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Pinball Rehab

pinball repair and restoration

Micro-Blasting Circuit Boards Micro-Blasting Circuit Boards Hot

The two primary reasons for micro-abrasive blasting a pinball circuit board are to remove the green solder mask (commonly, but mistakenly, called the conformal coating) or clean-up oxidation (see Images 1 and 2).  In the past this would have been done with chemicals, but due to regulatory and cost issues, micro-abrasive blasting has now become the standard solution. 

The problem with a mechanical approach is that ESD (electrostatic discharge) is created, which can damage board components.  In production environments specifically designed micro-abrasive blasters are used that minimize ESD.  Unfortunately with a price tag of several thousand dollars, this solution is out of the reach of most hobbyists.

The purpose of this article is to provide a cost-effective solution that will still minimize the chances of ESD damage by using the following techniques.

  • Use a micro-blaster to surgically control the application of blast media.
  • Remove IC's, which will also break the circuit and protect upstream components.
  • Connect 5v, 12v and circuit board ground to earth ground connection.

Electrostatic Discharge

It is estimated that up to 35% of total IC field failures are ESD induced.  Before you start thinking that you've never blown a chip due to ESD, the truth is you really don't know.  A large percentage of ESD damage does not immediately blow the chip, but rather causes internal damage which shortens its life.

Images 1 and 2 are microscopic views of components with ESD damage.  In Image 1 you can see in the upper circle where the circuit was damaged rather than completely destroyed as the others where.  In Image 2 the drain has been damaged on an NMOS gate and while still functional, it won't last as long as its buddies.

Image Gallery

Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards
Micro-Blasting Circuit Boards

ESD safe micro-abrasive blasters use an ionizer and a conductive work chamber to solve this problem.  In the Work Chamber section I will explain how to create your own anti-static work chamber, but there is no perfect work-around for the ionizer.  Instead we'll focus on using a blast media that creates the least ESD. 

The following information is excerpted from a whitepaper written by NASA, Evaluation of ESD Effects During Removal of Conformal Coatings Using Micro Abrasive Blasting.

Typical blast media options include wheat starch, sodium bicarbonate, plastic beads and glass beads. All of these media will remove oxidation and the solder mask, but will theoretically not damage soft metals (either copper or solder).

Following are the average ESD surface voltage ratings for each type of media when used on a urethane conformal coating (I could not find any hard data on solder masks, but these results should be similar) and the ionizer turned off (numbers in parenthesis are negative). 

  • Sodium Bicarbonate -- (1,423) volts
  • Wheat Starch -- 1,527 volts
  • Glass Beads -- (1,810) volts
  • Plastic Beads -- (3,076) volts

Note: Most IC's are designed to handle an ESD of 2,000 volts or greater, with CMOS chips typically being less than that.

With the ionizer turned on the average surface voltage with wheat starch was reduced to 16 to 66 volts.  The other media typically fell within a range of 150 to 682 volts with the ionizer on.  According to the paper, wheat starch also did the best job of removing the conformal coating, and glass beads the worst.  Although the same results may not be achieved with solder masks.

Equipment

If you've got an air compressor the only additional equipment you will need is an air eraser ($28 at Harbor Freight).  The one disadvantage of this solution is the rather small chamber for holding the media.  If you're doing a large area it will need to be refilled several times.

Media

At the time of writing this article I have personally used sodium bicarbonate and glass beads, but have not tried wheat starch.  I just ordered some though, and will be trying it in the near future and reporting back here.  It can be purchased at Crystal Mark, Inc. in a 1 pound bottle, Carbo Blast #25A (electronic grade) 160 micron.

Update: I switched over to wheat starch about a year ago and have found it to be slightly more aggressive in removing the solder mask while still limiting damage to component packages.  Since it also has the lowest ESD rating this is my recommended solution.

If you're going to use glass beads or sodium bicarbonate (I would not recommend the plastic beads) you will want to find a local source since it is usually sold in 25 or 50 pound bags.  Just Google "blast media." 

I should note that you can't use baking soda from the grocery store for a blasting media since the size of the particles is too large and will clog the air eraser tip.  The sodium bicarbonate blast media is also treated to reduce moisture absorption.

Glass beads come in various sizes, with #10 glass beads (100 to 170 mesh) appropriate for circuit boards.

