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SPC on SMT

Fredrik Hallen

#18209

SPC on SMT | 20 November, 2001

How to implement an easy SPC in the production line?

Im looking for relevant characteristics to perform SPC on electronics (e.g. PC-module with motherboard and UPS, power supply's, field bus cards, servo drive systems etc).

I understand that component-batches is one characteristic that can vary from unit to unit, as well as the solder quality.

But I have not started yet and appreciate if you have any know-how to give us.

Regards Fredrik H�ll�n

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Michael Parker

#18212

SPC on SMT | 20 November, 2001

First, determine a standard to follow. IPC/EIA J-STD-001C "Requirements for Soldered Electrical and Electronic Assemblies" is most current for electronic assemblies. Second, by your customers spec. determine which class of the standard to apply. Use IPC-7912 "Calculation of DPMO and Manufacturing Indices for Printed Board Assemblies" as a guideline to create representative defect type list. Collect defect data per assembly, daily. Create charts, reports that shares that data with the rest of the workforce, quantify defect types per reference location on the PCA. Be sure to get Manufacturing and Process Engineers involved to correct problems. Measure progress of Corrective Actions by seeing the defect rates go down. Congratulate everyone for their hard efforts, don't let up and keep pushing those that don't apply hard effort.

It's a dangerous job but somebody has got to do it.

Go to the IPC website for a wealth of information. http://www.ipc.org.

Stay connected to SMTnet. There are some really good people that can offer free advice that is most helpful.

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JoAnn

#18214

SPC on SMT | 20 November, 2001

The SMTA offers a book on this very topic that has been selling very well. It is titled SPC for SMT. You can find info on this at the Bookstore at http://www.smta.org. Bil Messina, the author, also teaches courses for us on this subject.

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#18218

SPC on SMT | 20 November, 2001

You're correct.

There�s a review of this book in the SMTnet Newsletter. Maybe last February???

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#18219

SPC on SMT | 20 November, 2001

There are lots of angles on this topic. Search the fine SMTnet Archives to review past discussions.

Don�t deceive yourself. Doing a good job with SPC requires work. �Easy SPC� ends-up being something that people do, get bored with, see limited value in, and stop doing <= another management fad.

Consider: * Doing SPC on a single item. This way you can focus in the issues on doing SPC correctly, train your troops, measure the trade-offs, and learn to implement SPC. * Selecting something that�s a problem, not the BIGGEST problem, but a problem where you can succeed and show benefits. Oftimes the BIG problems are such a mess that you do not have the luxury of fixing it correctly. [Er, maybe those very bright people that bull-in with their preconceived solution are just too smart.] Weird, eh? * Determining what you are measuring. You mention process characteristics. Measuring process characteristics is good, but product characteristics may be more meaningful. In the dead of the night that�s the goal in performing SPC. Who cares if you have good process, if you�re not shipping good product? Although, measuring process characteristics allow you to determine if the process is in control and calculate capability. No, No. Do product characteristics!!! No, No. Do �

From a process standpoint, people say that paste printing cases 70% of SMT defects.

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Fredrik Hallen

#18221

SPC on SMT | 21 November, 2001

Thanks Mr Parker! I understand that the basic "demming-wheel" needs to be on place first. My contract manufacturers list ALL adjustments, failures etc during the production, and we also discuss the statistics (pareto) and corrective actions for all defects. They have daily follow-up.

I agree 100% with your recommendation related to this!

Next step is now to sit down with my(our) contract mfg's to look into the FlowPlan for processes & tests. Then find out the characteristics that may vary from batch to batch and from unit to unit. Then see that these characteristics is measurable and finally see if it is possible to implement SPC on those.

My clue are to understand which characteristics that may vary during an production run: A. Different component batches may vary and can change the tolerances of Vref, Timing, Power-consuption, Start-up time. B. Solder paste thikness. Can be measured via e.g. laser. SPC is possible during a batch run. C. Solder quality.

But, we have not started this yet! So I'm out and seeking for persons that have don it and can stear me into the right direction.

We do also have many casting and mechanical partners to our company and thay are very good in SPC, but it is so easy to measure mechanical dimensions on the listed characteristics that may vary from unit to unit. Those suppliers have also saved a lot on their quality costs.

As I see, electronics and SMT mfg's is a long way after the mechanical mfg's. Some of our partners ony have "GO - NO GO" tests and do not collect any data or statistics of how "GO" the products are............

Thaks for your answer, I appreciate if you och any else have any reflections about this.

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Fredrik Hallen

#18222

SPC on SMT | 21 November, 2001

Thanks!!

I have bought this book. SPC for SMT by William S. Messina. It is an book with very deep calculations, I need to have the basics on place first - which characteristics may vary from unit to unit and batch to batch.

I think that we have a VERY GOOD potential of improvment, and quality cost savings.

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Fredrik Hallen

#18223

SPC on SMT | 21 November, 2001

Thanks Dave!

I agree 100% with you and try not to deceive myself! It is very dangerus to let it be a dam paperwork of must. Good id� to change focus frpm process to product-characteristics!!

