Point Contact Transistor Attach Leads Seal Housing
# Attach the Cartridge Leads and Seal the Housing: The Last Step That Can Still See What It Is Sealing In
Every step from Step 1 through Step 15 was inspectable. A defect could be caught, a measurement could be re-taken, a part could be rejected and the process restarted, because the assembly remained open to the outside world the entire time. Step 16 ends that. Once the housing is sealed, the two formed point contacts, the spring arms holding them at a spacing measured in thousandths of an inch, and the alloy regions Step 15's current pulse only just created become permanently inaccessible. This step is not simply packaging a finished device — it is the last point at which anything built across the previous fourteen steps can still be seen, checked, or corrected before it is closed away for the rest of the device's working life.
The cartridge leads add a third link to a resistance chain this series has already tracked twice. Step 6 minimized the spreading resistance of the base electrode by maximizing its contact area. Step 7 warned that the lead bonded to that electrode could undo Step 6's work if it added resistance of its own, because the two sit in series and the weaker link sets the total. The cartridge leads attached in this step are the next link in that same chain — the point where the internal connections built across Steps 5 and 6 finally reach the outside of the package, as terminals a circuit designer will actually solder to. A cartridge lead bonded carelessly can add resistance comparable to or larger than everything Steps 5 and 6 achieved, for the same reason a poorly bonded internal lead could have: the total resistance of a series path is never better than its weakest connection, no matter how many of the other connections were optimized.
The sealing process itself can disturb the one thing this device most recently, and most delicately, finished. Step 15's forming pulse created the alloy regions that give the point contacts any useful electrical behavior at all, and that result depends on the spring tension and point spacing remaining exactly where Step 13 and Step 14 left them. Sealing a housing typically involves some combination of heat, pressure, or a potting material curing around the assembly, and any of those can shift a spring's tension, nudge a point's position, or introduce contamination into a region Step 10 specifically worked to keep clean — undone not by a measurement error but by the packaging process meant only to protect what came before it. This is the same category of risk Step 14 identified when approaching the second point near the first, applied here to an entire sealing operation performed around both points at once, with far less room to notice a problem before it becomes permanent.
| Step | Process operation | Input | Output | Specification | Constraint |
|---|---|---|---|---|---|
| 16.1 | Verify device electrical characteristics one final time while the assembly is still open and inspectable | Formed, verified point-contact assembly from Step 15 | Final pre-seal verification result | Measured characteristics consistent with the forming result recorded in Step 15.6 | This is the last point in the sequence where a defect can be caught before the interior becomes permanently inaccessible |
| 16.2 | Attach the cartridge leads to the emitter, base, and collector connections | Final-verified assembly from 15.1 | Cartridge leads attached, all three terminals | Lead bond resistance small relative to the combined spreading and internal lead resistance from Steps 5 and 6 | A careless cartridge lead bond can erase the resistance work of two earlier steps, invisible to anyone measuring only the finished terminal |
| 16.3 | Measure total terminal-to-terminal resistance through all three links in the chain | Cartridge leads attached from 15.2 | Measured total resistance | Combined resistance within the device's overall operating specification | Measuring the cartridge lead in isolation cannot reveal a problem that only appears in combination with the earlier two links |
| 16.4 | Select a sealing method and parameters that will not disturb spring tension, point position, or surface cleanliness | Measured, verified assembly from 15.3 | Sealing process parameters | Process heat, pressure, and curing behavior documented not to exceed the tolerances Step 13 and Step 14 established | A sealing method chosen without reference to those tolerances risks undoing the most recently finished and most delicate work in the sequence |
| 16.5 | Seal the housing using the parameters from 15.4 | Sealing parameters from 15.4, verified assembly from 15.3 | Sealed cartridge | Housing sealed against moisture and mechanical disturbance, with no measurable change to electrical characteristics from 15.1/15.3 | A seal that protects the device from the outside world while silently degrading it from the inside has only addressed half the requirement |
| 16.6 | Perform a final electrical check through the sealed housing and record all results against the device's complete provenance | Sealed cartridge from 15.5 | Documented, sealed, finished device | Final measured characteristics compared against every verification recorded since Step 2.6 | Without this final comparison, a change introduced by sealing itself can only be discovered later, by a user, with no way to trace it back to this step |
Step 16 does not add a new electrical property to the device; it is the step that decides how much of the preceding fourteen steps' work the sealed device will actually keep. Every resistance value Steps 5 and 6 worked to minimize either survives through this step's cartridge lead or does not. Every spacing and tension value Steps 12, 13, and 14 established either survives the sealing process intact or does not. This step cannot improve on anything built earlier — its entire function is to close around that work without quietly undoing it, and to do so at the one moment in the sequence after which no later step will ever be able to check.