Flagship smartphone vapor chamber — 3.4°C lower skin temperature
A 0.35mm vapor chamber designed for an ultra-thin model where heat pipes could not fit, carried through to qualified production process conditions.
- Challenge
- Eight weeks from design freeze, rear skin temperature exceeded the internal limit by 2.8°C under sustained gaming load
- Approach
- Simulation identified spreading resistance as the dominant term; a thin local vapor chamber was applied only where spreading was needed
- Result
- 3.4°C lower skin temperature, 2.1× longer sustained performance
- Duration
- 11 weeks
Situation
The customer found the thermal issue eight weeks before design freeze. Twenty minutes into a sustained gaming load, rear skin temperature exceeded their internal limit by 2.8°C, and AP throttling began at the six-minute mark — well short of the target frame rate.
Two layers of graphite sheet and a TIM gap pad were already in the stack, and the mechanical design was days away from tool release. Very little design freedom remained.
Root cause
We began by decomposing the existing thermal resistance path by path. Detailed simulation showed more than half of the total sat in one place: spreading from the AP package top surface into the graphite layer.
| Path segment | Share of resistance |
|---|---|
| AP junction → package top | 18% |
| Package top → spreader (spreading resistance) | 54% |
| Spreader → rear cover | 21% |
| Cover → ambient | 7% |
Adding another graphite layer does not change spreading resistance itself, so the payoff is small. The customer had in fact already tried exactly that: 0.4°C improvement, matching our predicted 0.5°C.
Approach
Reducing spreading resistance directly required a layer with far higher effective conductivity. The remaining thickness budget was 0.4mm.
- A full-area vapor chamber exceeded both thickness and cost targets.
- A local vapor chamber — 0.35mm, covering only the AP and the region where spreading actually mattered — fit.
For the wick we chose stacked mesh over sintered powder. At 0.35mm total thickness, a sintered wick struggles to achieve usable effective thickness and carries larger production variation. Working fluid charge was settled over three prototype iterations guided by simulation.
Verification
Twelve prototypes were built and characterized for thermal resistance and Q-max.
- Prediction vs. measurement: within 1.2°C at junction
- Unit-to-unit spread: ±0.6°C at skin
- Anti-gravity tilt (5mm adverse): under 4% performance loss
Early units showed larger spread than expected, traced to insufficient vacuum evacuation time. Standardizing the evacuation profile halved the variation.
Transfer to production
Because performance reproducibility depends heavily on process, we defined the following as controlled characteristics with capability targets:
- Vacuum evacuation pressure and hold time
- Working fluid charge (by mass, with upper and lower limits)
- Seal strength and helium leak criteria
- Assembly contact pressure and TIM dispense volume
Across the first 3,000-unit production run, thermal-related defect rate was 0.8%, most of it seal leakage. After re-tuning the sealing conditions it stabilized near 0.2% by the third month.
Outcome
- Rear skin temperature down 3.4°C — 0.6°C of margin against the limit
- Sustained performance 2.1× longer (6 min → 12.6 min)
- Design freeze date held
Figures published with customer consent. Specific structures and process conditions can be shared separately under NDA.