Chapter 64: Copper Foil and Tolerance

06:30. Dormitory. The sky was still dark. A thin layer of frost clung to the windowpane. Lin Chen opened his eyes. He did not get up at once. First, he checked his left foot. Pale yellow tissue fluid had seeped through the edge of the gauze. The scab felt tight. The pain was dull. Manageable. He sat up straight. His back pressed against the wooden board. His breathing remained steady. In his mind, a timeline unfolded automatically.

Five hours and thirty minutes remained until the submission of the high-frequency probe and shielding box proposal. Nine hours and thirty minutes remained until the budget form had to be stamped and archived. Eleven hours and thirty minutes remained until the technical report required both signatures. His available cash was fourteen yuan and seven jiao. Estimated material cost: eighty yuan. Shortfall: sixty-five yuan and three jiao.

He threw back the blanket. Put on his shoes. Tied the laces. His weight settled on his right foot. He tested his left foot in the air before lowering it. The pain turned sharp the moment it touched the ground. He paused for three seconds. Adjusted his stride. Thirty centimeters. Avoid the frost line. He pushed open the door. The corridor was empty. The terrazzo floor reflected a cold sheen.

07:10. Behind the engineering building. Scrap recycling station. The iron gate stood half open. Old Sun wore a military coat and sat by a coal stove warming his hands. An aluminum kettle rested on top of the stove. The water was just on the verge of boiling. Lin Chen handed over the list. Old Sun scanned it once and frowned.

“Aluminum sheet. Copper mesh. BNC sockets. Variable capacitors. You’re building a high-frequency probe. No new parts in storage. Only salvaged junk from old machines. Sold by weight, not by piece.”

“By weight is fine. I only need specific specs.” Lin Chen crouched down and rummaged through the wooden crates. His fingers touched cold metal. He picked out the shell of a discarded oscilloscope. Aluminum alloy. One-point-five millimeters thick. No deformation. He selected half a roll of purple copper shielding mesh. Forty mesh count. Wire diameter zero-point-one millimeters. He picked out three broken BNC connectors. The center pins were intact. The insulators had no cracks. Finally, from a tangle of miscellaneous coils, he pulled out two ceramic trimmer capacitors. Capacitance range: five to thirty picofarads.

Old Sun brought over the scale and weighed everything. Aluminum sheet: one-point-two kilograms. Copper mesh: zero-point-four kilograms. Loose parts together: zero-point-eight kilograms. Total: two-point-four kilograms. At the scrap recovery rate for old parts, four yuan and five jiao per kilogram. Eleven yuan and three jiao.

“Cash or on account?” Old Sun asked.

“Cash. Also—” Lin Chen pulled a copper-clad board from the bottom of his canvas bag. Its edges were neatly cut. The solder joints were full. No oxidation. “The MF47 multimeter I repaired yesterday. Calibrated. Error under two percent. Count it as two yuan.”

Old Sun took it, inserted a battery, switched the dial, and watched the needle return to zero. He turned the knob. The damping was even. He nodded. “Worth two yuan. You pay nine-point-three. For the rest, go to the school-run factory sales office. Find Old Zhou. Tell him my name. He’ll let you buy on credit. Settle at the end of the month.”

Lin Chen pocketed the change. Nine-point-three plus fourteen-point-seven made twenty-four yuan. He was still short by fifty-six yuan and three jiao. He noted it down. Turned away. His stride unchanged.

08:00. School-run factory sales office. Glass counter. Coated with dust. Old Zhou wore reading glasses and was checking the ledgers. Lin Chen handed over the list. Old Zhou looked up and sized him up. Dark circles. Chilblains. But a calm gaze.

“First place in the provincial competition advanced group. Old Li mentioned you.” Old Zhou set down his pen. “A self-made high-frequency probe. Did you bring the drawings?”

“They’re in my bag. The parameters are already calculated.” Lin Chen pulled out a sheet of graph paper and spread it over the counter. Pencil lines. Clear. Fully labeled. Bandwidth: one hundred megahertz. Input impedance: ten megaohms. Adjustable compensation capacitance range. Ground terminal position. Conductive foam lining thickness inside the shielding box. Tolerances marked down to zero-point-one millimeters.

