I can't answer your questions regarding the OA flux and BGA's, but I can help with your other questions:
"Is there a rule of thumb for the pad surface area / part weight ratio to
maintain enough surface tension to correctly hold the part?" Yes, as stated in previous TechNet threads on the subject, the "rule of thumb" formula is the weight of the component in grams / Total pad mating area in square inches. The resulting value in grams per sq. in. must be less than 30.
"Does anybody use adhesives to stake heavy parts on the first pass?" No I don't, but it shouldn't cause you any worries. There are issues with thermal balance and back to back components that you will need to investigate to obtain the proper reflow.
"Does anybody have a solid rule of thumb for pin-in-paste apertures? On our
first try we used an 8 mil step up and had the aperture the same size as
outer edge of the PTH. Would an oversize print work better?" The key is to get a sufficient amount of solder fill in the hole and to provide for good fillets. A simple formula to use for paste required is: Vp = [Vh - Vl]x Fh x Sp ,where:
Vp = The "target" volume of paste
Vh = Volume of the hole (self explanatory)
Vl = Volume of the lead (again, self explanatory)
Fh = Through hole fill percentage (This depends on your own needs, ie:IPC class I,II,II)
Sp = Paste shrink factor (depends on your paste, I use 51%)
I have used this formula and have achieved great success reflowing through hole parts. I have overprinted the pads out of necessity to get the desired volume of paste, up to a .22" aperture, and the paste reflowed into the barrel, creating good top and bottom side fillets. The amount of overprint is dictated by the paste volume formula, and also by the PCB layout of nearby SMT components, the pitch of the device,vias, or other keep out areas. I created a spreadsheet that helps me design the apertures based on the above formula, and I can email this to you if you are interested. IPC-7525 contains some very useful guidelines for "Intrusive Reflow". Their formula is more detailed, but may be overkill. I hope this is of some help. Good luck!
Howard Watson
Manufacturing Engineer
AMETEK/Dixson
03/26/02 01:14 PM
Please respond to "TechNet E-Mail Forum."; Please respond to "Edward S. Wheeler"
To: [log in to unmask] cc:
Subject: [TN] Double Sided Reflow Flux and surface tension
Hello Tech-Net users! I am new to the group here, and would like your
assistance on a couple of matters concerning a double sided reflow process
we are undertaking. We are new to this process, so we would appreciate any
expertise on the matter.
We are using an OA type solder paste, and like to wash our boards within 2
hours, 4 being the maximum in house limit. To perform our double sided
reflow process in mass production, we may want to run all day on one side
and do the other side the next. My question is... does any of the residual
flux material aid the second reflow process, or can we wash it off before
the second reflow and get as good of results? I am concerned that the
solder joint may have an outer layer of oxidation or graininess due to the
lack of flux once it reflows again. Is there any flux left over that is
active, or is it simply the remainder of the carrier?
What luck are people having with BGAs populated on the bottom side? Is
there more of a concern with warp on the second pass?
Is there a rule of thumb for the pad surface area / part weight ratio to
maintain enough surface tension to correctly hold the part?
Does anybody use adhesives to stake heavy parts on the first pass?
Does anybody have a solid rule of thumb for pin-in-paste apertures? On our
first try we used an 8 mil step up and had the aperture the same size as
outer edge of the PTH. Would an oversize print work better?
Thanks in advance for any help.
Ed
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