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adhesive on pads


adhesive on pads | 29 March, 2002

We have a new assembly with bottom side components, and all the chip caps and resistors are 0603s. The dog boned shape of the caps necessitates a pretty big (and tall) dot to hold them still during placement, and when they get placed the dot squishes out onto the pads. I shudder to think about using the chip shooter for bottom side stuff....we're placing these with the pnp.

We're Class 2 and aren't violating IPC 610C specs yet, but I still worry about a hygroscopic material sharing real estate with electrical current, not to mention reducing my process window.

We use a water wash process in SMT but it's all no clean after that, so it's going to see a wave cycle after getting wet. I figure it'll get dry there, but will the epoxy absorb enough moisture down the road to create reliability concerns?

As to root cause, shouldn't I be able to expect enough green strength from my epoxy to hold 0603 caps in place without slathering it all over the place? What do you class 3 guys do?

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adhesive on pads | 30 March, 2002

You should not have glue "all over the place." Tou should not have glue on solderable surfaces. It sounds like you are stringing glue from one dispense location to the next. Search the fine SMTnet Archives [there might be something there]. Read a paper written by Chris Shea in the fine SMTnet Library.

Adhesive stringing can be attributed to various characteristics of the dispense nozzle: * Inside diameter of the nozzle: Called the nozzle ID. Dictates the smallest dot dispensed from that nozzle. A good rule of thumb is that the smallest possible dot that can be dispensed is typically twice the nozzle ID. * Distance between the nozzle and the board: Called the nozzle stand-off. The nozzle stand-off usually dictates the height of the dispensed material. * Correct volume of adhesive: Represents the amount of material dispensed through the nozzle at a given time. The volume of material dispensed is controlled by the stroke of the piston, when using a Piston Positive Displacement style pump, or by the amount of revolutions of the screw when utilizing an Archimedes Style pump.

All of that really relates back to how the 'dot' looks. Chrys says: The perfect glue dot looks like a Hershey's Kiss, and has a base to height ratio of 1:1. For an 0805 chip, the diameter and height should be 0.8 mm, for a 1206 component, 1.0 mm, with a tolerance of � 0.1mm for both. If the base/height ratio is: * Greater than 1:1, the failure mode of the dot will either be poor contact with the component or spread onto the circuit pads. * Less than 1:1, the dot will most likely collapse and appear to look more like a string than a dot.

The form of the nozzle tip strongly influences dispensing of material. Dispenser nozzles can have either conical or cylindrical nozzle tips. The conical form allows more material to contact the substrate surface than the nozzle, providing for releasing the proper amount of material intended to be dispensed, instead of fixing itself to the nozzle tip. A cylindrical form nozzle with 90� walls has more surface area touching the dispensed material and can cause material to be retracted to the nozzle during clearance, while returning to the z-retract height. This can cause stringing. Using conical nozzles eliminates these problems. In an emergency, reshape cylindrical nozzles into a conical form with a fine glass paper or engineers-paper.

There is no reason that you can not dispense two dots of glue, one on each side of the component to increase your volume.

So tell us: * What material are you using? * What is your cure profile? * Dot dimensions and tip? * Are the strings following the dispense head? Er what?

Help us understand your concerns about moisture better.

Search the fine SMTnet Archives fore a link to a paper done by CSL. Something er other about mircocanyons er some variant on that theme.

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adhesive on pads | 1 April, 2002

* What material are you using?

Loctite 3609

* What is your cure profile?

In excess of the 90-120 seconds at 150C that Loctite recommends.

* Dot dimensions and tip?

27g. conical tipped needle for the 0603's. I started with a 14 mil dot which looked great but didn't hold the caps still during placement. I needed 18-20 to get them stabilized and that also got me glue on the inside edges of the pads.

* Are the strings following the dispense head? Er what?

No stringing....placing the part spreads the material out further than I want, but reducing the volume of material results in parts jumping around on the board during placement.

I should also clarify what I'm seeing. My comment about it being all over the place was an exaggeration. I don't actually see any adhesive in the connection. The arc of the compressed glue dot extends onto the pads slightly, but is not visible post-wave, and not typically (when centered) under the termination of the part. All that is visible is adhesive flush with the pads, since solder fills in on the sides of the pads. I just know the adhesive is in there from pre-wave inspection. In other words, it's not a defect, but I don't much like it.

