Help on ball grid array bending during reflow

[JBond.3]

Hi, I work for a CEM in the UK and we've run into a problem when reflowing an STM32H757XIH6 BGA onto a PCBA. Under optical microscope inspection we're seeing that the center of the component is further away from the PCB surface than the corners. The component is bowing in the middle, with the solder balls more visible in the centre than at the corners, I've attached the below picture to show what we're seeing.

 

To give greater context, we're currently using an Essemtec RO300 reflow oven (3-zone), we're using Alpha OM353 Type 5 solder paste (SAC305 lead free), the components were placed using a Europlacer iineo pick and place machine, the components were baked prior to reflow as per IPC/JEDEC guidelines for MSL 3 parts.

We were able to lessen the degree to which these BGAs were bending by reflowing the PCBAs a second time. While this does help decrease the warping, it also slows our production down by half so is not an ideal solution.

Any tips, tricks, advice or pointers would be greatly apprecaited, thank you for your help

[Peter BENSCH · Accepted solution]

Welcome, @JBond.3​, to the community!

Your description sounds like what one finds with your favourite search engine under terms like "reflow bga bowing" or "... warpage" and can have a wide variety of causes: temperature profile, temperature differences between top and bottom, solder paste consistency, etc, see e.g. here.

Unfortunately, you can't see the large components well in the somewhat poorly exposed picture on the left, but I could imagine that they cast a heat shadow on the STM32H757XIH6.

Regards

/Peter

[JBond.3]

Hi Peter,

Many thanks for your input on this, I really do appreciate it. We've been doing a lot of Googling and we're trialing some different oven profiles at the mo, fingers crossed we can alleviate the issues.

Thanks again,

Jack

[LCE]

And you also might want to have a look at the PCB and its internal layers.

Big copper areas which might not be symmetric, combined with the big connectors (? or whatever big comps) might cause the trouble with heat distribution.

But I guess the connectors here are worse, so you you might try a reflow without these.

[JBond.3]

Thank you for this, it's much appreciated