Contact Force in Stamped Parts: Normal Force, Wipe and Reliability
Short answer: contact force in a stamped contact is the normal force the spring exerts at the mating point — typically 0.3–1.5 N for signal pins, 1–5 N for power contacts, and 3–10 N for board-to-board or battery terminals. It must be high enough to break through oxide and contamination, low enough to avoid wear, and stable across temperature and cycles. Wipe — controlled sliding during mating — scrubs the interface clean. An ISO9001 source factory designs the strip temper, contact geometry and plating together so the force you measure at first article is the force you still have at 5,000 cycles.
A contact is not a metal touching metal; it is a controlled spring. Every electrical and signal connection in a connector, terminal or battery holder lives or dies by how much force that spring applies, where it applies it, and how long it holds. Get the force wrong and the part fails in the field for reasons no one can see.
This guide covers the design variables that decide contact reliability: normal force, wipe, contact geometry, material and plating, and how to verify all of it before shipping.
What Is Contact Force and Why Does It Decide Reliability?
Contact force — engineers often say normal force — is the perpendicular load at the point where two surfaces touch. It serves three jobs: it breaks through the oxide and contamination film on the surfaces, it keeps the true contact area stable, and it resists vibration that would otherwise cause intermittent opens.
Too little force and the connection is intermittent. Too much force and the contact wears, the plating wears through, and mating force becomes high enough to damage the housing. The engineering window between those two failures is narrower than most designers expect, and it is why contact force belongs in the drawing, not in a guess.
Normal Force: The Number That Keeps a Contact Clean
Every metal surface carries an oxide film in air. Contact force has to deform the contact asperities, penetrate that film and create a clean metal-to-metal path. Below roughly 0.2 N, many plated contacts never reliably break through; above about 10 N you start trading reliability for wear.
| Application | Typical normal force | Why |
|---|---|---|
| Signal / fine-pitch pins | 0.3–1.5 N | Low mating force, many pins, gold plate |
| Power contacts | 1–5 N | Higher current, tin plating needs more wipe |
| Battery terminals | 3–10 N | Shock and vibration, low-resistance path |
| Board-to-board | 0.5–2 N per contact | Arrayed, matched to housing |
| Grounding clips | 2–8 N | Must stay bonded under vibration |
These bands are starting points, not rules. The right number depends on plating, expected vibration, mating cycles and the current the joint must carry. A tin-plated power contact at 2 N may open under vibration; the same contact at 4 N will not.
Wipe and Contact Geometry: Movement You Must Design In
Wipe is the small sliding motion that occurs as a contact mates. It is not a defect; it is a feature. Wipe scrubs away oxide and debris, exposing fresh metal exactly where the two surfaces will settle. A contact with force but no wipe can still fail, because it presses contaminated film together instead of clearing it.
Design for roughly 0.2–0.5 mm of wipe on a signal contact so there is meaningful scrubbing without excessive wear. The geometry that produces wipe is usually a curved beam — a contact that looks like a bow or a bent cantilever. Flat-to-flat contacts wipe poorly; add a crown or dome to concentrate the force and control the sliding path.
Tuning wipe and force at the same time is why contact design is iterative. Change the beam radius and you change both the force curve and the wipe distance. If you want the spring mechanics behind this, our stamped terminal design guide covers beam force and deflection math.
Hertz Contact Stress and Why the Contact Radius Matters
Force by itself says nothing about pressure. A sharp point concentrates force into a tiny area and yields the surface; a broad dome spreads it and may not break the film. The quantity that governs this is Hertz contact stress, roughly the force divided over the real contact area, and it needs to sit above the film-breaking threshold while staying below the material yield point.
Two practical levers. First, control the contact radius: a small radius raises stress and improves film breaking but accelerates wear; a generous radius spreads load and lengthens life. Second, keep the contact point away from edges, where stress concentrates and plating is often thinnest. Good stamped contacts put the working point in the middle of a formed dome, not on a sheared edge.
