Reflow parts launching.

[itsmoked]

After my hideous oven runaway I've got things back to where they were with a new ramp and soak controller that plots on the screen and all that stuff. Fine.

What isn't fine is that the same old problem is back. A lot of boards I do are about 4 x 2 or 4 x 3". I use 0603 parts wherever I can but naturally I have to use 0805 and 1206 etc for either power dissipation needs or cost (big C values in small packages are expensive).

These bigger than 0603 parts have a penchant for bailing out. It's been so consistent that the policy (grating as it is) is to not even bother placing 1206 or larger parts because almost all of them fly. Now I'm seeing a lot of 0805 parts fly too, probably about %50. Hunting down the flyers and relocating them is painfully time consuming.

Looked at a board today thru a microscope and had the feeling I was looking at reconnaissance photos after an extensive bombing run thru a small town.

First crack at trying to logic this, it would seem like the paste has absorbed water that is flashing to steam and launching victims. So I changed the profile to soak at 50C for a while before proceeding. No real improvement was seen.

I've heard to fix this you need to "sprint from the start really fast to the peak". I haven't tried this because it seems like it would increase a rapid vaporization launching problem. On the last 10 boards I ran 3 boards, 3 boards, 2 boards, and 2 boards, and there was a marked reduction in flyers, perhaps by half on the 2 board runs. Not an insubstantial improvement! The difference would be that the temperature can probably move a bit faster on 2 board runs so maybe faster temp ramping has something to it.

Of course when you get down to tiny things that are overwhelmed by forces like capillary action it can all get 'unexpected'.

I use [URL unfurl="true"]http://www.chipquik.com/datasheets/SMD4300SNL10.pdf[/url]
paste.

We often take three days to complete placement so the paste is out for a while, however we don't really see a launched part difference between 5 hours and 72 hours.

Does anyone have suggestions.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

I remember baking desiccant blue and loading it into compressed air dryers for our laboratory air supply. ... which had to be done a couple times a day, because our compressors were outside, in Florida's excessive humidity. It took maybe a week for me to give up on the whole idea, and just instruct my guys to blast the accumulated water out of the air guns before pointing them at anything important.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

No No Mike you needed a refrigerated air dryer. Desiccant for compressed air is a horrible idea, for the reasons you just stated.
B.E.


You are judged not by what you know, but by what you can do.

 

[itsmoked]

Actually the very best compressed air dryers ARE desiccant but they're regenerative types. Essentially two in parallel, one in-circuit until the media approaches saturation then switched to the other. The saturated one is then heated(if a heated regen) or dry air is used to purge and drive out the wet (non-heated regen) and has air bled thru it to carry away the vapor. Tiz all automatic. I spec'd one out for a huge controller for Texaco's San Ardo field. It worked like a champ.

Typical regen dryer: [URL unfurl="true"]http://www.airdryers.com/air-dryers/vsa.html[/url]

But if you just need dry enough air for a shop and the air needs to be just dryer than the local immediate dew point the refrigerated ones work really reliably and don't have a lot of problems with contaminants that a regenerative desiccant unit would hate.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

If you are in a pinch and can't get it to work Advanced Assembly in CO can help. They specialize in prototype quantities for a reasonable price, so our company sends a lot of work through them. I especially like that they can work with cut tape instead of a full reel. I no longer shy away from BGA parts because I know they can place them.

Z

 

[itsmoked]

Nice tip zapped! I usually cringe at 10 piece builds because assemblers hate them.

Keith Cress
kcress -

http://www.flaminsystems.com