Double sided SMD assembly

[itsmoked]

I have access to a pick and place machine and an order for a bunch of double-sided boards. In the past I've been placing one side and reflowing it to completion, then hand soldering all the parts on the bottom (only about 1/4 the parts). But with a large order I'd like to avoid hand soldering the bottom at all.

I believe regular board houses have some way to dab glue on the bottom side then they place the parts and then they can flip the board and do the top side then reflow the whole thing one time without the parts on the bottom falling off.

Not having the glue application equipment I'm searching for an alternative.

One thought is different melting-point solders. The first thing that comes to my mind is using lead-free and leaded solder. Lead-free takes another 100F to melt over leaded. Has anyone tried doing one side with lead free then doing the other side with lower temp leaded at it's lower melting point and so having the first parts be unaffected by the second reflow?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

If the passives are small/light enough (like 0201/0402), solder surface tension will hold them on the bottom side. For larger components, a glue dot may used.

Another technique is to first reflow the highest-density side of the board. With a slight modification of the reflow profile, the second side can be reflowed without warming up the higher-density side enough to bring it past the solidus temp again. You have to do this day in and day out, though, to get a feel for it... unless you want to do a lot of experimenting, I'd suggest sticking with the glue dots and/or manual soldering.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

Thanks Dan. It's pretty scary. A lot of expensive parts involved and a nasty deadline. Parts are mostly 0603 0r 0805 nothing as small as 0402.

I think I'll try the two solder pastes. The oven blows hot air around but the IR only comes from above so I suspect we can prevent solidus temp from being achieved on the bottom.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Sparweb]

An idea more likely to display my ignorance than be helpful, but here goes:

If you use a template to locate the components on the first pass, then wouldn't a template such as that serve to hold them in place during the heating of the other side?
For instance, if you left the template in place, then the only way the components could move is away from the board (not slither around on its surface).
If every component was exactly the same thickness, then you could place it on a flat table and 100% position control would be maintained.
Since the components will surely have varying thicknesses, then you need a table with raised places to match.
I can easily visualize an machined aluminum plate with pockets and pads that your first side rests on neatly, supporting every fat IC and skinny diode, meanwhile holding the template and board positions while they are all upside-down using (I assume) the mounting holes in each as locating holes.
There may also be beneficial heat-sink properties in that aluminum plate, but that's adding speculation onto an already wild invention.

I was also going to ask if it's possible to encapsulate the first side with a (very) high temperature resin? Re-flow temperature is... 250C... probably not...

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[itsmoked]

Hi Spar, That's not a bad idea. My associate was pressing for that. My problem is that my layout tool doesn't do 3D so I don't really know what the part heights are in any useful-to-feed to a CNC router way. Furthermore the parts are drawn more for their footprints since that's really important to layouts. There is not really even a 2D explanation of the part bodies since it's all about their little footsies.

Fear not, I've got a viable solution. Apparently this is a common problem all over the industry. Since even the big houses hate taking the time to glue-dot they pressed for a solution.

Typical Pb free solder melts around 220C. This is also too hot for some parts so a different alloy has been formulated. This 'low temp' Pb free alloy melts at 138C. I'll do the tough parts on the bottom of this board with the 220C stuff then flip it over and do the top parts with the 138C alloy. I'd been forced to hand solder two 16pin DIP switches on the top because they were destroyed at 220C and even at the lower Pb solder temps they got distorted. Now, this 138C alloy should keep them happy. The boards operate at 20 to 30C only so issues like having hot parts soldered with only 138C alloys won't be a problem.

Keith Cress
kcress -

http://www.flaminsystems.com