My concerns about moisture arose when reading about adhesive's tendecy to absorb it. It doesn't sound like a good idea to have a moisture absorbing material on solderable surfaces, even if IPC allows it to some degree. Maybe it stays dry enough with use....dunno.

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adhesive on pads | 1 April, 2002

Oh. [Howabout if I smack you?]

If ahma connectin� wif whut yer sayin�, you: * Don�t like the appearance of the glue slobberin� out from under the component even though there is no apparent glue on solderable surfaces. * Concerned about the hydroscopic nature of glue, in that it could concern the LT reliability of your product.

APPEARANCE OF THE GLUE SLOBBERIN� So, we talkin� cosmetics here, eh? Why not try a different shaped glob of glue? Maybe: * Two smaller dots between the pads. * An oblong dot [head moving while dispensing]

... Or * Buy BIGGER caps. * Take a deep breath, look around, fix something that will make money. * Analog of the previous one: Pimp slip the one that has you chasing on this. Well, unless it's yer boss, yano. * It hurts to say this, replace your 3609.

HYDROSCOPIC NATURE OF GLUE How is this characteristic of your glue more concerning than the hydroscopic nature of your: * Solder mask? * FR-4 construction of your boards?

True. Epoxy is hydroscopic. It is used for many purposes in electronic manufacture. But the real issue isn�t moisture lovers. Electrochemical failures are the result of contaminants residing on the surface of the laminate and/or components of your assembly. To create the reverse plating cell that produces electromigration, required components are: * Bias voltage * Moisture * Active ionic material.

Increasing any one of the these required components will increase in electrochemical migration.

BIAS VOLTAGE. The circuit design dictates the bias voltage present, which leaves you with control of the active ionic material present and to some extent the moisture penetration through the conformal coating (type and thickness).

MOISTURE. Conformal coating does not completely stop moisture penetration, so any surface ionic material will react under bias to create the electromigration condition.

ACTIVE IONIC MATERIAL. Proper cleaning of the laminate surface during the fabrication processes, adequate flux activation during all thermal excursions, and subsequent cleaning (as required by flux vehicle used) in the assembly process will produce a finished assembly with low risk for electrochemical failure. Halide residues are usually the root cause of dendritic growth. Halide residue may be from fluxes in the assembly operation or from HASL fluxes used in PCB fabrication. You should investigate both areas, bare board and assembly, as either or both could be contributing to the problem.

Closure on your moisture sensitivity concern: Use surface insulation resistance [SIR] testing as the bottom line of whether a residue is harmful. If SIR testing can show no detrimental leakage currents under humid conditions, no corrosion, no metal migration, which are the big factors in figuring out electrochemical reliability, your product should be good in the field. People often have a greater comfort with passing SIR than they do with passing other tests [ie, ROSE, IC, etc]. Of course, SIR testing takes a while and so it is best to do both ion chromatography and SIR at the same time so you can determine the correlation between SIR performance and cleanliness levels for troubleshooting the process later.

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adhesive on pads | 2 April, 2002

"Oh. [Howabout if I smack you?]"

Like I ever said anything 'bout strings. Sheesh.

I'm gonna save this one for future reference. Oughta take a week or two to figure out whether yer trying to make a fool of me or not. ;)

Criminy. And I was thinking about going to 3621 because of the (3X) shelf life advantages. I find out today it's got less than half the green strength of 3609. Phooey.

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adhesive on pads | 2 April, 2002

Whatever I've said, my intent has never been to insult you. Smack you yes, insult you no.

True, you never said the word �strings� but you did say � * �� shouldn't I be able to expect enough green strength from my epoxy to hold 0603 caps in place without slathering it all over the place?� * �� necessitates a pretty big (and tall) dot to hold them still during placement, and when they get placed the dot squishes out onto the pads.�

A DOE on glue bond strength showed that by far the most significant factor on holding the SMD on the board is volume of glue. Other factors were glued part height, time of cure, temperature of cure, and operator tweaking of part after placement in glue.

Yano, the another thing we like about 3609 is its very low chloride content. [Returning to your concerns about moisture absorption by your glue.]

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