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BGA underfill necessary with conformal coat?


K

#19297

BGA underfill necessary with conformal coat? | 27 March, 2002

Does anyone have experience doing conformal coating over Plastic BGA in High-Rel PCB assembly?

Is underfill necessary to avoid problems with entrapped air, etc?

Our application is a 17x17mm BGA, 256 I/O. We are assembling with a water soluable paste & washing. The assembly will be in an aircraft.

Thanks

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#19307

BGA underfill necessary with conformal coat? | 27 March, 2002

There is two major BGA coating camps. * Camp #1: "We require the coating to be under low stand-off devices but with no filleting. So we apply a thinned dip coat first, and then top-off with a proper spray coat" * Camp #2: "We want no coating under the components so we only dip with high viscosity material"

We don't do this, but regardless of you camp, you should be OK with acrylic conformal coating over plastic BGA components without using under-fill for avionics use environments.

Rockwell Collins did a fair amount of testing looking both at paraylene and acrylic prior implementation. This work was published (paper was "An Investigation of the Effects of Printed Wiring Board Surface Finish and Conformal Coating for Ball Grid Array Assembly") at the IPC APEX 2000 Conference in Long Beach.

They saw no "entrapped air" issues after 2 years of production experience.

Regardless of your camp, if you plan use silicone coatings, you should be concerned. * Silicone TCE: jygundous 300-350 ppm/�C * Solder TCE: ~16 ppm/�C

Bartholomew in "An Engineer's Handbook Of Encapsulation And Underfill Technology" suggests that the TCE problem with silicone may be reduced by silica filling, but that high filling ratios may result in micro-cracking during high temperature cycling.

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BTaylor

#19348

BGA underfill necessary with conformal coat? | 1 April, 2002

We had a problem with conformal coating getting under an IC and with Temp. and vibration would cause cracking of the solder bump, underfill solved the problem.

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#19363

BGA underfill necessary with conformal coat? | 1 April, 2002

Regarding your underfil ... * What material did you use as an underfil? * What machine and technique did you use to apply the underfil? * What type of component were you underfilling? * What was the end-use environment?

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BTaylor

#19367

BGA underfill necessary with conformal coat? | 2 April, 2002

davef I assume this question is for my reply and me. We used a dexter\hysol underfill and a Camalot 5700 to dispense.Single line technique next to a small IC with 14 bumps around the perimeter.The conformal coating was applied with a small brush around the IC to protect it from moisture intrusion, a silicone type. It is a automotive application.

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#19372

BGA underfill necessary with conformal coat? | 2 April, 2002

Tks BT

Can't be many places tougher than automotive, 'cept maybe oil field, down pipe.

We use Dexter Hysol flow underfills for both 1 thou and 3 thou gaps, but they wouldn't come close to filling under a BGA. Most BGA are sitting about 16+ thou from the board. [Not that those are the only DH ufills out there.]

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BTaylor

#19378

BGA underfill necessary with conformal coat? | 3 April, 2002

Any time Dave I can tell from your responses you have been around this block as many times as I have.Your right automotive is tough nowadays not only from the quality standpoint, now they want everything for free.

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