How to Avoid Design Problems in Photo Etching
![photo etched microelectronics leadframe](https://blog.conardcorp.com/hs-fs/hub/239673/file-21536654-jpg/images/img_4603-resized-600.jpg?width=150&height=160&name=img_4603-resized-600.jpg)
The biggest design problem is that the design requirements for photo etching are not widely understood. The five most common etching design problems are:
1. Choosing metal that is thicker than needed
Specifying material thickness in excess of what the application requires. It is not uncommon to see drawings asking for much heavier gauges of metal than the parts function may require. Frequent situations include parts that do not have functional or structural roles being designed with 18 or 20 gauge steel because the designer thought it was common. Heavier gauges are common in sheet metal applications, but are not suited to photo etching. In stainless or carbon steel, 20-gauge (about .036) is the upper limit. In aluminum sheet, the photo etching process can handle up to 12-gauge (.080). In copper sheet, the etching limit would be 14-gauge (.065). Understand that with every mil (.001) of thickness comes another fixed increment of etching time. Use the lightest material suitable for the process.
2. Making tolerances tighter than needed
Although the metal etching process can generally achieve +/-15% of the metal thickness, tighter tolerances affect costs. +/-.0015 is the minimum absolute tolerance and can only be achieved if the metal is .010 thick or less. +/-.005 is suitable for metal thicknesses up to .030. Tolerances for thicker materials will be +/-20% of the metal thickness. The tighter the tolerance band, the smaller the sheet size, and the higher the costs. The economies of photo etching are best realized by being as generous as possible with tolerances. Location tolerances are defined by the CAD data and will consistently be +/-.001 to drawing nominal.
3. Designing features (holes, slots, ligaments) that are too small for the metal thickness
Metal thickness also defines minimum feature sizes in photo etching. The minimum dimension for etching holes is at least 110% of metal thickness. The photo etching process uses etching solution to dissolve the metal to be removed. In order to etch all the way through the metal, there needs to be enough space for the etchant to flow. Similarly, the land area between holes needs to be at least equal to the metal thickness. The reason here is that the etchant will undercut the photo resist and cause the land area to be etched away.
4. Ignoring industry standards for metal gauges and tolerances
Another problem we encounter with customer designs is lack of attention to industry standards with regard to raw material. Strip metals are generally rolled to a thickness tolerance of +/- 10% of the nominal. Precision stainless strip is available at +/-5% of nominal. Drawing call outs that are tighter than industry norms can lead to a lot of excess cost. Many alloys can be precision rolled to +/-.0001 on thickness, but expect to pay upwards of $1000 per pound and wait 12 weeks for it. There is no industry standard for flatness of rolled metals, even for tension leveled material. This applies to all rolled metals, not just for photo etching.
5. Not checking metal availability before specifying
In the wake of the 2008/2009 economic downturn, the metal making industries sharply "rationalized" their production. Gone were the days of running 50,000 pounds of a particular alloy or gauge that would take 20 years to sell through. That was a pretty painful lesson for many.
In the wake of the 2020 pandemic, the further "rationalization" of production plans and inventories has narrowed the product selection even further.
There are many more alloys covered by standards than are being made. In the past 5 years alone, the stock situation for raw materials has been significantly rationalized both at the distribution levels and at the mills.
The mills will still make whatever you want, but you won't like the cost or the minimum weight. It becomes a major challenge when a legacy part for a prime contractor is calling for what we call "un-obtainium." Before specifying an alloy based on its data sheet, contact us to see if it's really available.
"When you want to stop digging a hole; the first rule is to put down the shovel."--Anon
These are the five most frequent problems we see with new designs being considered for photo etching. One of the reasons we're here is to answer those questions before they become specifications.
The simple solution: Talk to us first!