How Plating Interacts with Contact Force
Plating and contact force are one system. A soft plating deforms under force, increasing real contact area; a hard, brittle plating can crack and expose the substrate. Choose them together.
| Material | Typical plating | Force behavior |
|---|---|---|
| Phosphor bronze | Gold 0.5–1.5 µm | Stable force, low resistance, premium |
| Beryllium copper | Gold flash over nickel | High force retention, best spring |
| Copper alloy | Tin 3–8 µm | Higher force needed for film break |
| Brass | Tin or nickel | Cheap, lower spring strength |
| Stainless steel | Gold or nickel | High force, higher resistance |
For a deeper look at matching substrate and coating, our contact plating guide explains gold, tin and nickel behavior in stamped contacts, and our take on shrapnel and contact materials covers the substrate side.
How Do You Measure and Verify Contact Force?
Never accept a contact design that has not been force-tested. Three checks catch the vast majority of problems.
First-article force measurement. A force gauge or load cell pushes the contact to its working deflection and records the force. Do this on a sample from the actual die, not a prototype blank.
Spring-rate check. Measure force at two deflections to confirm the beam behaves linearly. A nonlinear curve signals yielding, a bad radius or a material problem.
Life cycling. Mate and unmate thousands of times and re-measure force. A well-designed contact loses very little force over 5,000 cycles; a bad one relaxes quickly. For the mechanics of relaxation versus fatigue, see our spring relaxation guide.
Design Rules for Stamped Contact Springs
| Rule | Target | Reason |
|---|---|---|
| Force window | 0.3–5 N typical | Break film without wear |
| Wipe | 0.2–0.5 mm | Scrub oxides on mate |
| Beam stress | Below 60% yield | Avoid relaxation |
| Contact point | On dome, off edge | Stable stress, thick plate |
| Plating thickness | 0.5–1.5 µm Au, 3–8 µm Sn | Balance cost and life |
Follow these and a stamped contact will behave predictably for the life of the product. Ignore them and you will meet the field-failure mode that is hardest to diagnose: an intermittent open that only appears under vibration, at temperature, after months in service.
Frequently Asked Questions
Q: What is a good contact force for a stamped contact?
A: It depends on the job. Signal pins usually want 0.3–1.5 N, power contacts 1–5 N and battery terminals 3–10 N. The rule is the lowest force that reliably breaks the oxide film and stays stable under vibration. Ask for a measured force value on the first article rather than a design target.
Q: What is wipe and why does it matter?
A: Wipe is the small sliding motion as two contacts mate, typically 0.2–0.5 mm on a signal contact. It scrubs away oxide and contamination, exposing fresh metal where the surfaces settle. A contact with force but no wipe can still fail intermittently, because it presses contaminated surfaces together.
Q: Can I increase contact force just by thickening the material?
A: Thicker or harder strip raises force, but it also raises mating force and can over-stress the housing. The cheaper levers are the beam geometry — shorter, narrower or more curved — and the material temper. Change geometry first, then pay for higher-grade material only if the design truly needs it.
Q: How does plating choice affect contact force requirements?
A: Gold needs less force because it does not oxidize, so fine-pitch gold contacts run at low force. Tin forms a harder oxide and needs more force and wipe to break through, so tin contacts are usually specified with higher force and deeper wipe. Match the force target to the plating.
Q: How do you verify contact force in production?
A: We measure first-article force with a load cell on parts from the production die, check the spring rate at two deflections, and run life cycling to confirm force retention. Send the drawing with your force and cycle targets to sc@bquq.com, and we will quote a stamped contact that meets them within 12 working hours.
Related Resources
- Stamped terminal design guide: beam force, deflection and geometry for stamped contacts.
- Stamped terminals and contacts: progressive-die contacts, terminals and clips with controlled force.
- About BQUQ: an ISO9001 source factory in Dongguan running stamping, CNC, springs and heat sinks under one roof.
- Contact BQUQ: send your drawing and get a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
