When I ran an R&D lab, I found you could make almost anything work in small quantities with patience and time. One "needy" BGA had to be re-worked which took us about 20 tries to get right, but we did come up with a process which worked repeatably using hot air:
-Don't use solder paste (all of that surface area forces more aggressive flux action/higher temperatures). Pre-flow any deposits (requires creativity to avoid trying to set a sphere on a sphere)
-Minimize flux
-Hand wrap as much of the part as possible with copper tape to force a uniform temperature
-Add a stick-on temperature trigger on top of the copper tape (trigger temperature at the max allowed temp of the part)
-Loosely cover the top of the part with another layer of copper tape, creating an oven over the temperature trigger
-Pre-heat until the circuit board is "close" to the reflow temperature ("close" depends on the board material/layup and the vertical temperature profile tolerance of the assembly)
-Design a top side heat profile which will hit the required specs
So maybe you don't need to spend a long time finding a VP partner if you only need small quantities.
As noted by other responders, while VP initially sounds like perfection, the science is that the heat is transferred to the part by condensation. So instead of perfectly bringing everything up to temperature at the same time, the system is providing varying amounts of thermal energy to each of the components and the board, depending on thermal mass and exposed surface area. On the other hand, air/inert convection heats stuff up by hurling excited gas molecules at the surface, with a maximum achievable temperature of the chamber temperature (which has to be significantly greater than a comparable VP system). But the gas system is similar to VP in the sense that the temperature profile of each part is determined by its surface area and thermal mass.
Anyway, using the VP with a higher than necessary condensation temperature starts to make the operation more closely resemble convection because the maximum temperature can be significantly above the optimal reflow temperature. But since each droplet carries a lot more energy than an air molecule, there is a larger opportunity for local overheating/temperature gradients.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Monday, April 17, 2017 7:23 AM
To: [log in to unmask]
Subject: Re: [TN] Shouldn't the Fluid Be Changed for VPhase?
Hi Bob,
Well, the next step down with Galden heat transfer fluids is the HT200 which has a boiling point of 200 C. 3M makes some 215 C. fluorinert, called their FC70 if you can find it. Last time I looked was up close to $1,000 a gallon...
Steve
On Sat, Apr 15, 2017 at 1:12 PM, Bob Wettermann <[log in to unmask]> wrote:
> All:
>
> We are running an experiment on the use of vapor phase reflow for a
> rework process due to the excessive warpage (confirmed by and
> quantified by Shadow Moire' measurements) of a specific component.We
> want to keep the temperature of the component, which is an Sn63
> soldered component, as low as possible. The balance of the board is all soldered using a SAC305 alloy.
>
> We are "renting time" on a local company's VP reflow oven to run the
> experiments.
>
> Last week went over the DOE and the engineer who is responsible at the
> company that owns the vapor phase claims that they profile tin lead
> and lead-free boards using the same fluid in the bed and just adjust
> the time and heater conditions to get the right profile.
>
> I understood that you were to match the fluid to the solder
> alloy/liquidus temperature-right?
>
> Being used in the machine is the Solvay S 230 230°C for unleaded
> solder (e.g.. SnCuAg)
>
> Comments would be appreciated.
> --
> Bob Wettermann
> BEST Inc
> [log in to unmask]
> Cell: 847-767-5745
>
--
This email and any attachments are only for use by the intended
recipient(s) and may contain legally privileged, confidential, proprietary or otherwise private information. Any unauthorized use, reproduction, dissemination, distribution or other disclosure of the contents of this e-mail or its attachments is strictly prohibited. If you have received this email in error, please notify the sender immediately and delete the original.
|
