Is there any relationship between Brookfield and Malcom Viscosity Values?

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#29364

Solder balls | 3 July, 2004

Q1: What's the difference between static and dynamic viscosities? A1: Static viscosity is the viscosity of the solder paste as it is dispensed off of a tube or syringe onto a screen printer. Dynamic viscosity gives the variation in the thixotropic behavior of the solder paste during the course of the screen printing process. Dynamic viscosity is gained by adding shear to solder paste. Shear is what the solder paste encounters on a screen printer. [Malcom]

Q2: Is there any relationship between Brookfield and Malcom Viscosity Values? A2: None

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yukim

#29372

Solder balls | 4 July, 2004

Thanks.

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yukim

#29390

Solder balls | 6 July, 2004

The manufacturer confirmed that it is humidity - the solder paste is too sensible to humidity. Now I tried Koki solder paste with good result. But, it does leave flux residue in good quantity. Does anyone have experience with Koki solder paste?

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blnorman

#29393

Solder balls | 6 July, 2004

We have now switched to Kester EP256. During our solder paste evaluation, resistance to humidity was one of the selection criteria. We printed multiple boards, then exposed them to 85�F/65% RH. At 30 minutes, 1 hour, 2 hours, and 4 hours exposure we reflowed the boards. 256 showed no solder balls after 4 hours exposure. The rest showed solder balls at different times.

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Koki

#29405

Solder balls | 7 July, 2004

Please let me know exactly the Koki specification that has managed to solve the balling issue and we can look at the level of residue or the cosmetic may be what you wish to improve.

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yukim

#29570

Solder balls | 20 July, 2004

Hi, The solder paste is SE5-M951X-9.

Comparing with the previous one we used, we are having much more tombstoning problems. The reflow profile is such as: room temp to 140C: 85~95 sec 140 to 170 C: 80 sec 170 to 200 C: 20 ~ 23 sec Above 200 C: 44 ~ 48 sec

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RDR

#29607

Solder balls | 22 July, 2004

What is your pad dimensions/spacing at components you are having trouble with

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