I agree with you, but just keep in mind that laser is darn near infinitely controllable. I can easily "warm up" a surface by adjusting the power, lase speed, focal spot, dwell time, etc.
But warming up or pre-heating a large surface area (larger than 4" square, for example) to reflow temps is difficult in that the laser head needs to oscillate over such a large area that it is difficult to heat the area when it is cooling down just as fast, or faster.
35 mm sq. BGAs, no problem. 4" square area of Teflon, I'm not so sure.
However, it would be very easy to perform solder ball attachment to pads on flex circuits, and then reflow the components to the solder balls using convection.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric CHRISTISON
Sent: Thursday, February 04, 2010 8:53 AM
To: [log in to unmask]
Subject: Re: [TN] Laser soldering of BGA
I'd be very surprised if you could. Two reasons:
1. For the balls heat to melting point the opposite side of the flex
would have to be much hotter as most PWB materials have a low thermal
conductivity. Thus you'd probably damage the flex material.
2. Lasers I've seen apply heat very quickly. The flex material would
probably burn before the balls melted.
My stupid suggestion would be to get the BGA device without the balls
and attach it to the flex with a hot bar and conductive paste or film.....
Regards,
On 04/02/2010 14:41, Stadem, Richard D. wrote:
> Trying to heat flex to the temperature required to melt solderballs on the other side, I am not sure if that can be done using laser, nor would it be feasible due to the amount of time required versus convection reflow.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea
> Sent: Thursday, February 04, 2010 8:21 AM
> To: [log in to unmask]
> Subject: Re: [TN] Laser soldering of BGA
>
> Technos,
>
> There are 2 main ideas I can gather from your answers:
>
> 1. Why do I bother with laser when I could reflow? Well, as I said, reflow seems to warp either the component or the PCB and having no idea about laser soldering myself, I thought it would be a good idea to inquire. Since it seems to have no benefit, I believe I better get back to reflow. But not before asking another one: the BGA is soldered onto flex that is backed by a 12 mil rigid something; are there lasers that could solder from the bottom, through the substrate, rather than through the part, thus minimizing the heat seen by the assembly?
>
> 2. Steve K, could you please elaborate regarding the metal stiffener vs. FR-4. To my surprise, it seems that brass stiffeners yield better results than FR-4 indeed; shouldn't they give a bigger CTE mismatch?
>
> Thanks,
>
> Ioan Tempea, ing.
> Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
> T | 450.967.7100 ext.244
> E | [log in to unmask]
> W | www.digico.cc
>
> N'imprimer que si nécessaire - Print only if you must
>
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Eric Christison Msc
Mechanical Engineer
Consumer& Micro group
Imaging Division
STMicroelectronics (R&D) Ltd
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