Tuesday, October 28, 2014

3 Reasons the SMF6533 5kV Diode Makes the World a Better Place

The SMF6533 5kV diode is one of VMI's most popular devices.  It offers several advantages over the axial-leaded 1N6533, which it is based.

Hermetically Sealed

The thing that this writer really likes about the SMF6533 is that it uses the hermetically sealed diode (1N6533), over-molds it with a rigid epoxy, and then forms and trims the leads.  You get hermeticity in a plastic. surface mount package.  This can be a big deal if you're running the diodes in a dielectric fluid.

Reduced Hand-assembly, Pick-and-Placeable
SMF6533

An added bonus, because it's over-molded, is the fact that the body is smooth, and pick-and-placeable.  That means reduced hand assembly, which is a big deal if you've ever worked with a combination of through-hole and surface mount devices on the same printed circuit board.  Most customers specify the tape-and-reel packaging option which makes automatic insertion even easier.

What's Not to Like?

So basically, the SMF6533 makes the world a better place because you can place it in your assemblies faster, it reduces hand assembly, and it saves you time AND money.  Plus, you get the reliability of a hermetically sealed, glass-body diode in a plastic package.  What's not to like about that?

Other voltage, current and Trr ratings are available in the same package style, offering the same advantages.  More info.

Tuesday, October 21, 2014

Cleaning High Voltage Rectifier Assemblies the Right Way

Cleaning high voltage rectifier assemblies can be a big deal! There are environmental concerns (VOC limits), safety concerns (vapors), contamination concerns, process compatibility concerns, and well, there's always the question about just how well the cleaning process works.  

Process Compatibility
Image Credit

Some ICs, components, and diodes are susceptible to moisture absorption. VMI’s glass body diodes are not.

Epoxy diodes are not hermetically sealed. They are resistant to moisture, but given enough time, temperature, or pressure, they can be affected. That’s why it’s a good idea to use hermetically sealed diodes in wet environments.

But anyway, back to cleaning rectifier assemblies….it’s especially important to clean assemblies that are going to be potted. Cleanliness means the encapsulation material will better bond to any surfaces. Contamination on component bodies or along pcbs or substrates, may cause a weak spot in the adhesion, resulting in less-than-ideal voltage isolation and higher voltage stress. Higher voltage stresses can impact long term reliability, so it’s very important to have a good – very good – cleaning process.

Sources of Contamination

Lots of things can be sources of contamination. Skin, hair, chipped fingernail polish, skin oil, machine oil, dust, dirt, the presence of silicone, silicone oil used in pumps to pull vacuums….they’re all possible sources. A good cleaning system will get rid of most of the above, and what it can’t fix can be handled in other ways.

True Story

Once, long, long ago, this writer was working on a hi-rel, high visibility project. It was a product type VMI had a lot of experience building, but it was pushing the envelope on mechanical stresses of some rather large capacitors. The parts kept failing temp cycle. A failure analysis determined that the capacitors were fracturing right at the second potting line in a multi-step potting process.  It was so bad, our customer's customer showed up to help us find the problem.  (In actuality, they were very helpful).  

 Further examination revealed uncured potting materials at the interface. Even further examination determined that the proper cure schedule for the glue used to attach pads to the caps was not being followed. The pads were mechanical buffers used to keep the caps from fracturing. Not following the recommended cure schedule resulted in out gassing of the glue during the heated cure schedule that wreaked havoc at the potting interface. The out gassing contaminated the potting layer, and inhibited the cross-linking process. Once the cure schedule was followed, the problems went away.  It was an easy fix.  That is precisely why it was so embarrassing.  It only took our customer's customer showing up to shed light on the problem.  Sometimes it helps to have an extra set of objective eyes looking at the problem.  Once we got over our initial embarrassment, we went about the business of making things better, including a corrective action to prevent that same thing from ever happening again.  So far it hasn't.      

