Ed
With respect to your comment about vias in pads - you overlook your own
point about discussion with the designers about special requirements.
Vias in pads would be acceptable on a part that is second side mounted
_provided_ there is no wavesolder exposure to those vias (either no wave
process, or a partial wave fixture that blocks those sites). The dialog
with the designer is essential if any design out of the ordinary is to
be attempted.
regards,
Graham Collins
Process Engineer,
Litton Systems Limited, Atlantic Division
(902) 873-2000 ext 215
> -----Original Message-----
> From: Ed Holton [SMTP:[log in to unmask]]
> Sent: Wednesday, November 26, 1997 2:10 PM
> To: [log in to unmask]
> Subject: Re: [TN] Double Sided Assembly
>
> Excellent discussion so far, this one is going in my history file for
> future programs.
>
> Two general comments:
> As the manufacturing engineer, make sure that you are telling the
> designers
> about the special requirements for the process. Since you have to
> build
> it, make sure you tell them what you need, for example, distribution
> and
> types of parts between top and bottom. You do not mention if this is
> dual
> reflow or reflow/wave. If the latter, and you are wave soldering
> Quads or
> SOIC, verify pad design etc.
>
> Second, there was a comment about vias in pads. This is an example of
> what
> not to have in your design for double sided assembly. It will lead to
> problems in the second solder process, be it reflow or wave, with
> reflowing
> of solder joints. (it is my understanding that vias should never be
> in
> pads of double sided boards because of the possibility of reflowing
> the
> first solder joint in the second solder process, or starving the
> solder
> joint because the printed paste was sucked down the via!)
>
>
>
>
>
> [log in to unmask] on 11/26/97 01:38:31 PM
>
> Please respond to [log in to unmask]; Please respond to
> [log in to unmask]
>
> To: [log in to unmask]
> cc: (bcc: Ed Holton/Hella North America Inc.)
> Subject: Re: [TN] Double Sided Assembly
>
>
>
>
> Hi! Don't apologize for disagreeing with what I wrote - disagreements
> can
> be good things too. Yes, you are correct that there are some designs
> that
> the pwa is too thin or the internal signal construction is such that
> achieving a temperature less that 183 C is just not possible. However,
> we
> routinely run pwas in which many of the components do go above 183 C
> and
> due to the surface tension of the solder stay positioned right where
> they
> should be (no fixturing required). We do some profiling on these type
> of
> pwas to insure that we don't generate excessive intermetallic layers
> in
> our solder joints. Also, some of the larger SMT components, coupled
> with
> gravity, negate the surface tension factors. We have pwas that I do
> not
> hit reflow temps on the bottom side throughout the entire reflow run -
> it
> just takes a robust oven design and some extra profiling effort.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
>
>
>
> [log in to unmask] on 11/26/97 08:35:03 AM
>
> Please respond to [log in to unmask]; Please respond to [log in to unmask]
>
> To: [log in to unmask]
> cc: (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
> Subject: [TN] Double Sided Assembly
>
>
>
>
> Dave you wrote|-
> Hi Lee - two items to think about as you venture into double sided
> assembly: your reflow oven capabilities and the robustness of your pwb
> surface finish. Reflow ovens that have both top and bottom side
> heating
> provide some recipe flexibility to keep your "first pass and now
> bottom
> side" of the pwa from reaching reflow temperature. Surface tension
> will
> keep many components attached to the pwa so avoiding bottomside reflow
> isn't a requirement but it does keep the solder joint intermetallic
> thickness minimized. Secondly your "second pass and now top side" of
> the
> pwa has seen one thermal exposure - the surface finish (OSP, tin/lead,
> gold/nickel, etc.) needs to maintain its solderability so that
> successful
> soldering can be completed during the second pass. Good Luck.
> Dave Hillman
> Rockwell Collins
>
>
> This simply isn't the way I see things.
> Peak reflow temperatures are usually around 212 - 222 centigrade.
> Reflow temperature of solder 179 - 183 degrees.
> That's 40 degrees difference.
> Copper vias the thickness of the board 50 - 60 thou.
> How do we create 40 degrees differencial across this distance?
> Even if we do, vias with direct connections,( such as vias in pads )
> to the
> top side pads will be cold, while other pads will have no problem
> achieving
> reflow.
> Assemblies need to be heated up completely, especially multi-layer
> with
> lots
> of copper planes.
> The issues of tombstoning are currently being discussed. Thermal
> differences
> for whatever reasons, (maybee inadequate heating/convection in IR
> ovens(
> are
> likely to be the cause.
> Surface tension usually does hold components on underneath, but if it
> doesn't then some means of support is needed while it reaches reflow
> temperatures. We have used tooling jigs during reflow on our complex
> assemblies, while adhesive is simpler solution.
>
> Sorry to argue Dave but I do feel passionate about this point.
> Thanks
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