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EPC Fets and Bondit

The tiny EPC GaN fets, especially the smallest 4-ball ones, are very fragile. If you nudge one in handling a board, the part can fracture, or the balls can break away from the board or from the fet itself.

Someone here recommended Bondit, so I got the kit. It works great. Just put a small drop over the fets and hit it for a few seconds with the blue LED. It hardens nicely before it has time to spread out. The viscoscity is just right.

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There are probably more commercial UV epoxies and illuminators (we have a serious UV gun here, which I can try) but the standard kit works great and will protect hundreds of little fets.

It filled the vias but not enough to make a convex bump on the bottom, which matters to us since we will use a pogo fixture for testing.

I wonder: what happens to places, like under the fets or inside the vias, that don't get illuminated much? Does the UV cure effect spread out into the dark regions?

I've also wondered the same sort of thing about 2-part epoxies. Do they have to be mixed down to molecular level?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin
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Sorry, Bondic not Bondit.

The stuff at the bottom of the vias is hard.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Does it release easily with alcohol like hot glue does?

Reply to
John S

...

Dentists use a lot of UV-cure materials. For cosmetic work, they get applied in multiple thin layers, cured one at a time. I believe this is so folk don't end up with uncured resins in their mouth - so it's is quite likely a problem.

On the other hand, anywhere you can't get UV is somewhere you can't see, so if there's uncured resin there, you'd only know by cutting into it.

Improperly mixed (or wrong ratio!) epoxies form a microscopic mesh containing uncured resin components. If the ratios are close, the result is almost as strong as properly cured epoxy, but professionals still take extreme care with proper weighing and mixing of parts.

I learned this from a neighbour who spent his whole career in this industry. His techniques are being used in Melbourne to make airfoils for Boeing jets, and his technique&design for making refrigerated shipping containers is the industry standard. Vale Bill Maloney. He was a lovely chap.

CH

Reply to
Clifford Heath

Not very fast, if ever. In optical assembly it's common to tack parts together by partly curing a few areas, so that if there's some screwup you can get them apart again.

IIRC it depends on the catalysis mechanism. If the catalyst is a small molecule, it eventually diffuses around pretty well. OTOH it isn't at all difficult to get mixing on molecular scales. The exponential effect of folding does a very good job even for epoxy putty.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

While that may be true of UV cure acrylic adhesives, there are UV cure adhesives designed to cure in unexposed areas such as under components, for example:

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Unlike the Bondic site

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they provide specs on uncured viscosity and cured modulus, and have a wide range of properties available. Bondic won't even tell you that ~365nm is optimal for curing

- they claim it is proprietary!

There are other mfgrs of UV cure adhesives designed for electronic component encapsulation which provide good specs too, some of which seem to be readily available in small quantities from the usual suspects

BTW I have found that half a bottle of UV cure optical adhesive will cure in the bottle if left in a dark refrigerator for a decade :-).

Most Epoxy manufactures claim that under mixing is the most common cause of failure to achieve rated properties, and many recommend mixing until the mix visually appears to be complete, then continue mixing in the same manner for the same time again, mixing twice as long as appears necessary. The catalyst is rarely a small molecule in epoxy, it won't diffuse far and is not purely a catalyst responsible for initiating polymerization but also an essential part of the cured co-polymer resin.

Interestingly many of the UV cure adhesives tailored specifically to glob- top encapsulation and BGA underfill (UV + anreobic cure) are one part epoxies, not acrylics.

Reply to
glen walpert

Yeah, I'm mostly familiar with Norland 65 and 85, which I standardized on long ago.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

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