Reflowing SMD PCBs with suspected dry-joint components on them?

[Flyback]

Hello,

We have some all-SMD, FR4 PCBs which comprise LEDs and Driver components, or some PCBs which have just driver components. These PCBs are either double sided or 4 layer. They are 1.6mm in thickness. The components range from D2PAK FETs down to 0402 resistors, and 4mm by 4mm SMD LEDs.

We are sometimes finding that these PCBs, when delivered to us, don't work because of dry-jointed LEDs or other components.

We find this out because we can often make PCBs work by simply 'dunking' on/near the component pads with a big , wetted soldering iron.

However, we really need a better way to reflow these SMD components. Do you believe a hot-plate would be best?, or some kind of little SMD oven?, or an IR oven?
Sometimes the dry-jointed components have connection to a thermal copper pour on the PCB bottom. -But sometimes this is not the case.

We are actually wondering if the best & quickest way of doing it could be to simply put the PCB on a cheap hot-plate and waft over the suspect dry-joint components with a "gas gun" soldering tool?..(blowing scorching hot air on the suspect components)

 

[Colin]

Whatever you do, it has to be done quickly because the components have already gone through a hot bath.
A fine heat gun can deal with a few components at a time but a very fine soldering iron will work faster.
The problem is the solder is high temp and it stresses the LEDs and chips and transistors enormously.

 

[DerStrom8]

It's also very important to consider that some components were probably hand-soldered, and are not designed for reflow temperatures. I once saw a video showing someone trying to reflow cracked solder joints on a board and didn't realize that most of the connectors were low-temperature and they completely melted. I agree with Colin in that a heat gun with a fine tip is probably your best (and safest) option.

 

[JimB]

We are sometimes finding that these PCBs, when delivered to us, don't work because of dry-jointed LEDs or other components.

That sounds like it is time to have a serious talk with your supplier.
It could be:
you are not specifying the correct quality,
they are a cheap and cheerful cowboy outfit,
there is a fundamental problem with the "buildability" of your design.

JimB

 

[schmitt trigger]

Have you seen a reflow profile like the attached figure?
Any soldering shop worth its salt must create an oven recipe for each board type they run, to meet the desired reflow profiles.

The mistake some cheesy outfit make, is to create a single oven recipe and then attempt to solder all sorts of different boards using that single recipe.

This is wrong. All boards have different thermal masses, layouts, densities, etc. The oven recipe MUST be tweaked for the different board types.
You may request, and they SHOULD BE able to show you, what was the actual profile the boards were soldered to.

 

[Flyback]

Thanks, i wonder if we should make them test some of the boards after they come off the reflow machine...then they can tweak their process till they get less failures?

 

[DerStrom8]

Thanks, i wonder if we should make them test some of the boards after they come off the reflow machine...then they can tweak their process till they get less failures?

If your supplier is even half-decent they should catch bad solder joints during their optical inspection. If they don't offer optical inspection, then you shouldn't be using them.

Also, you shouldn't be reworking these yourself. You should be sending the parts back to them to rework. If the supplier is not willing to fix their shoddy work, then once again, you shouldn't be using them.

Who is your supplier?

 

[schmitt trigger]

Do they have ICT capabilities?
Or at least flying probe?

 

[Flyback]

If your supplier is even half-decent they should catch bad solder joints during their optical inspection. If they don't offer optical inspection, then you shouldn't be using them.

Thanks, they said because the solder resist is white, the optical inspection doesnt work well.

Also, you shouldn't be reworking these yourself. You should be sending the parts back to them to rework. I

Thanks, the thing is we dont actually know its dry joints till we "resolder" them and then the board works.

 

[DerStrom8]

Thanks, they said because the solder resist is white, the optical inspection doesnt work well.

Then that is a problem with their process and they need to resolve it or you need to find someone else.

I ask again, who is your supplier?

 

[DerStrom8]

Let me tell you what I think:

I think you're not naming your supplier because you know they're no good. They're the cheapest supplier you could find (probably some low-profile Chinese company) because:

This is very cost sensitive. As you know, British lighting is a Chinese dominated market.

You're not willing to design the products the right way because doing it the right way costs too much and becomes non-competitive with the Chinese manufacturers. You think that it's easier and cheaper to go with an el cheapo, low-quality manufacturer and rework their mistakes yourself than to go to a slightly more expensive supplier who gets things right the first time. This is your mistake. As a company you're throwing away more money at your engineers and technicians who need to spend their own valuable time fixing the mistakes made by a crappy supplier than you would simply paying a decent manufacturer to do the job in the first place. Your'e sacrificing quality for cost, and for little to no gain. Companies in the Western world who try to compete with cheap, low-quality manufacturers in China by cutting corners eventually die out because, even after sacrificing the quality of their designs, cutting down on employee count, etc. it's still impossible to compete with Chinese prices. The companies that flourish are the ones that maintain high-quality products and make sure their customers know the value of high quality at a slightly higher cost vs. the unreliability of low quality at a lower cost. They keep a good reputation because of quality, not because of price.

My advice to you: Get out of where you are and find another job. Whatever company you work for is cheaping out and if it keeps it up, it will go under in a very short amount of time. Customers don't like products that fail frequently, and that's exactly what you're designing if you keep refusing to do things the right way just because of cost.

That's my $0.02