Non wetting spots of flex PCB pads

Subscribe to Thread

Today during the regular microscopic inspection of delivered... - May 11, 2023 by The

This is a defect in all IPC classes. Solder has not wetted t... - May 12, 2023 by MagyarT

Have you tried cleaning these spots with anything? ... - May 12, 2023 by Evtimov

I agree with the two other replies. Left alone, it is a defe... - May 26, 2023 by Auriga2001

The

#89342

Non wetting spots of flex PCB pads | 11 May, 2023

Today during the regular microscopic inspection of delivered flex-PCBs i found a few pads that showed some kind of residue. 3 out of 240 inspected PCBs were affected. From the color of these residues my best guess would be, that it is adhesive from the coverlay. After performing a wetting test with these PCBs it showed that these spots do not accept solder. Would you still use these PCBs? Have you seen this type of anomalies on flex PCBs?

Attachments:

reply »

MagyarT

#89349

Non wetting spots of flex PCB pads | 12 May, 2023

This is a defect in all IPC classes. Solder has not wetted to the land where solder is required.

reply »

Evtimov

#89353

Non wetting spots of flex PCB pads | 12 May, 2023

Have you tried cleaning these spots with anything?

reply »

Auriga2001

#89388

Non wetting spots of flex PCB pads | 26 May, 2023

I agree with the two other replies. Left alone, it is a defect.

If it's just excess adhesive or coating, you can use IPC-7711/7721 'Rework, Modification and Repair of Electronic Assemblies' for guidance on removing. Section 2.3.1 explains how to test for what kind of coating it is. The removal method: solvent (2.3.2), peeling (2.3.3), thermal (2.3.4), scraping (2.3.5), micro blasting (2.3.6). Then use section 4.1.1 for land (pad) preparation.

I've had to remove f.o.d. of excess conformal coating, water soluble coating, chip bonding epoxy, board masking, coverlay... I've been able to remove most with thermal and scraping methods. A hot clean solder tip should be able to rub the offending material off. Since it's on a pad, that will make it easier for you. Just be careful not to overheat the surrounding area too much, just like normal soldering operations. Clean the pad and inspect that all f.o.d. is removed. Tin the pad again to test for wetting.

However, if it's an improperly made pad, and there's just no base metal to attach to; you will need section 4.7.1 or 4.7.2 for 'Surface Mount Pad Repair'. The pad would be cut out and replaced. In most cases this is acceptable per 1.5.2 (Compliance) "Products that have been subjected to rework need to comply with the functional requirements for the product and any other attributes that may be required by the customer. In the absence of other defined acceptability criteria of J-STD-001 and/or IPC-A-610 to the reworked actions."

and

"Modifications and repair, by their nature, do not have industry established requirements and acceptance criteria"..."case by case basis". It's up to the customer if that would be acceptable.

reply »