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BGA not level when placed

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BGA not level when placed | 25 February, 2008

Hi ...

We use DRS24C Air-Vac machines for placing BGA devices, and have been having problems with shorts on corners of one device in particular, its a PBGA and its size is 4.5cm square and 15grams weight.

When the component is picked up and the component is not sitting perfectly flat in the nozzle, it is placed at an angle, so during reflow, instead of the device leveling it dives into the board, at the lowest corner, and creates shorts, the opposite end lifts causing opens.

I have cured this by adding 4, removable, BGA supports at the corners made from cut offs from squeege blades and insulated with high temp tape.

I was wondering what the industry has to say about the use of supports?

Is there any where I can find out more information about supports or the above issue?

And what are your thoughts and experiences on this matter?

Is there a technical term for this, its not coplanarity?

Many thanks for your time and trouble.


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BGA not level when placed | 25 February, 2008

It sounds more as though you have a profiling problem primarily, however, if your placement is not square to the board, then you should get your machine setup correct first. After that you can get to the profiling.

If the BGA is brought up to temperature in a more correct fashion the collapse will tend to be more even. This is also directly related to the amount of bottomside heating you can provide for the PCB.

You mentioned a DRS24. Single or double diffuser? Do you have the correct nozzle size and type for the BGA? Is this a lead free or tin/lead Process? Size of the PCB? Layer count?

On the short side, I have seen this method of using cornmer standoffs used successfully, but in my experience it is not a substitute for correct profiling, even more so when using a rework machine to mount a large BGA. If you have enough horsepower you can certainly get it done, but the DRS 24 is a little more limited in lead free processes, simply due to the bottom side heating design. This is promarily why the new gen DRS25 uses a far more efficient bottom heating system.

The same mantra for productiion as well as for rework. Repeat after me... "profile...profile...profile"


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BGA not level when placed | 26 February, 2008

Thanks for the reply Hege,

I calibrate the machines every 3 months and have checked that the board carrier to nozzle parralelism is ok.

Have tried many different profile settings using a MOLE for readings but still collapsing on one corner.

I did a study with a thermal camera of the DRS24 diffuser plates and they have very patchy heating characteristics, so a tip is to always place your boards in the same location relative to the diffuser plate.

I am using double diffuser machines, the process is tin/lead. The size of the PCB is 42cm x 50cm and the number of layers is 18

We will be upgrading the 24C to 25 soon, the heating on the diffuser plates for the 25 is perfectly uniform.

So would you think that if the component was placed very slightly at an angle it would level out during reflow, my experiance is that it does not.



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BGA not level when placed | 26 February, 2008

We have both DRS24 and DRS25 here. I think you will be much happier with the DRS25 as the underboard heating is superior, of course after a year of using the 25, it has its caveats as well, though minimal. I was a little surprised that you are having difficulty, but you are asking a lot of that old style diffuser for an 18 layer pcb of that size. Didn't realize your PCB was that large. DRS25 will solve the problem regarding the underboard heating. If you have calibrated the parallelism of the PCB to the nozzle, and you are still having planarity problems during the placement, it only seems to leave the nozzle tip (where the O ring mounts) as the only variable. I would be curious to the key once you figure it out.

Good luck. I am told they can retro fit the new design panel heater to the DRS24, if true we are likely to do that to the old machine here.


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