Goop on small components, what is this? - Page 1
Forum section: electronics / general

author: angdis
author_rank: Newbie
timestamp: March 25, 2015, 05:52:17 pm
content: I've always wanted to know this but I am too embarrassed to ask at work.Boards that come from the EMS we use have red goop on some of the 0201 components. I've seen this before but there is more of these on this product.It looks like some kind of glue perhaps intended to keep the part from tombstoning? But how is this applied? I can't imagine that it would be manual and how would they know to apply it on some parts but not others?

author: Falcon69
author_rank: Super Contributor
timestamp: March 25, 2015, 06:21:08 pm
content: It looks like these could be components on the bottom side of the board? Just guessing here, as I see the bottom side of what looks to be a header or something. Anyway, if these are on the bottom of the board, my guess is that the goop could be adhesive to hold the chips on while they wave solder the board.

author: angdis
author_rank: Newbie
timestamp: March 25, 2015, 06:31:14 pm
content: Thanks! Yes, these are always on the bottom side of the board. But not all components have the goop, just _some_ of the small ones.I am not sure if the board is traditionally wave-soldered on the bottom(that would have to be done first). It might be "selective wave soldered" with a little fountain of solder that is robotically moved to the few through-hole components.As I write this, I am now thinking that perhaps the goop is intended to keep these small components from moving when soldering the through hole stuff. That would be a big heat source.

author: max666
author_rank: Frequent Contributor
timestamp: March 26, 2015, 06:13:10 pm
content: I don't understand this.If the bottom side is selectively soldered so precisely, that exposed via pads right next to the through-hole solder joints don't even get a smidge of solder, why would they even need to glue the smd resistors?And why aren't the ceramic caps glued? It seems to me they are far more likely to fall off than the tiny resistors, due to bigger mass per solder joint surface area.And if the caps are placed in a second step afterwards, so they wouldn't have to be glued, then why aren't the resistors placed in a second step as well?Also, doesn't this look like way too much glue? Those red blobs are bigger than the resistors itself?!

author: AndyC_772
author_rank: Super Contributor
timestamp: March 26, 2015, 06:44:33 pm
content: The board doesn't look like it's been wave soldered; all those gold vias are still free of solder, and there's no way they were masked off that accurately.That red adhesive is probably a Loctite Chipbonder variant. It's usually dispensed very carefully and selectively; I've never seen it applied quite so generously to such tiny components before. If anyone can suggest how it's managed to avoid contaminating the solder joints I'd be interested to know.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 26, 2015, 06:53:45 pm
content: Quote from: AndyC_772 on March 26, 2015, 06:44:33 pmIt looks like it was appliedaftersoldering the components, but I have no idea why you'd do that.

author: free_electron
author_rank: Super Contributor
timestamp: March 26, 2015, 07:07:45 pm
content: that board has gone through a selective soldering machine.the parts with goop on them are so small and light , and the traces so thin, and placed too close, that the local heat of the selective soldering machine could melt the solder on the part and have the part fall off the board.So they squirt Loctite Chipbonder on em (characteristic red color of Loctite).interesting link below : ( including video)http://www.ersa.com/selective-soldering-en.htmlThere are specific design rules to use such machinery ( distance from pad to components etc )

author: SeanB
author_rank: Super Contributor
timestamp: March 26, 2015, 07:20:28 pm
content: I would guess the board has been screen printed with solder, and then the red glue was applied with a robotic applicator, but the droplet volume is too large for the small components ( or the operator mixed up the settings for the small components so they got a little too much) then the PNP placed the parts and then they were oven cured to cure the glue, flipped over and the top placed before going through either vapour phase reflow or dual side IR reflow. The bigger parts likely hide the glue spots well, and have the right volume of glue applied, but the small ones have way too much. Only reason they work is because these passed the power on testing phase, while those where the glue covered the pads and isolated it from the solder failed and were weeded out by ATE.