Phil,
You probably know 99% of this but here is my input.
With not having the luxury of populate boards you have only experience and
bare board to work with. Though I would think you could get one of the
large ferrite transformers to work with. I do a lot of proto types builds
with EFD20 sized parts and getting a complete good reflow around these has
been a problem. I suggest try running a with any similar component. The
issue will be getting a sufficient high temp for that component. Run the
oven hotter and longer, but you MUST reflow that large part.
If you cannot use the real board use something similar.
Then you need to backup and watch for overheating of small light parts. My
experience running on the hot side is OK except for some parts like LEDs.
I have a board with an aluminum backing and it spreads heat well, it helps
even out the thermal difference over the board. As an idea try placing the
board on a board sized piece of Al, say 1 or 2 mm thick. Kind of a false
thermal spreader. Turn up the bottom side oven temp to deliver faster
heating to the Al.
In my other life (aapcb.com) doing protos is all we do, we never have a
spare board and we often run using our "Standard" profiles. We do 10 to 20
different jobs (completely different boards) a day. There is one oven setup
for leaded, the other oven setup for lead free. A vapor phase oven is
available, I am a strong believer there, but in reality the standard
profiles work well. But that large transformer will cause problems and you
need to heat up and stretch out the run through the oven.
If this is lead free you have another issue and that is flux burn off. Watch
out for that.
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nutting, Phil
Sent: Friday, February 05, 2016 9:42 AM
To: [log in to unmask]
Subject: Re: [TN] developing thermal profile without a populated board
This thread leads me to a slight variation. We have a 4 layer board, copper
planes on both sides with a bunch of 0603 and SOIC components in two small
areas and some large ferrite transformers that are about 1 inch square and
1/2 inch high. I can't believe this would use the same profile. Do you run
this to mount the big parts and follow with a different profile that will
reflow just the small parts leaving the big parts untouched?
I'm in disagreement with other engineers on how to handle this. And no I
have not yet looked at the manufacturer's recommended profiles of the
important and different parts. Other fires to put out first.
Also, how do you handle a profile for a batch of ten boards where you don't
have the luxury of having a populated test board? The ten boards could
potentially be the run for the entire year.
Phil Nutting
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yuan-chia Joyce Koo
Sent: Friday, February 05, 2016 9:01 AM
To: [log in to unmask]
Subject: Re: [TN] developing thermal profile without a populated board
all well said. one addition, check the recommended thermal profile of key
components from supplier. you will have list of different
requirements: process, amplifier, RF etc. you end up with narrow range of
ramp, TAL, plus the board Cu weight, more less you only have to try few
conditions within the window (don't forget different surface finishing, that
need more or less considered as well).
as for wetting of the components, if you do have small glass top viewable
reflow oven with 5-7 zone (R&D table top), you can see the paste/components
reaction under either air or nitrogen condition (provide you have
correlation between your production reflow oven and your R&d oven). if you
done homework, it might be able to cut down your DOE. my 2 cents. a Lot of
work, but pay off in long run (if your product is follow the platform design
- with few generation use similar stuff).
jk
On Feb 5, 2016, at 8:41 AM, David Hillman wrote:
> Hi Tom - well, nothing beats the real board but we often have the same
> issue. You have a couple of options that have been shown to get you in
> the right ball park: (1) there is profile software that does a pretty
> reasonable job of getting a valid thermal profile which you then tweak
> as you run your first couple of boards; (2) If you add these factors
> together:
> board thickness, total amount of copper weight, component technology
> type and component density - you should be able to create an initial
> working thermal profile. I put together a set of "golden" boards
> reflecting these variables many moons ago, thermal coupled them and
> recorded their profiles.
> That action provided me a thermal profile comparison window I could
> then use to estimate what a new design might demand for thermal
> inputs.
> Over
> time, we have gained enough comparison experience that we can created
> a thermal profile for a new design that is fairly accurate and only
> needs small tweaks.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
> On Thu, Feb 4, 2016 at 4:49 PM, Tom Gervascio
> <[log in to unmask]>
> wrote:
>
>> Many times we have to create an oven profile but only have a bare
>> board.
>> Many times the boards are complex and have many bottom terminated
>> components on them and wanted to avoid having to guestimate how a
>> profile for a bare board would actually perform on a populated board.
>> Can't imagine
>> that we are the only persons to face this problem. Wondered how other
>> users have worked around this problem?
>>
>
>
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