Do not reuse the blast media since it will be contaminated with alkali.

Work Chamber

The first step is to provide a path to ground for the static charges.  In Image 4 you can see an anti-static bag sandwiched between the circuit board and some scrap wood.  The assembly is then held together with some rubber bands.  Note: You want to use conductive anti-static bags.

In Image 5 you can see where the components, leads and pads I want to protect from the blasting are covered with electrical tape.  In some cases I left the leads and pads exposed so I can clean off the oxidation.  Since the blast area is very small, and as long as you're careful, you only need to protect those areas adjoining where you will be blasting.

The same approach is used for the back side of the board, although it is not quite as effective since we don't have the advantage of the component leads pressing into the conductive material.  You also need to apply electrical tape on the back, especially on the IC leads, which need ESD protection.

In Image 6 you can see the assembly placed in the work chamber.  I have added two ground straps, one going to the anti-static bag and the other to a ground point on the board.  Both of these then need to be connected to earth ground.  You also want to ground the 5 volt and 12 volt traces (not shown in photo).

Since this board has empty sockets on it I've added an additional anti-static bag (see Image 7) to keep media from bouncing around and lodging in the sockets.

At this point you've got a reasonable work chamber, but if you really want to protect the board you could buy a gun ionizer, which will cost you about $500.  Not cheap, but still better than a full-blown ESD micro-abrasive blaster.  While blasting you just point the ionizer gun at the area that you are working on.

Of course, you could make a fancier work chamber by covering the top with some plexiglass and cutting two holes in the side for access to the the chamber.

Safety

When working with an open blasting chamber you will want to wear gloves and a full face shield.  Invariably you will get some media on the garage floor and the glass beads are about like walking on ball bearings, so clean the area thoroughly when you're done or you'll end up on your ass (I speak from personal experience).

Preparation

Unless the board has a very small amount of alkaline damage, I always remove the battery holder.  In addition I will troubleshoot the board before blasting and remove any IC's that are damaged or that are covering up potentially damaged traces.  By removing the components and battery holder first, you will have a better idea of the extent of the damage and can ensure all necessary areas are micro-blasted.

The more components (especially IC's) you remove the better job you will be able to do of cleaning up the board.  So if in doubt, pull it.

Micro-Blasting

Before we start, it's important to understand what we want to avoid and what we want to achieve.  Components, leads and solder pads should normally be avoided while blasting.  You also want to be very careful around small traces.  While theoretically the blast media should not remove copper or solder, it will if you excessively work an area.

Other than that, we want to remove the following:

  • Any corrosion (the blue/green stuff) that was not removed during chemical cleaning.
  • Any portion of the solder mask that has turned dark green or brown.
  • Any oxidation on component leads and pads.
  • The solder mask on any traces that are cracked and will need repair.
  • The solder mask on any suspect areas that require further inspection.

It's up to you if you blast the component leads and pads.  If you do they will likely need to be re-flowed and, especially on the IC's, you risk ESD damage.  Note: It only takes a couple of quick passes to clean oxidation off of leads and pads.

I used glass beads in the following example with the air pressure set at  70 PSI and the nozzle tip about 1/2" away from the board.  (Note: The maximum air pressure rating of the Harbor Freight Air Eraser is 65 PSI).  Adjust the throttle on the air eraser until you get a good flow of media and proceed just like you were sand blasting, working a small area at a time.

In Images 7 and 8 you can see the completed board after about 30 minutes work.  In addition to removing the solder mask where there was oxidation I've cleaned up some of the solder joints.  You can see how much shinier the solder joints are compared to the before photos.

In Image 9 you can see some "bubbling"  in the middle bottom part of the ground trace, and a hole with raised edges on the bottom right-hand side.  This will typically happen if the corrosion has eaten through the solder mask and began to eat away at the trace. This portion of the traces should be removed and replaced.

The trick is to clean the trace without doing more damage.  Unfortunately though, once you get a hole and the blast media gets under the copper it will peel it up very quickly.  To prevent this I stop once I see the bubbling and then finish the area mechanically.

Finishing Steps

After removing the solder mask you will need to neutralize any alkali that has now been exposed, complete our repairs and seal all bare copper.  See the references for more info on this topic.