Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Dave Weihrauch

#10306

Parylene Coating of Plastic Parts | 4 August, 1999

Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy.

reply »

#10307

Re: Parylene Coating of Plastic Parts | 4 August, 1999

| Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy. | Dave: Sounds expensive ...

1 Mask components taking care that the coating will not affect solderability or fit 2 Have SGS coat components 3 Assemble components onto boards 4 Test boards 5 Have SGS coat boards 6 Test boards

I can appreciate the need for parylene coated boards, but I'm uncertain why you want to coat components twice.

my2�

Dave F

reply »

Dave Weihrauch

#10308

Re: Parylene Coating of Plastic Parts | 4 August, 1999

| | Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy. | | | Dave: Sounds expensive ... | | 1 Mask components taking care that the coating will not affect solderability or fit | 2 Have SGS coat components | 3 Assemble components onto boards | 4 Test boards | 5 Have SGS coat boards | 6 Test boards | | I can appreciate the need for parylene coated boards, but I'm uncertain why you want to coat components twice. | | my2� | | Dave F | Clarification from Dave Weihrauch: You are right in questioning coating twice. My wording was misleading. I do not intend to parylene the boards a second time. I only want the plastic parts pre-coated so that I won't have to coat the entire board with all its components just to protect a few plastic parts. We may or may not have to coat the boards with urethane after assy & test (which can be done at a lower cost than applying parylene to the board (masking factored in).

reply »

Graham Naisbitt

#10309

Re: Parylene Coating of Plastic Parts | 5 August, 1999

| | | Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy. | | | | | Dave: Sounds expensive ... | | | | 1 Mask components taking care that the coating will not affect solderability or fit | | 2 Have SGS coat components | | 3 Assemble components onto boards | | 4 Test boards | | 5 Have SGS coat boards | | 6 Test boards | | | | I can appreciate the need for parylene coated boards, but I'm uncertain why you want to coat components twice. | | | | my2� | | | | Dave F | | | Clarification from Dave Weihrauch: | You are right in questioning coating twice. My wording was misleading. I do not intend to parylene the boards a second time. I only want the plastic parts pre-coated so that I won't have to coat the entire board with all its components just to protect a few plastic parts. We may or may not have to coat the boards with urethane after assy & test (which can be done at a lower cost than applying parylene to the board (masking factored in). | | My thanks for the prompt by Dave F. - Hello Dave! Now, Dave W..

I understand the logic but the purpose of conformal coating of a finished assembly is to protect the exposed solder joints/legs etc.. There would be no advantage to pre-coating the plastic components as you MUST leave these legs "clean" to ensure good soldering. The coating will not therefore give you any process advantage - only extra cost.

BTW, why Paralene? Why not use a more conventional material? Can you elaborate?

Hope this helps, Regards Graham

reply »

Dave Weihrauch

#10310

Re: Parylene Coating of Plastic Parts | 5 August, 1999

| | | | Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy. | | | | | | | Dave: Sounds expensive ... | | | | | | 1 Mask components taking care that the coating will not affect solderability or fit | | | 2 Have SGS coat components | | | 3 Assemble components onto boards | | | 4 Test boards | | | 5 Have SGS coat boards | | | 6 Test boards | | | | | | I can appreciate the need for parylene coated boards, but I'm uncertain why you want to coat components twice. | | | | | | my2� | | | | | | Dave F | | | | | Clarification from Dave Weihrauch: | | You are right in questioning coating twice. My wording was misleading. I do not intend to parylene the boards a second time. I only want the plastic parts pre-coated so that I won't have to coat the entire board with all its components just to protect a few plastic parts. We may or may not have to coat the boards with urethane after assy & test (which can be done at a lower cost than applying parylene to the board (masking factored in). | | | | | My thanks for the prompt by Dave F. - Hello Dave! | Now, Dave W.. | | I understand the logic but the purpose of conformal coating of a finished assembly is to protect the exposed solder joints/legs etc.. There would be no advantage to pre-coating the plastic components as you MUST leave these legs "clean" to ensure good soldering. The coating will not therefore give you any process advantage - only extra cost. | | BTW, why Paralene? Why not use a more conventional material? Can you elaborate? | | Hope this helps, Regards Graham

Response to Graham from Dave Weihrauch

Actually in the case of the plastic parts, corrosion is far more likely to occur inside the plastic package, rather than at the soldered leads. This is the desired reason for wanting to coat the part prior to soldering - to slow down the pentration of moisture. Since the bare semiconductor devices are not housed in a hermetic package (the plastic package is not impervious to water vapor & ionic contaminants), there is a justifiable fear that even at low level exposure to a humid atmosphere, corrosion of the semiconductor's aluminum pads could occur. On the other hand ceramic and metal packaged devices do not have this problem. It may be justifiable not to conformal coat the solder joints on this type.

Thanks for your response.

