For engineers working with mission-critical parts, boreholes, inner diameters (IDs), and tapped holes are often where performance lives or dies. But when it comes to plating these internal features, even small design decisions can have outsized consequences for quality, cost, and timeline.
Electro-Spec has been helping engineers solve ID-based plating challenges for decades. Whether it's poor coverage, high rejection rates, or unexpected failures in the field, we've seen how hard it is to get consistent results, especially if design for plating isn’t factored in from the beginning.
This article unpacks what engineers need to know to design bore-intensive parts for plating success.
Why are deep inner diameters and boreholes so difficult to plate?
At first glance, an internal thread or bore doesn’t seem like a plating risk. But the physics and chemistry of plating tell a different story.
1. Reduced Plating Thickness Inside the Bore
Electroplating is a current-driven process. Inner diameters naturally receive less electrical current than outer surfaces, often only 50% as much. That means a surface that appears fully coated may be under spec where it counts, leading to:
- Poor corrosion resistance
- Soldering failures
- Long-term reliability issues
To compensate, platers will often overplate the rest of the part just to hit spec in the ID, risking oversized outer diameters and increasing rework or scrap rates.
2. Air and Solution Trapping
If the bore is too deep, narrow, or closed off, it can trap air or chemistry during plating or rinsing. This causes:
- Voids or stains inside the bore
- Uneven adhesion
- Field failures
Our team recommends designing weep holes at the bottom of bores to allow trapped air and solution to escape. This simple addition can drastically improve plating uniformity.
3. Cleaning and Rinsing Complications
Cleaning is critical before plating, but bore geometries can trap:
- Cutting oils from machining
- Particles and residues
- Cleaning chemicals
If these contaminants aren't fully flushed out, they can react during plating, causing pitting, poor adhesion, or complete failure. As one of our engineers puts it: “If you can’t rinse it clean, you can’t plate it right.”
4. Tolerance Issues (That Can’t Be Fixed in Post-Processing)
Bore features are often specified with tight tolerances. But if your drawing calls for something like +0.000/-0.000 and the part gets plated—even lightly—it will be out of spec.
We’ve seen customers reject thousands of parts due to tight ID tolerances that didn’t account for even minimal plating buildup. That’s expensive, and usually preventable.
The “Solo Cup” Effect: Nesting and Locking During Barrel Plating
When bore diameters are too close to their mating components, small parts can nest or lock together during barrel plating. Our team calls this the “Solo cup effect,” where parts stack like disposable cups and stick together.
If they can't be separated without damage, the parts must be scrapped entirely. This isn’t a rare event; it’s a common reason for the failure of bore-based designs.
Recommended Processes for Bore and ID Plating
Electro-Spec applies several specialty processes to overcome these challenges. Here’s how we recommend matching your specified process to the part geometry:
Spouted Bed Electrode (SBE) Plating
Best For: Small parts with deep bores, counterbores, or complex IDs.
Why it Works:
- Fluidized bed plating prevents parts from nesting
- Ultrasonic action and continuous flow ensure even plating, even inside bore walls
- Ideal for connectors, pins, and tight-tolerance parts where ODs and IDs both matter
Rack Plating with Custom Fixtures
Best For: Midsized parts with controlled internal features
Why it Works:
- Precise control over part orientation
- Reduces rthe isk of trapped air and uneven current
- Fixtures can be customized to ensure optimal coverage in ID zones
Design Guidelines for Engineers: What to Do Differently
To design bore-intensive parts for plating success, Electro-Spec recommends:
1. Add Weep Holes
These allow trapped air and solution to escape, improving rinse and plating uniformity.
2. Adjust Tolerances for Plating Growth
Work with your plater to determine realistic post-plate tolerances and machine to the low end of spec if necessary.
3. Use Thread Forms That Tolerate Plating
Plating buildup affects thread pitch and diameter. Avoid zero-clearance threads unless you plan to re-machine or chase them afterward.
4. Communicate Application Requirements
Let us know:
- What surface is functional?
- Will the part be soldered?
- Is there salt spray or high-frequency exposure?
Even if you provide a spec, we can help optimize the process to avoid overengineering or underperforming.
Want to talk about your application?
We specialize in helping engineers get better plating results on hard-to-plate parts. We’ve seen the most common failure modes and can advise you on the smartest ways to avoid them. Talk to an expert today!
