Reflow soldering of lead-free components with leaded solder paste

#90586

Reflow soldering of lead-free components with leaded solder paste | 16 February, 2024

I am confused about the parameters of the reflow profile needed to solder lead free components. We assemble boards for automotive modules required with leaded solder paste. The boards have ENIG finish. The problem is that I see some wetting issues, very erratic, can be a single resistor with one side not soldered, not in the same place and not the same one, so for sure is not a component oxidation. The profile is set according to the solder paste manufacturer data sheet, however the dealer recommended to increase the peak temperature and bring it to the lead free level. While this makes sense as the components pads are coated with lead free tin, this exceeds the solder paste limits. The solder paste is SnPbAg

#90612

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

How have you created the oven profile?

Have you taken into account the heat soak from the components/board/carrier etc.

Is the paste being mixed correctly before use, is the paste in its use by date & at ambient temp etc?

I thought almost all the automotive sector had moved away from leaded paste.

#90613

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

We have the standard profile as a template made according to the solder paste specs. For some boards we measure the result at 2 points, on the board itself and on top of a sensitive SOT263 module and adjust accordingly. The solder paste is well kept, mixed with a paste centrifugal mixer and we never use remains from previous jobs. Regarding the automotive market, as you said... almost. Heavy duty and construction equipment still require leaded soldering. The thing with increasing the peak temperature as the KOKI dealer suggested is not out of the blue, there is a well known problem with soldering lead-free components with leaded solder paste, practically in most cases even if the joint looks perfect inside is not. Aerospace industry for example require all lead-free components to be sent for re-plating. I recently saw a solder paste data sheet from Kester where they clearly specify on some of the leaded solder paste that they are made to work up to 235°C in order to reach the melting point of the components coating. We are talking only about the peak temperature, the flux activation area remains between 140-170°C. The problem is that the KOKI solder paste allows up to 225°C peak.

#90614

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

Got you. 225C for peak does seem low to me.

Henkle is the main one i've used with a peak of 245c & Alpha around the same although I try to keep that at no higher than 235C as we use it for LED arrays.

If the peak is that low, is the liquidus temp still around 217C? as that doesn't leave you with much "wiggle" room

I would generally measure 6 points to try to cover more of the board though.

#90615

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

Well, the story is a little more complicated. The product is also an LED lamp made on an aluminum PCB with the LEDs on one side and the drivers on the other. The board itself is not big, about 90x90mm and most of the components are soldering well. Usually we were first assembling the components and the LEDs on a second reflow pass. But some 2 years ago TI made a surprise, resulting in about 30% failure of a DC-DC module after reflow. It took them some efforts and complaints from other customers to admit that there was a production issue as a result of an internal change in the component. Meanwhile we switched the order and now pass the LEDs first and the components second, just to be sure we are not damaging the module. This is exactly the module I mentioned in my previous reply on which we measure the temperature. It is designed to stand 245°C but I don't want to get close to that. The liquidus area for the leaded solder paste is 180°C or 200 with other solder pastes.

#90616

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

Would you be able to post an image of the joint in concern?

Sounds a good problem to resolve.

Guessing you have used an external profiler & not the oven built in type?

If 100% certain the profile is correct I would focus on the paste from my experience of a similar issue it was the paste found at fault.

#90617

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

Unfortunately it is not possible to post pictures here. It looks the same as in these photos: https://yic-assm.com/wp-content/uploads/2019/06/dewetting-1.jpg https://res.cloudinary.com/engineering-com/image/upload/v1577777622/tips/20191216_174750_1_azrqf2.jpg As much as some think of it as tombstoning, it is not that. The pads are equal, it happens mainly with resistors but sometimes also on capacitors or diodes. Always one pad and the strange thing is very erratic, not on all the boards, not on all the resistors of the same kind and in different locations. Very strange. I have both external and the oven's option to run the thermocouple with the board.

#90619

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

That link doesn't work for me.

From the more you explain - it does sound like an issue I've had in the past on a few different builds.

In the end we traced it to a fault with the paste (they were having mixing issues in the chemistry) but they as you say would just say increase the temp - it did "resolve" the error but brought another with the temps impacting some other components.

In the end we switched to a different paste supplier & have never had a repeat.

#90620

Reflow soldering of lead-free components with leaded solder paste | 23 February, 2024

Just another thought - how long is the printer idle for between boards? is it long enough to have the chemicals start to separate within the paste?

#90626

Reflow soldering of lead-free components with leaded solder paste | 24 February, 2024

Not very long, minutes

AlexV

#90630

Reflow soldering of lead-free components with leaded solder paste | 26 February, 2024

Leaded solder is liquidus at 183C, 220C is max. Sounds to me your boards might be contaminated or the plating is poor and allows oxidation. Without seeing the boards or profile, I recommend you slow down your oven for a longer soak, to allow the activated flux over more of the pad.