I. Definition of Defect Phenomenon
After vacuum brazing, the crests of triangular‑wave fins tilt to one side; fin columns are non‑perpendicular to separators. Localized large‑area fin collapse, dislocation of triangular apexes and uneven flow‑channel width occur. In severe cases, brazed joints between fins and separators peel off accompanied by cold solder joints, which reduces the effective flow cross‑section of the heat sink and leads to deterioration of heat‑exchange performance.
Structural characteristics of triangular fins: continuous triangular columns, classified as thin‑walled compression members. Compared with serrated fins, they feature weaker lateral anti‑buckling capacity and are highly susceptible to lateral buckling and tilting at high temperatures. Base material: 3003 aluminum alloy; cladding material: 4004 brazing sheet.
II. General Failure Mechanism
Aluminum alloy suffers drastic strength degradation at the brazing temperature range of 590‑605 °C. Non‑uniform thermal stress is generated throughout heating, holding and cooling cycles. Combined with insufficient lateral restraint, improper assembly clearance, unbalanced tooling pressure, dragging force induced by surface tension of liquid brazing filler metal and gravitational load, triangular fins undergo lateral plastic instability and retain permanent tilting after cooling.
III. Root‑Cause Analysis (Prioritized by Troubleshooting Sequence)
(1) Incoming Parts & Fin‑Related Factors
- Poor forming accuracy of triangular fins Out‑of‑tolerance fin wave height, asymmetric triangular apexes and wavy fin edges lead to inherently inclined fin columns after assembly. Such distortion cannot be self‑corrected under brazing high temperature and further aggravates tilting defects.
- Excessively thin fin foil & high height‑to‑thickness ratio Higher fins with thinner base foil deliver lower critical buckling load for compression members. Minor lateral force at elevated temperature will trigger fin collapse and tilting.
- Residual stamping stress on fins Without stress‑relief treatment after stamping, residual stress releases during heating and causes spontaneous twisting and tilting of fins.
(2) Core Stack Assembly Process (Most Frequent Inducement)
- Non‑uniform assembly clearance
- Excessive clearance: Poor contact between fins and separators results in insufficient lateral support at high temperature. Liquid brazing filler metal flows and generates lateral dragging force, tipping over fins.
- Insufficient clearance: Thermal expansion during heating squeezes fin crests and triggers lateral buckling of fin columns.
2. Dimensional deviation of sealing bars
‑ Effective height of sealing bars ＜ fin wave height: Tooling clamping force is fully exerted on fins, leading to compressive instability and lateral tilting. ‑ Inconsistent height of sealing bars: Localized stress concentration on core assembly and batch‑mode tilting of fins on one side.
3.Mis‑alignment and offset during manual stacking Triangular fins are mis‑positioned during core stacking. Mis‑registration between upper‑layer and lower‑layer fins generates extrusion and tilting under high temperature.
(3) Tooling Fixture & Clamping System
- Improper clamping pressure
‑ Over‑pressure: Aluminum matrix loses most strength at high temperature; triangular columns are bent and topple laterally.
‑ Insufficient pressure: Fins and separators lack effective constraint; flowing liquid brazing filler metal drives fin displacement.
2.Poor flatness of pressure plates & uneven pressure distribution
Deformed pressure plates or eccentric counterweight placement cause uneven pressure across core assembly, resulting in large‑scale fin tilting on the high‑pressure side.
3.Absence of lateral limiting structures
Conventional tooling only applies vertical compression without lateral stop bars. Triangular fins have no lateral restraint and are prone to lateral buckling.
Key distinction: Straight fins possess decent lateral stiffness; triangular‑wave fins
require dedicated lateral limiting structures.
4.Mismatched thermal expansion between tooling and aluminum components Stainless‑steel tooling features lower thermal expansion coefficient than aluminum alloy. Clamping status shifts during heating and induces abrupt local pressure fluctuation.