Dave

|

reply »

Graham Naisbitt

#10311

Re: Parylene Coating of Plastic Parts | 6 August, 1999

| | | | | Has anyone out there tried to parylene coat plastic parts prior to soldering them to a pcb? I'm looking at several potential applications where fewer than 5% of my components on a pcb are plastic, yet from a reliability aspect (environmental conditions) I need to mask and parylene my entire pcb assy. | | | | | | | | | Dave: Sounds expensive ... | | | | | | | | 1 Mask components taking care that the coating will not affect solderability or fit | | | | 2 Have SGS coat components | | | | 3 Assemble components onto boards | | | | 4 Test boards | | | | 5 Have SGS coat boards | | | | 6 Test boards | | | | | | | | I can appreciate the need for parylene coated boards, but I'm uncertain why you want to coat components twice. | | | | | | | | my2� | | | | | | | | Dave F | | | | | | | Clarification from Dave Weihrauch: | | | You are right in questioning coating twice. My wording was misleading. I do not intend to parylene the boards a second time. I only want the plastic parts pre-coated so that I won't have to coat the entire board with all its components just to protect a few plastic parts. We may or may not have to coat the boards with urethane after assy & test (which can be done at a lower cost than applying parylene to the board (masking factored in). | | | | | | | | My thanks for the prompt by Dave F. - Hello Dave! | | Now, Dave W.. | | | | I understand the logic but the purpose of conformal coating of a finished assembly is to protect the exposed solder joints/legs etc.. There would be no advantage to pre-coating the plastic components as you MUST leave these legs "clean" to ensure good soldering. The coating will not therefore give you any process advantage - only extra cost. | | | | BTW, why Paralene? Why not use a more conventional material? Can you elaborate? | | | | Hope this helps, Regards Graham | | Response to Graham from Dave Weihrauch | | Actually in the case of the plastic parts, corrosion is far more likely to occur inside the plastic package, rather than at the soldered leads. This is the desired reason for wanting to coat the part prior to soldering - to slow down the pentration of moisture. | Since the bare semiconductor devices are not housed in a hermetic package (the plastic package is not impervious to water vapor & ionic contaminants), there is a justifiable fear that even at low level exposure to a humid atmosphere, corrosion of the semiconductor's aluminum pads could occur. | On the other hand ceramic and metal packaged devices do not have this problem. It may be justifiable not to conformal coat the solder joints on this type. | | Thanks for your response. | | Dave | | | | | Dave,

By your message, I aasume you are a military contractor.

FYI the automotive industry and utility metering industry, impose specifications that are 1 whole order of magnitude, tougher to pass than NASA. ABS, AirBag and EMU's in cars, have NO backup or redundant systems, and are safety critical.

I say this because they have been using plastic packages for years with great success. I suspect therefore, that your fears are somewhat unfounded. I do respect them though, don't get me wrong.

As to the issue of coating before soldering, you are going to have to clean these devices after soldering, so your coating material must continue to work after the cleaning process. Can you be sure it will? I would suspect you are over-guilding the lily.

Talk to the componenet supplier and get their view -or is this their recommendation? If it is, you might want to talk to a few others?

Regards, Graham

reply »

Brian

#10312

Re: Parylene Coating of Plastic Parts | 8 August, 1999

Dave

Frankly, I think you may be barking up the wrong tree.

Like DaveF, I suggest that the process would be horrendously expensive with masking and so on.

The $64,000 question is whether what you propose will serve any useful purpose. I venture to suggest that it won't. When a plastic IC is moulded, the leads are wetted by the plastic. These leads (I'm assuming a J-, gull-wing or SOIC package or similar, but analogue arguments may be applied to others) are usually tinned copper or alloy. When they are soldered, considerable heat is conducted upwards to where they enter the package. Assuming the tinning doesn't melt (it could happen if your IR oven is badly adjusted), there will be a differential expansion which may be sufficient to break down the weak metal-to-plastic bond, leaving a crack of sub-micron, almost molecular, thickness, which would be the subsequent purported ingress path of contaminants (I personally have doubts whether this is common, although sporadic reports have been published). I have customers who have been soldering plastic packages with amine-hydrochloride water-soluble fluxes for donkey's years, often with less-than-adequate cleaning, their products being used in all climates, without experiencing any problems of this nature. However, it COULD happen.

Another point is that parylene is itself a polymer and it is effective only if the substrate to which it is applied is clean. Most components are more heavily contaminated on delivery than the assembled boards, having often been hot-tinned with a chloride-containing water-soluble flux and very perfunctorily cleaned (see "Cleaning and Contamination of Electronics Components and Assemblies"). This presupposes you will have to clean the components before parylene-coating. The differential expansion between the lead/parylene/package will not guarantee that you are hermetically sealing the part.

I agree with you that parylene is a coating par excellence, provided that it is well applied on a perfectly clean assembly. IMHO, one of its main positive features is that it must be applied in vacuo: this means that almost all volatile residues are removed before the stuff is evaporated onto the assembly and this includes some types of flux activator which may have escaped cleaning (although not chloride ions, of course).

My advice, therefore, is not to parylene-coat the individual parts. I suggest you may care to solder them in normally and clean off the flux residues with an adequate method, according to the flux/paste type, not forgetting that essential final spray rinse in the purest, non-recirculated, DI water you can lay your hands on, a thorough drying by clean air-knifing followed by evaporative drying at 105�C and then parylene-coating the assembly. This will give you a better reliability than what you are suggesting with a subsequent polyurethane coating at a far lower cost (I suggest that the cost of coating each component would be nearly equal to that of the whole assembly). Any chloride ions which have entered the crack prior to coating would have little chance of migrating to the die.

BTW, did you know that you can have ions from the gold plating trapped in ceramic packages as well? 10,000% reliability is a pipe dream. The best that can be done is to strive for 99.9999%!

Good luck

Brian

reply »

PCB machines

SMT Machines