I have a DRS22 and a DRS24 here at GD, and installed and used DRS24s at
four other sites. I also currently use a Summit 1100, as some of the
assemblies I need to rework are .250" thick, and 34" wide by 24" high.
Others I have done in the past were smaller, but were .300" thick with
30 layers of 1/2 oz. copper.
The key to successful reflow of BGAs on the large/massive pwbs is the
bottomside preheat capability of the particular machine being used.
Attempting to reflow BGAs without sufficient bottomside preheat leads to
overheating of the topside area around the component, and either
damaging the component or the pwb.
Typically, I try to get the topside pwb temperature up to 140-150 deg.
C. and then kick in the topside heater. This board topside heat is too
high of a temperature for standard .062" thick pwbs, as the glass
transition temp is about 140 C for most FR-4.
But for thicker boards such as .093", .125", and .250" you can get away
with more heat up to 150 deg. C, but you really should go no higher.
I prefer the nozzles with a straight cutoff, and I bring the nozzle down
around the component, but off of the pwb, perhaps .200". My only reason
for this is to be able to see the solder balls go into reflow. I have
modified more than one DRS24 so the microscope can swing down to about a
10 degree viewing angle from horizontal. It does not seem to make a
great deal of difference on the AirVac, but you need to have the Z-axis
height set in the software so it is in the same place everytime, as it
does have some effect.
Another option for the pre-heat on massive boards, the use of metal
shields around the assembly. These trap the heat from the bottomside
preheater and focus it toward the board. They are available from AirVac.
They fit over the board holder and leave only the assembly exposed, thus
directing all of the bottomside preheat into the assembly.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Macko, Joe @ IEC
Sent: Tuesday, July 25, 2006 11:06 AM
To: [log in to unmask]
Subject: [TN] AIR VAC DRS24 users
Fellow AIR-VAC Users,
Good morning.
Occasionally, it is a challenge to heat small high mass boards (with a
lot of copper in the inner layers) sufficiently to achieve an acceptable
solder temperature under a larger BGA and maintain component body
temperature within the manufacturer's limits using an EZ style nozzle.
Would any of you DRS24 users like to share your experiences?
What nozzle styles have you found to work best - EZ, ES, MX, or DVG? I
have heard pros and cons concerning using nozzle that basically cover
the BGA and stand off a few mm from the board (e.g., the delta T between
the BGA and surrounding area is too high because you do not have enough
radial heating).
Has anyone modified their station to include some type of post reflow
cooling like the newer Air-VAC machines offer?
Look forward to your reply. Thanks again
Joe
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