I will see the benefits and cost savings as well as the time savings.

On our casting and mechanical paretners we have seen that (we detected problems during Full Run Test with SPC before releace of Full Production run).

But I'm seeking for good examples and know-how.

Thanks!

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Michael Parker

#18225

SPC on SMT | 21 November, 2001

Now that we know where you've been and what you wish to accomplish next, I can still give you some suggestions.

1. Get IPC-7912. Use it to evaluate your CM's. Especially if you have multiple contractors supplying like products. You will quickly see who has the lower PPM's. Find out why. Use the "why" as a benchmark to get the others up to quality spec.

2. You are asking for attributes of why. Good question. Once the attributes are identified, then you have to establish what variations are possible.

Your example listed an attribute of: A. Different component batches may vary and can change the tolerances of Vref, Timing, Power-consuption, Start-up time.

This is your job to determine and should not be part of the CM's responsibility to decide. If they have specific experiences and can offer suggestions, listen and take the advice back to your engineers. The point is, a CM must follow your BOM, either consignment provided or turn key purchased for you. Your engineers should have considered lot to lot variations during Design Review. Those component types deemed acceptable should be on the AVL of the BOM.

B. Solder paste thickness. Can be measured via e.g. laser. SPC is possible during a batch run.

This is an attribute that you could be investigating. Better yet, consider volume. Use IPC 7525 "Stencil Design Guidelines" to determine if your CM has the right apertures in the stencil. The apertures are dependent on land pattern design. Land patterns are a variable that can affect paste volume. A good CM will alter the apertures to accomodate lands that are not sized correctly. Only so much can be done, you may have to change the fab to match the parts and paste volumes needed to get a good solder joint. Other variables for printing include make and model of printer, squeegee type and pressure, sweep speed, angle, seperation rate. These all can vary from CM to CM, depending on how they set up their process.

C. Solder quality.

Another attribute. Variables include: shelf life, temperature, viscosity, sphere size (screen mesh), flux chemistry (water soluble, no clean, aggressive, long tacky life, etc.)

Next is placement. More attributes such as placement accuracy (dependent on make and model of machine), speed, etc. Variables include maintenance (clean nozzles, feeder calibrations, x-y offset (differs from each individual machine), etc.)

Reflow. Attributes: make and model, technology (IR, vapor phase, convection) Variables - reflow profile, maintenance, calibration, nitrogen.

This is a basic outline, many more details exist. You need to go to each process step, find the attributes, determine the variables, decide what may possibly promote defects you are experiencing, SPC (Pareto of defects - concentrate on the highest rates - in my experience, if you solve for the 3 highest defect rates, usually the other random events are solved or reduced significantly).

I'm sure my collegues here at the 'net can provide more details of attributes and vaiables.

I will stop here because this is turning into a windy novella.

Good luck

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ianchan

#18239

SPC on SMT | 23 November, 2001

Hi All,

agree 100% on product characteristics SPC, and do find process indicators such as paste Volume, useful to monitor via SPC, the printing process, as printing constitutes "70% cases of SMT defects".

We juz purchased a 2nd hand CyberOptics 3D paste inspection machine (what a mouthful...) and am seeking info :

1) what formulae is applicable, to calculate the deposited solder paste volume onto the pre-defined PCB pads?

2) want to pre-calculate the required volume (cubic-cm) measurements, so will know what USL/LSL to program the CyberOptics machine, and trigger the "process-error" alarm...

3) for those guys with experience in similar 3D volume paste inspection machines, any suggestions as to whether we are in the right direction in application of this machine? as really need this machine to measure the paste volume and reflect into SPC charts, thus cutting down time wastage for my operators (who measure paste Z-height manually)

Suggestions and directions appreciated, guys, thanks!

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kgroen

#18247

SPC on SMT | 23 November, 2001

Some comments on the previous post:

Be careful when looking at the proportion of SMT defects that are solder defects - you may very well find that only a small few of them originate at the screen printing step.

As for specification limits for paste volume - that's a tricky business: The paste release characteristics of stencil apertures are related to the aperture area ratio, the manufacturing process for creating the stencil and other parameters. When you design your stencil, do not expect 100% paste release. Without getting into a deeper understanding of your stencil design or processing, let's assume that an 85% volume release is typical. Consider this your target volume. Determining spec limits for paste volumes is difficult because you will find for many paste volumes that sometimes they result in a post-reflow defect and sometimes they do not. Instead of battling with the notion of spec limits, why not use SPC in full and design your process to be ON TARGET with MINIMUM VARIATION.

We've been working with inline 3D paste inspection for years and this has been our best approach:

1) Wide volumetric spec limits on individual paste features in order to detect gross defects (which an operator can visually verify) 2) SPC on key reference designators(volume measurements, again) 3) Data analysis for stencil qualifications, designed experiments, process development.

BTW we have also used offline paste height audits and the information is not useful at all. 1) It is difficult to find a tool that is gauge repeatable 2) Paste height variation is a smaller proportion of volume variation than area variation. 2D inspection would be better than height measurement. 3) Paste heights usually vary more on the same card than they do on the same location from card to card.

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