Saturday, December 31, 2011

Multiple Lithography in Homemade PCBs

Earlier this week I decided it was about time to try making a board for some of the 24AA16 EEPROMs I had sampled from Microchip a year ago. It's a 16kbit I2C EEPROM with five pins: power, ground, I2C data and clock, and write protect.

Five pin package - piece of cake, right? But, just to add to the fun, the package I picked was CSBGA with balls about 250μm apart!

From the packaging specification, it can be seen that the balls are 150 μm diameter and spaced in a 2x2 grid 570μm x 520μm with one more in the center. This is a little smaller than my laser-printer contact lithography process can comfortably resolve. What to do?

Conveniently I have a metallurgical microscope that I've managed to coax into service as a projection lithography system. The field of view is, however, far too small to do an entire PCB.

After a little thinking I decided to try a multiple-exposure technique. The first step was to design a board layout in ExpressPCB (my preferred CAD tool is kicad but Express is a little easier for super simple layouts) with a 4-pin SIL header going to a rectangle of copper a little bigger than the CSP footprint. I also made a second mask containing the BGA footprint and tracks going out to four large pads, and printed it at 4x actual size. The center ball is WP# so I tied it to Vdd rather than breaking out to a separate pin.

Mask design

I then printed a mask on my printer, exposed onto precoated PCB, developed (1% w/v NaOH in distilled water), and etched (6 parts 3% H2O2 : 1 part 32% HCl at low heat) as with my standard PCB process.

The next step was to strip the existing photoresist since it had been exposed to light during the etch process. A few drops of acetone did the trick nicely.

I then spin-coated the board with fresh photoresist, using my standard mixture (Shipley SP24 photoresist diluted 50% v/v with acetone for a thinner layer), soft baked on a hot plate, and exposed the BGA mask onto the copper rectangle. After developing, this was the result:

Second photomask on top of etched metal1

Closer view showing edge quality
Not surprisingly, the resolution and edge roughness were vastly better than the contact lithography process. (I've scaled the same technique to 20 μm half-pitch on silicon and there's room to go a lot further.) In retrospect the traces were a little too small considering that the copper layer they're sitting on is 35 μm thick, but this was a mask design error and not a process issue.

Since the thin photoresist I use is harder to see on copper than the thick stuff the board came coated with, I tossed it in the etchant for a couple of seconds to make it more obvious what was being masked.

After a couple seconds in the etch bath

The copper pad was also a bit larger than it needed to be and exceeded the FOV of the lithography system (note the unwanted photoresist shorting the pads together). I gently scraped this away with a #11 scalpel blade under 30x magnification and briefly etched to confirm good separation.

Surgery time!

After etching, no shorts

I then etched for a couple of minutes and removed the board to see how it was doing.

Almost done etching
Things still looked very good, all traces were intact. For traces with such a high aspect ratio (about 40 μm wide in 35 μm thick copper) things looked surprisingly good, but it needed a little more time.

Overetched. (photoresist stripped before taking this pic)
Unfortunately I overetched, one of the traces was gone entirely and another was seriously damaged. Some residue was still in place between two of the pads. Perhaps better agitation would help?

In either case, had the traces been a little larger (perhaps 75 μm) or the copper a little thinner it would have worked beautifully. The lithography itself was flawless and even though the board was not usable it appears the technique is feasible. Given a mask respin this same board could be fabricated with little difficulty.

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