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PCB overmolding

Page history last edited by PBworks 13 years, 7 months ago

 

The initial question:

We were trying to injection mold a part with a PCB imbedded in it, but were running into problems with the temperature and pressure damaging the PCB. What other processes (casting, etc) and vendors can you suggest for about 3000 to 10,000 parts per month?

 

 


Here's what we ended up doing:

 

I found a company in Reno called Cavist (cavist.com) and they use a low-pressure molding process with a material called Macromelt.  It's essentially a reformulated hot-melt glue, so it runs very easily into the mold, and high pressure (which damages PCB's) is not needed.  They were very eager for the work, and made some prototype parts for us.  They look really good.  They did a two shot molding process - first with a clear material over the LED, then they held the part by that light pipe to mold the rest of the connector in black.  By doing this, there are no pin holes for holding the board.  On the other end of the cable, they molded one side and then the other, so that there are no pin holes on that side either.  The final parts have nice definition and the material is slightly soft and grippy - a nice feel on the plugs.  And because the material is an adhesive, it is waterproof.  They said that another product they do has a spec that it can survive going thru the washer 12 times.

 

Here's the contact info:

http://cavist.com/a_contact.html

 


And here are the other comments I got: 

 

Here are a couple ideas:

 

- I met the General Manager of Piranha Plastics (Santa Clara) somewhere and I recall that they were embedding pcbs in inj. molded parts. Ralph Lower is his name, ph. 408-855-9650;  www.piranhaplastics.com

 

- Here are some materials my tooling guy recommends that run at lower pressures-- acrylic (thicker is better because it's brittle), ethylene, polypropylene , TPR, add a foaming agent (2% agent from Reed or LNP) to Lexan or ABS (wall thickness will have to be thicker than standard thin-wall inj. mold material).

 

I know it can be done. I have a USB memory stick that is done this way.

 

For an alternative approach...ultrasonic weld a thin-wall clamshell together. Are SD memory cards done this way...rip one apart?

 


There are three common ways to encase PCBs in plastic we've seen used successfully.

 

1) Overmold the same way cables are made. A first shot of polypropylene is molded over the PCB, encasing it. PP is relatively low melt temp and low viscosity, but you still need to be thoughtful with gating, pressures, and temperatures to avoid blasting parts off the board. The PP stabilizes the PCB and its components for a second shot, often PVC, that provides cosmetic and structural properties, as the PP and PVC bond together during the shot. Consult any sophisticated, value-added cable manufacturer for advice, such as Northstar.

 

2) Many ICs today are encased in a thermoset plastic loosely referred to as "molding compound". This material is strong but brittle and has limited color choices, but it injects at a cool temperature and low pressure, due to its low viscosity. It's gentle on the components it encases (wire bonds, etc) so it should be no problem to encase a full PCB in it, but I'd ask an IC fab for more details.

 

3) A third forgiving overmold material is polyimide. Certain grades can be injected like PP in the above example but don't require an overmold if you don't mind the feel of bare resin (it's a little gummy). This application is more rare and I can't suggest anyone who I'm sure molds it today, but they must be out there. I'd try starting with manufacturers of high-end connectors.

 


We had great success avoiding PCB damage using ISC Engineering in Pomona. http://www.overmoldtooling.com/index.html They specialize in cable assemblies with electronics embedded. Our application, as Dave mentioned was a fully embedded PCB with some delicate shutoffs to retain access to some pads on the PCB. The volumes were going to be in the 100,000’s per year, so volume shouldn’t be a problem. Materials are limited to lower melt temp stuff to avoid desoldering components or breaking parts off the board due to high shrink rates.

 

There were some limitations, though. The shrinkage was hard to control since plastic thicknesses were all over the map. Often, ISC uses plastic caps over parts of the board to standardize thicknesses.

 

I dealt with the VP and the president at various times. Both were great guys:

Steve Burk (President)

 


You may want to try taking a look at this low pressure and temperature molding material and technique.

 

http://www.loctite.com/int_henkel/loctite/binarydata/pdf/lowpressuremolding_LT-4145_Final_030605.pdf

 

In a more general sense if you are interested in having Flextronics Medical take a look at the application. We have experience in insert and overmolding applications.

 


I'm not sure if you have looked into the molding compounds used in the semiconductor packaging industry. PC boards are routinely overmolded. There are also a number of lower temperature encapsulants that are used. I'm not sure where you are located, but there are a few prototype operations in the Bay Area that might be able to look at your application. I've worked with Pantronix. They might be a good place to start. You can find them at http://www.pantronix.com/index.asp .

 

For general info on molding compounds, you can check out Henkel's website at http://www.henkelelectronics.com/int_henkel/loctite_us/index.cfm?pageid=14&layout=1


Overmolding of Embedded Electronics

 

By Steve Burk

 

This look at overmolding embedded electronics addresses current technological capabilities and the choices available to designers.

http://cs.pennnet.com/Articles/Article_Display.cfm?&Section=Articles&SubSection=Display&ARTICLE_ID=98416&PUBLICATION_ID=42&VERSION_NUM=1


Cavist has info and does low-pressure molding with Macromelt from Henkel.  They do molding for SanDisk and they do cell phone batteries.

http://cavist.com/index.html 

 

(They say if overmolding PVC can be made to work, then there's no need to use macromelt.)


The information below is for processes that were ruled out for my application, but included here for thoroughness


 

Try RTV molding.

 

http://www.forecast-3d.com/

 


You could consider RIM (much lower pressures) but it usually requires pretty chunky wall thicknesses, greater than 1/8" http://www.rimnetics.com/ is one good RIM vendor I've worked with.

 

Your quantities make soft tooling (silicone tools) pretty impractical.

 


From http://rimnetics.com/rimFAQmain5.html

 

Is my part right for RIM?

By Size?

RIM is ideal for large parts. Generally speaking, Rimnetics can mold a part 38"x60"x20" and weigh 100 lbs.

 

By production quantities.

Annual quantities of a few hundred to four or five thousand generally fit the RIM method. Part design features, together with tool cost amortization issues come into play. The easy way to settle the quantity question is to RFQ the part and compare.

 

By structural design needs.

RIM, with its unique molding abilities, is often employed to unitize design, reducing part count. Designers use RIM to avoid the dreaded "thermoplastic sink" that occurs when there's a mix of wall thicknesses caused by the called for thick ribs for structure.

 

By structural property needs.

For high stressed parts, RIM offers tough polyurethane formulations. Such formulations may be further enhanced through fillers, metal encapsulation or a SRIM composite. RIM provides the designer with the advantage of combining the efficiency of a molding with the advances in polymer chemistry.

 

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