Old Zhou studied the drawing. His finger traced along the lines and stopped at the compensation network. “Here. You didn’t include parasitic capacitance. The distributed capacitance from the BNC socket to the probe tip is at least three picofarads. Your trimmer capacitor bottoms out at five picofarads. The compensation will overshoot. The waveform will peak at the high-frequency end.”

Lin Chen said nothing. Three seconds. He pulled out his pencil and rapidly recalculated in the margin. Distributed capacitance. Lead inductance. Equivalent circuit at the probe tip. He crossed out the original values and rewrote them. Lower limit of the trimmer capacitor changed to two picofarads. Add a fixed capacitor in parallel to offset the distributed capacitance. Recalculate the compensation time constant.

“It’s corrected.” He pushed the drawing back.

Old Zhou nodded. “Drawing approved. Student rate for materials. Aluminum sheet. Copper mesh. BNC socket. Trimmer capacitor. Enameled wire. Insulating sleeve. Total: fifty-four yuan and eight jiao. You pay twenty-four. Remaining balance: thirty yuan and eight jiao. It goes on your work-study account. Deducted next month from your lab stipend. Overdue, and you lose access to consumables.”

“Understood.” Lin Chen signed and pressed his fingerprint down. The paper was rough. The ink was still wet. He took the bag of materials. It was heavy, but its boundaries were clear.

09:30. Library. Third floor. Electronics engineering section. The shelves were old. The spines faded. Lin Chen pulled out High-Frequency Circuit Design, Principles of Electromagnetic Compatibility, and Oscilloscope Probe Technical Manual. He spread them open across the desk, laid out fresh graph paper, and began drawing the formal process instructions.

The pen tip rasped softly over the paper. He drew the unfolded layout of the shielding box. Marked bending lines. Drilling positions. The routing path for the grounding copper foil. He calculated springback in the aluminum bends and reserved a zero-point-two millimeter compensation. He marked the sequence for applying the conductive foam: line the inside first, then press the edges, then secure the ground terminal. He wrote the assembly steps. One: cutting. Two: deburring. Three: bending. Four: drilling. Five: applying foam. Six: soldering the ground wire. Seven: continuity testing. Eight: high-frequency response calibration.

Every step came with tolerance requirements. Every step came with inspection tools. Every step came with a procedure for handling defects.

11:15. Proposal bound. Held together with paper clips. Pages flat. No creases. Lin Chen stood up. The stabbing pain in his left foot intensified. He steadied himself against the wall and moved forward slowly, making no sound.

11:40. Old Li’s office. The door was ajar. Lin Chen knocked. Entered. Handed over the document pouch. Old Li took it, opened it, read the drawing, read the process instructions, read the parameter calculations. His finger stopped at the correction for distributed capacitance.

“You added the parasitic capacitance,” Old Li said, looking up. “Grounding for the shielding box. Single-point or multi-point?”

“Single-point. Tied to the signal source ground. It avoids power-frequency interference caused by ground loops.”

“How much bend tolerance did you leave?”

“Zero-point-two millimeters. Compensation for aluminum springback.”

Old Li closed the file, stamped it, and handed it back. “Proposal approved. Bring the physical build to the lab tomorrow. Joint debugging in the afternoon. If it fails, you’ll be replaced.”

“Understood.”

14:00. Academic affairs office. Duty room. Lin Chen submitted the budget form. Source explanation. Attachments complete. The duty teacher checked the figures. The stamps. The signatures. The closed-loop logic. Then stamped it and handed it back. The red ink on the edge of the paper was still damp.

16:00. Laboratory corridor. Chen Hao leaned against the window holding a draft of the technical report. When he saw Lin Chen, he straightened.

“I’ve reviewed the drawings. You’re responsible for the probe section. I’ll follow up on the shielding box and load matching. Double signatures.” Chen Hao held out a pen.