What to Look For

Some things to look for in a cleaning system include how well it removes solder solids, flux, oils, and other surface contaminants. How FAST does it work? Does it have to heat the parts to get them really clean? Does it give off any volatile organic compounds? If yes, how much? And how does much does this magic cleaning solution cost? Some of them can be quite expensive. But sometimes, it’s totally worth the cost.


What are some of your weird “cleaning” experiences?

Thursday, October 2, 2014

Sweet Consequences in High Voltage Manufacturing


Image Credit
Lately it seems the word "consequences" has a negative connotation.  Well, such is not the case this time!  This time, the consequences are sweet.

September 2014 turned out to be VMI’s biggest booking and shipping month EVER (knock on wood).

Everyone worked very hard to make it happen. I mean everyone – the folks in incoming inspection, production assembly, production testing, diode assembly, diode testing, potting, maintenance, engineering, QA, sales, shipping, machine shops, drafting, purchasing, accounting, HR, management and non-management alike.  From the guys and gals who empty the trash and clean out refrigerators, to production support engineers, to lead-people, temp employees, and shift supervisors.   

As is customary at VMI, the first Friday following a record-setting month, VMI provides pizza and sodas for all during the (extended) lunch hour.  For everyone.     

This Friday we can celebrate our success and prosperity with a full belly and smiles all around.  It's a great way to lead up to the weekend, no?

Go Team!!

Wednesday, October 1, 2014

Thermal Resistance in High Voltage Rectifiers - Part IV

In Part III, we discussed using alumina substrates, formed lead and surface mount diodes as ways to achieve higher thermal transfer, and to keep parts running cooler. Regardless of the type of assembly - axial-leaded, surface mount, substrate, or pcb - one of the most important things to consider is how to CLEAN them. 

Sources of Contamination

Contaminants can range from flux residue, flux solids, to wire clippings, or even fingernail clippings.  Yes, it happened - many, many years ago - and resulted in better assembly practices overall which are now standard.  Solvents work great, but there is increasing pressure from environmental groups and government agencies to reduce VOC. That’s nothing new to manufacturing companies in the U.S. Remember the good ol’ days of MEK? Great stuff. Just toxic. 

Methods of Cleaning Rectifier Assemblies

Nowdays there are many suitable replacements that work well and are not toxic to the environment. Coupled with no-clean solders that have improved over time, cleaning rectifiers has become easy to achieve. Several methods are available, including vapor degreasers and aqueous systems. Both are suitable for certain product lines. 

Post-Cleaning Bake-out

Another source of contamination is that of cleaning material absorption. Have you ever found yourself in a situation where potting material didn’t adhere to a component because of the presence of water? Or maybe the water or solvent absorbed during the cleaning process prevented the potting material from curing. Post cleaning bake-outs can be used to drive out any trapped water or solvents. Depending on the mass of the device and other factors like maximum temperature rating on components, VMI generally starts with one hour at 100C.   Review the maximum storage temperature ratings for ALL the components.  (One time there was an issue with a capacitor where the curing temperature of the potting material exceeded the storage temperature of the cap.  It resulted in a circuit redesign and delayed delivery, but in the end, it turned out okay).   

Impact of Inhibited Cure or Adhesion 
One Example of a Vapor Degreaser

Cleanliness of a rectifier, or lack of it, can impact adhesion or curing of encapsulation materials. Adhesion of encapsulation materials to diode leads, diode bodies, substrates, and pcbs can be critical in high voltage applications. Many applications rely on encapsulation to provide additional voltage isolation between components, or between mponents and the ‘outside world’.   

Cleaning an assembly prior to encapsulation can have a huge impact on long and short term reliability.  Cleanliness won't save a bad design, but it can help prevent a good design from going bad.   

VMI works very hard to stay green. Our cleaning processes use methods and materials that are effective and environmentally safe. We’re very proud of that. Coming up in Part V is “rectifier testing”.