Alloy Junction 1952 Mount and Inspect the Assembly

# Mount and Inspect the Assembly: A New Kind of Isolation Risk

Step 36 confirmed the bare device passed isolation, rectification, and transistor action on the bench, with nothing else nearby to short against. Mounting the verified device into its package header changes that picture entirely: the three leads Steps 33 through 35 attached now have to be routed, bent, and secured near a metal case and near each other, and the mounting process itself can reintroduce exactly the kind of short Step 36 just certified absent — not through any defect in the device, but through insufficient physical clearance introduced by the mount itself. This step's real job is verifying that the terminal separation achieved during mounting is enough to survive the rest of this device's life, not just enough to pass inspection on the bench today.

## 1. Clearance Has to Survive Tolerance Stack-Up and Thermal Expansion, Not Just Today's Measurement

A terminal separation measured right after mounting looks adequate today, but two separate effects can close that gap over time. The first is simple manufacturing tolerance stack-up — the same root-sum-square combination Step 17 used for disk-alignment error — applied here to however many individual positioning tolerances compound between the lead-forming operation and the header's own hole spacing. The second is thermal: the lead material and the header material have different coefficients of thermal expansion, the same CTE mismatch that drove Step 15's interfacial stress concern, except here it drives a dimensional change in clearance rather than a stress at a bonded interface. The real design requirement has to account for both, leaving a true minimum separation of:

$$ d_{\text{min}} \;=\; d_{\text{nominal}} \;-\; \sqrt{\sum_i \delta_i^{2}} \;-\; L\,|\alpha_{\text{lead}} - \alpha_{\text{header}}|\,\Delta T $$

where $d_{\text{nominal}}$ is the as-designed separation, $\delta_i$ are the individual positioning tolerances feeding into the mount, $L$ is the lead length exposed to the thermal swing, and $\Delta T$ is the full temperature range this device will see across its qualification and service life. A clearance that looks fine at room temperature immediately after mounting can still close dangerously at a later temperature extreme if this margin was never actually checked.

## 2. Real Diagram: Where the Margin Actually Goes

Nominal Clearance Minus Two Separate Risks Tolerance stack-up and thermal expansion both eat into the same gap d_nominal, as-designed terminal separation tolerance stack-up RSS of mount positions CTE mismatch over full Delta T d_min, what actually remains

## 3. Clearance Also Has to Satisfy an Electrical Requirement, Not Just a Physical One

Even a mechanically stable clearance that survives tolerance stack-up and thermal cycling is only sufficient if it is also wide enough to withstand the voltages this device will actually see in service without arcing or leakage across the gap. A simple air-gap voltage-withstand criterion sets a second, independent lower bound on the same separation distance:

$$ V_{\text{bd}} \;\approx\; E_{\text{bd}}\,d $$

where $E_{\text{bd}}$ is the breakdown field strength of the medium between the terminals (air, or whatever fills the package interior before Step 38's encapsulation) and $d$ is the actual clearance. This step's real check is therefore not a single number but two independent requirements on the same measured distance — a mechanical one from Section 1, protecting against a short developing over time, and an electrical one from this equation, protecting against a short happening instantly at the device's rated operating voltage — and the as-mounted clearance has to clear both simultaneously, not just whichever one happens to be checked first.

## Real Diagram: Two Independent Floors on the Same Measurement

Mechanical Floor vs. Electrical Floor on the Same Gap The measured clearance must clear whichever line sits higher operating temperature or rated voltage required clearance mechanical floor, d_min(T) electrical floor, V_bd / E_bd as-mounted clearance

## Mount and Inspect the Assembly's Place in the Process Lineage

Mount and Inspect the Assembly follows Step 36, Check the Contacted Device, which certified the bare device electrically sound before any of this step's mechanical handling began; it precedes Step 38, Encapsulate or Enclose, which will seal in whatever clearance this step actually verified, for good. It is the first step of Phase 5 and the point where this series' running concern with isolation — tested electrically at the bench in Step 36, and protected structurally against a stray surface bridge since Step 30 — becomes a geometric and mechanical property of the finished package instead, one that this step's two independent requirements, mechanical durability and electrical withstand, both have to confirm before anything gets sealed shut around it.

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