Lin Chen took it and read the report. The division of work was explicit. Responsibility boundaries were clear. He signed. His handwriting was neat, no cursive connections. Chen Hao signed too, his pen strokes sharp and decisive.

“I’ll cover half the materials cost,” Chen Hao said. “For the provincial competition travel, my family can approve the money. It’s not a big deal.”

“No need.” Lin Chen put away the report. “The account is already balanced. Work-study deduction. Settled at month’s end. Your part—build it according to the drawing. Don’t exceed tolerance.”

Chen Hao nodded and did not insist. The two of them looked at each other for three seconds. No extra words. Then they turned away and left in opposite directions.

17:30. Dormitory. The desktop cleared. Lin Chen spread out insulating paper and laid down the materials. Aluminum sheet. Copper mesh. BNC socket. Capacitors. Enameled wire. Insulating sleeve. He took a steel ruler and a scribing needle and marked lines on the aluminum. The depth was even. No slant. He took shears and cut along the lines. The edges came out neat. He took a file and deburred them. Light motions. No cut hands. Fine aluminum shavings fell in glittering fragments.

He bent the metal along the lines. Ninety degrees. Close fit. Flat. He drilled holes with a hand drill, turning slowly, never letting the bit wander. Hole positions aligned. Ground terminal, M4. Threads smooth. He applied the conductive foam, peeling the backing first, aligning the edges, pressing it flat. No bubbles. He soldered the ground wire. Copper mesh. Overlap joint. Soldering iron heats. Solder fed in. Withdraw. Full joint. No cold solder. He tested continuity. Multimeter. The beeper sounded. Resistance: zero-point-one ohms. Up to standard.

19:00. Probe assembly. BNC socket. Center pin. Soldered. Insulating sleeve. Heat-shrunk. Heated. Tightened close. He connected the enameled wire and wound the compensation coil. Twelve turns. Even spacing. He connected the trimmer capacitor and a fixed capacitor in parallel, matching the calculated value. He installed them into the housing. Secured them. Tightened the screws. No stripped threads.

He connected the oscilloscope. Signal generator. Ten megahertz. Square wave. Amplitude: one hundred millivolts. He looked at the screen. The waveform. Rising edge. Slight upward flare. Overshoot: eight percent. Out of tolerance.

He stopped. Did not panic. Switched paths. The actual parasitic capacitance was higher than estimated. Coupling from the distributed inductance of the leads. He removed the probe and recalculated the compensation network, the time constant, the adjustment. He replaced the fixed capacitor with a smaller value. Resoldered. Tested again. Waveform. Smooth rising edge. Overshoot: three percent. Within spec.

He recorded the data. The sheet lay flat. The handwriting was neat.

20:30. A letter slid through the crack under the door. Kraft paper. Worn edges. Postmark: Qinghe County. Sender: Wang Guiying.

Lin Chen opened it. The paper was thin. The handwriting crooked. Written in pencil.

“Chen. Xing. Last night. Another fit. Half an hour. The village doctor said the medicine is almost out. The township clinic has none in stock. Must wait till next month for the county to send more. You focus on your studies. The family can still bear it. Don’t worry.”

He finished reading. Folded it carefully. Put it into his pocket. His fingers touched the ledger there too. Its edges curled. The paper was rough against his skin.

He sat upright and looked out the window. The night was deep. No stars. Wind slipped through the gap in the frame with a low whistle.

He took out graph paper and drew a new set of axes. Horizontal axis: time. Vertical axis: parameters. He marked the probe calibration curve. The shielding box grounding impedance. The load matching points. He connected the lines. Smooth. No steps.

He stood up. Put on his shoes. Tied the laces. Shifted his weight to the right. Stride: thirty centimeters. Do not step on the frost line.

Ahead, the lab light was still on. Old Li was waiting for the joint debugging.

He pushed the door open. Went out. Wind rushed in. Cold.

He closed his eyes. In his mind there were no curves. Only solder joints. Temperature. Time. Molten tin. Solidification. Three lines crossing in the dark without colliding, without tangling, each moving forward on its own.

He opened his eyes. Stride: thirty centimeters. Do not step on the frost line.

Forward.