Questions about reflowing solder on a motherboard

Question

One of my laptop's DIMMs is dead and working with less than my usual 8GB of RAM is brutal. Multiple sources suggest reflowing the board. I quite frequently tinker with/hack my electronics, but I've never attempted to reflow a board, especially whilst putting my beloved computer at risk. I have a few questions before I start:

Temperature? I know it depends on what kind of solder alloy is used, but I don't know how to check it. Is there a de facto standard I can resort to?
Time?
Oven or heatgun?
How come the processor (among its other chip buddies) don't burn out at 200°C+ temperatures while being (essentially) baked, but burn out when operating at near 100°C?
Any advice you can offer on the basis of dramatic/entertaining anecdotal evidence of how this can all go terribly wrong?

"burn out when operating at near **10°C?**". Did you mean 100°C? — Hennes, Mar 26 '13 at 07:38
It's a somewhat risky process, and I wouldn't apply it to something other than as an alternative to throwing it away. I have a board on my desk that was reworked "professionally" but that came back minus a couple of components and with some of the plastic parts brown and crispy. If it's just a DIMM isn't it removable? — pjc50, Mar 26 '13 at 13:36
The DIMM is the slot itself, not the memory module within it. I agree with your caution, I think i'll only do it if i can obtain access to a proper reflow oven — Alexander, Mar 26 '13 at 18:17

score 6 · Answer 1 · edited Jun 20 '19 at 21:31

It's perfectly possible to reflow your motherboard, but you have to do it really carefully to avoid destroying it. Several things can go wrong during soldering: thermal stress caused by different parts of the board being at different temperatures, and chips getting too hot. The reflow soldering process is designed to avoid these problems. This is the temperature profile of a typical reflow soldering process:

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The idea is to pre-heat the whole board to a high, but safe temperature. It is then held at this temperature for about a minute, allowing everything, even the insides of the chips to reach the same temperature. Then the temperature is briefly brought up to the melting point of the solder, and then the heater is switched off, and the board is allowed to cool slowly. Fast cooling will cause thermal stress, and can shear off solder joints, especially on large BGA chips.

This is what you should do:

Pre-heat the board in an oven. Try to raise the temperature fairly slowly, taking about three minutes. Get a laser temperature probe, and check that the board is about 180ºC all over.

Temperature probe

Now get your heat gun, and heat up the suspect component, taking care to heat it evenly all over. Keep the temperature sensor pointed at it the whole time, and stop heating when you see the temperature hit 245ºC, or when you see the solder melt all over.

Put the board back in the oven, and switch it off. Allow it to cool gradually, and take it out when it's safe to handle, about 40ºC.

Your processor can operate reliably upto 80º or so. Above this it will not operate reliable, but won't suffer permanent damage. Above the reflow temperature may cause permanent damage.

That is the profile for lead solder. No-lead goes up to 245°C, and chances are the motherboard is no-lead. — Scott Seidman, Mar 26 '13 at 12:38
if there is a suspicion that the problem is bad joint below the bga, should not he use flux for that? Will not heating up the chip create a bad joint? — , Jun 09 '17 at 11:13

score 5 · Accepted Answer · answered Mar 26 '13 at 07:50

5

Heatgun with aluminum foil warped around the rest of the board loosely, to act as a heat shield. You only want to heat the bad chip. No need to reflow everything else. If there is anything on the other side of the board, that might be an issue. This is just based on what I've read, no personal experience on this.

The reason that the chips don't die when baked at a high temperature is because the temperature is external, and takes a while to transfer through the ic package. Not only is the package designed for this in the first place (SMD/SMT was created for both size and ease of machine automatization. It allows for simple reflow after using a pick and place machine, instead of requiring manual soldering), the reason high temperatures kill ics is because the internal junction temperature is too high. Without proper cooling, the inside cooks itself. It's the reverse of normal baking.

answered Mar 26 '13 at 07:50

Passerby

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Very hard to believe that the inside of an 0402 diode doesn't get hot during a typical no-lead solder ramp/soak profile. – Scott Seidman Mar 26 '13 at 10:55
1

I don't think this is true. The insides of chips *do* get to the soldering temperature. The whole point of the reflow temperature profile is to bring everything up evenly to a temperature close to the melting point. The melting temperature lasts for several seconds, and only *very* large chips would have a significant temperature difference between the inside and outside. – Rocketmagnet Mar 27 '13 at 10:08
Also, I don't understand the last couple of sentences. How does the inside of a chip cook itself during reflow? – Rocketmagnet Mar 27 '13 at 10:09
@Rocketmagnet OP asked why chips will be fine at reflow temperatures, but die if operated without heatsink/fans at a operating temperature. – Passerby Mar 27 '13 at 13:14
@Passerby - Aah yes. I see that now. However, the answer is still misleading about the temperature inside the chips during reflow. That is why I downvoted. – Rocketmagnet Mar 27 '13 at 13:30
If you can find some proof or evidence for that, I'd very much appreciate looking at it @Rocketmagnet – Passerby Mar 27 '13 at 15:50
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@Passerby - Since you made the original assertion that chips don't die because they are somehow protected from the heat, the burden of proof is on you to show that. However, proving you wrong is easy. Simply look at the many chips which take a heatsink, and therefore have an excellent thermal connection to the outside. Like this [Intel P55](http://images.bit-tech.net/content_images/2009/09/intel-lynnfield-details-and-architecture/p55s.jpg). You can actually see the die right there. This chip can clearly survive full reflow temperature. – Rocketmagnet Mar 27 '13 at 16:29
@Rocketmagnet that proves nothing when you consider running a processor without a heatsink or fan will kill it into the 100°c range. But your asserting that it can survive the reflow, yet have no proof that the core junction reaches those temperatures. You also don't take into account that reflow is done in seconds. Or that Junction and Case temperature often differs. – Passerby Mar 27 '13 at 21:00
Besides, @Rocketmagnet, you made the counter claim, you need to provide evidence, you have the burden of proof. – Passerby Mar 27 '13 at 21:02
We can look at thermal diffusivity to estimate how long the heat takes to get from the surface to the inside. – Kevin Kostlan Sep 08 '17 at 20:21

score 0 · Answer 3 · answered Mar 26 '13 at 07:28

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You definatley want to use a heatgun to remove the chip, the data sheet should give you the reflow profile if you use it to re-attach as well, if using an iron than try to use some flux and have some wick handy to handle any bridges. this should be a pretty easy job theres really not much that could go wrong, chips are more resiliant than you'd think...

answered Mar 26 '13 at 07:28

alxcpa01101

1

I'm not actually replacing the chip, just reflowing the board ti establish broken connections – Alexander Mar 26 '13 at 18:20
Does this procedure have any chance of wetting both surfaces? If the joint is cracked it could have already oxidized. – Jun 09 '17 at 11:14

Questions about reflowing solder on a motherboard

3 Answers3