SMD hot air rework, how not to overheat chips and other components?

[neazoi]

Hi, I have a good quality hot air rework station with quite thin hot air nozzle, but I have never soldered components with it (desoldered some though).

As general guidelines, I would like to know how hot can the chips and the passive components be made before damaged?
My station can go to 1000F and at different PSI.

Hot air seems to heat up the whole part very much for a successful soldering, especially if it is a small QFN.

 

[jiripolivka]

General temperature specifications can be found from component data sheets. Soldering temperature profile is specified for tunnel ovens.
Use your tool carefully to desolder as well as resolder. ICs are more sensitive than passive components, and some failure rate should be expected. Work fast and train yourself on discardable boards first.
Use a digital temperature meter to learn what your hot air tool generates, and use control to keep it within limits.
Different solders require different temperature levels, too.

 

[D.A.(Tony)Stewart]

1. A good hotplate prevents CTE issues on substrate and assists from cooling losses. Then preheat the board to 150C for short duration & practise with thermocouple and kapton tape to mimic reflow profile on a scrap board. The nozzle air pressure may cause the part to move so, a custom nozzle directed to pads works best for each size.

Failing that, a pick to gently hold part on paste with nozzle to reflow around pads with an FR4 air dam to shield other parts would be excellent. otherwise it takes practise like calligraphy to apply heat.

Paste may dry out faster so minimize air time exposure and keep refrigerated. Heat time of 10 sec or less at liquidus stage to drive out volatiles.

 

[neazoi]

Thank you all,
From your experience, are there some general rules that one has to follow for time/temperature?
For example, in general smd capacitors can tolerate that hot air temperature for that time.
Or for example, in general you can heat a QFN on the whole body, or TQFP on the pins, for that much time, in these range of temperatures.

I do not think preheating the board will be useful in my case, as I do not seem to have large grounds. I am building this one https://4.bp.blogspot.com/-jPG-dxTS...E/s1600/mchf_uiboard_soldered_mcu_side_1a.jpg

 

[D.A.(Tony)Stewart]

Thank you all,
From your experience, are there some general rules that one has to follow for time/temperature?
For example, in general smd capacitors can tolerate that hot air temperature for that time.
Or for example, in general you can heat a QFN on the whole body, or TQFP on the pins, for that much time, in these range of temperatures.

I do not think preheating the board will be useful in my case, as I do not seem to have large grounds. I am building this one https://4.bp.blogspot.com/-jPG-dxTS...E/s1600/mchf_uiboard_soldered_mcu_side_1a.jpg

TCE can be most stressful on vias.

 

[dick_freebird]

I've watched the nice ladies in the lab do this and never could
match their "feel" for it on my own. I'd start practicing on junk.
Preheat minimizes the time@temp needed on the parts and PCB
features to get the components off, maybe letting you stay
below a critical temperature where it counts. Getting the right
distance and combination of global vs local heat appears to me,
the key.

The ladies put the board upside down in one hand and work the
petite heat gun in the other, and the parts just shake loose
when they're in the groove.

But you've got to find the groove, that they practice in daily.

 

[neazoi]

I've watched the nice ladies in the lab do this and never could
match their "feel" for it on my own. I'd start practicing on junk.
Preheat minimizes the time@temp needed on the parts and PCB
features to get the components off, maybe letting you stay
below a critical temperature where it counts. Getting the right
distance and combination of global vs local heat appears to me,
the key.

The ladies put the board upside down in one hand and work the
petite heat gun in the other, and the parts just shake loose
when they're in the groove.

But you've got to find the groove, that they practice in daily.

you cannot really practice on junk and make sure that you do not destroy them.
of course you will solder them eventually, but how are you sure that they are not burned out?
you have to work on the real circuit then, that is my problem and that is why I am asking about practical guidelines.

I have seen some videos on youtube, and they tend to keep the gun for about 20-30 sec on a qfp's pins and at the same time move the gun back and forth for each pinside.
Would that be the sort of time to expect? What would in general be considered as too much time?
My station temps vary from 850-1000F to get an idea.

 

[D.A.(Tony)Stewart]

Several years ago a simpler, faster method for reworking leadless devices was developed using polyimide stencils. In this method a polyimide stencil is placed over the land patterns on the bottom of the device. Solder paste is then rolled in to the apertures. After reflow, the stencil is peeled off leaving “bumps” on the bottom of the device. A stencil permanently affixed to the PCB filled with paste flux or solder paste then acts as the receptacle for these device “bumps” to fit in to.

IPC 7711 method 5.8.1.2. was used to “bump” the parts.

 

[EzPCB-Nila]

From our experience,the temperature of rework station is up to several factors:
1.the thickness of board
2.the part's type,package and datasheet infomation
Before you use hot air to heat up the soler pads,please use high temperature sticker to protect around areas.
This will not lead to broke other parts.Hope this info can help you.