(4) Vacuum Brazing Thermal Profile
- Excessive heating rate
Large temperature gradient exists between surface and interior of core assembly.
Differential thermal expansion among components generates shear stress and tilts fins.
2.Excessive peak brazing temperature & prolonged holding time
Aluminum base matrix over‑softens with further deterioration of deformation resistance. Prolonged residence of liquid brazing alloy brings about flow scouring and dragging effect on fins.
3.Non‑uniform furnace temperature field
Components closer to heating elements heat up faster. Differential thermal expansion on both sides twists core assembly and drives fin tilting.
4.Improper cooling rate
Rapid gas‑quenching cooling creates sharp temperature difference, generating massive residual stress and deforming softened fins.
(5) Cleaning & Environmental Influences
- Inadequate cleaning leading to uneven wetting
Oil contamination and residual oxide film cause non‑uniform wetting of brazing filler metal. Local excessive spreading of liquid braze alloy creates asymmetric surface tension and pulls fins sideways.
- Workpiece placement and gravity effect
Vertical brazing orientation: Softened fins deform and topple under self‑weight. Large‑size heat sinks without intermediate support suffer higher risk of fin collapse in central zones.
IV. Systematic Improvement Solutions
- Incoming‑quality control
‑ Tighten tolerances for fin wave height and apex symmetry; adopt thicker foil for tall fins.
‑ Implement low‑temperature stress‑relief pre‑treatment for stamped fins to eliminate stamping‑induced internal stress.
2.Assembly process optimization
‑ Stabilize height tolerance of sealing bars to ensure sealing bars bear most clamping load while fins only transmit minor pressure.
‑ Apply standardized positioning fixtures for core stacking to eliminate manual mis‑alignment.
‑ Control assembly clearance to achieve uniform light contact between fins and separators.
3.Brazing tooling upgrade (Most effective countermeasure)
- Calibrate flatness of upper & lower pressure plates; place counterweights symmetrically to realize full‑area uniform pressure.
- Install lateral stop bars to restrict lateral displacement of triangular fins.
- Determine optimal clamping load; avoid over‑compression. Add intermediate auxiliary supports for large‑size cores to prevent central collapse and tilting.
- Brazing thermal profile optimization (General guidelines for aluminum vacuum brazing)
1.Reduce heating rate; add isothermal holding stages at 200 °C and 450 °C to minimize temperature gradient inside core assembly.
2.Operate peak temperature at lower limit (598‑602 °C); shorten holding duration to avoid excessive softening of base material.
3.Adopt gradient slow cooling instead of one‑shot high‑flow gas quenching.
4.Periodically verify furnace uniform‑temperature zone. Keep workpieces away from heating elements and reserve gaps between parts for thermal circulation.
5.Cleaning & braze‑material management
‑ Deploy stable ultrasonic degreasing plus pure‑water rinsing with full drying to eliminate surface contaminants.
6.Loading scheme optimization ‑ Prioritize horizontal brazing orientation. Avoid suspended single‑layer mounting for large heat sinks; apply layered loading for even load‑bearing.
V. Quick Troubleshooting Guide
- Tilting confined to one single side of heat sink → Eccentric tooling pressure, missing lateral limit, uneven furnace temperature.
- Defects concentrated in core central area → Insufficient intermediate support, self‑weight‑induced collapse.
- Massive fins incline in identical direction accompanied by local braze‑alloy accumulation → Liquid braze dragging effect, excessive assembly clearance.
- Universal fin bending across whole batch → Excessive clamping pressure, over‑high peak temperature or over‑long holding time.
- Random good‑to‑bad performance within one batch → Inconsistent assembly quality, dimensional fluctuation of incoming fins.
VI. Supplementary Risk Remarks
Triangular‑fin tilting is far more than cosmetic defect. Tilted fins alter flow‑channel geometry and raise flow resistance. Brazed joints on distorted positions sustain uneven stress, which easily triggers joint cracking and leakage under long‑term thermal cycling, giving rise to high‑volume field failures.
