What temperature profile is used for BGA rework?

BGA rework follows a four-zone reflow profile matched to the solder alloy. For SAC305 (Sn96.5/Ag3.0/Cu0.5, melting point 217-220 C): Zone 1 Preheat ramps from room temperature to 150 C at 1-3 C/s over 60-90 s. Zone 2 Soak holds at 150-200 C for 60-120 s to equalize thermal mass across the BGA and PCB. Zone 3 Reflow ramps to peak 245-260 C and holds above liquidus (220 C) for 30-60 s (time above liquidus, TAL). Zone 4 Cooling descends at 2-4 C/s. Critical constraint: adjacent components must not exceed 200 C (delta-T from ambient). The rework station achieves localized heating via focused nozzles matched to the BGA package outline (within 1-2 mm). Bottom preheaters bring the entire board to 100-150 C to reduce thermal gradient and prevent PCB warpage. For leaded (SnPb, eutectic 183 C): peak 220-235 C, TAL 45-75 s. Mixed-alloy boards (lead-free BGA on SnPb board or vice versa) require peak temperatures sufficient to melt both alloys, typically 245-255 C, with careful TAL management to avoid excessive intermetallic compound (IMC) growth at the interface.

How is X-ray inspection used after BGA rework?

X-ray inspection is mandatory after BGA rework because the solder joints are hidden beneath the package and cannot be visually inspected. 2D X-ray provides a transmission image showing ball presence, bridging (shorts between adjacent balls), and gross voiding. 3D X-ray (computed tomography, CT) provides cross-sectional slices through individual solder joints, enabling precise void volume measurement, head-in-pillow detection (partial ball collapse where the sphere did not fully wet the pad), and IMC layer thickness assessment. IPC-7095D specifies the acceptance criteria: void area must be less than 25% of the ball cross-section for Class 2 (general electronics) and less than 10% for Class 3 (high-reliability, military, aerospace). For RF applications, voiding requirements are often tighter (less than 15%) because voids create impedance discontinuities that degrade signal integrity at high frequencies. A void of diameter d in a ball of diameter D causes an approximate impedance perturbation of delta-Z/Z0 proportional to (d/D)^2 multiplied by the void position factor. Voids near the pad interface are most critical because they concentrate current density and create localized hot spots under thermal cycling, leading to accelerated fatigue cracking.

What RF-specific concerns exist during BGA rework?

RF assemblies present unique rework challenges beyond standard SMT considerations. Impedance continuity: BGA packages on RF boards often carry controlled-impedance signal paths (50 ohm microstrip or stripline transitioning through BGA ball vias). Rework that alters the solder joint geometry (void content, ball height variation, pad wetting angle) can shift the characteristic impedance by 2-10 ohms, causing return loss degradation. Post-rework S-parameter verification (S11, S21) should confirm return loss better than -15 dB and insertion loss within 0.2 dB of pre-rework values at the operating frequency. Substrate damage: Rogers, PTFE, and ceramic substrates used in RF boards have different thermal expansion coefficients (CTE) than FR-4. Rogers RO4350B (CTE 14 ppm/C x-y, 50 ppm/C z) is more resistant to thermal shock than PTFE (CTE 70-100 ppm/C z-axis), which can delaminate during aggressive rework profiles. Maximum rework cycles: IPC-7095D recommends a maximum of 3 rework cycles per site for Class 3 assemblies. Each cycle degrades the copper pad adhesion, increases IMC thickness (approximately 1-2 um per reflow), and risks microcracking in the substrate dielectric. For RF applications, many manufacturers limit rework to 2 cycles and require a full RF functional test after each attempt. Ground plane integrity: BGA packages with thermal/ground vias require careful attention during site preparation. Over-aggressive solder removal can damage plated through-holes, increasing ground inductance and degrading decoupling performance at microwave frequencies.

Manufacturing / Assembly

BGA Rework

/bee-jee-ay REE-wurk/

Removal and replacement of defective Ball Grid Array packages on assembled RF PCBs using IR/hot-air rework stations with focused nozzles. Lead-free SAC305 peak: 245–260°C, TAL 30–60 s. Adjacent delta-T < 200°C. Site prep: mini stencil at 50–80% paste volume. X-ray verification: void fraction <25% (IPC-7095D Class 2), <10% (Class 3). Post-rework RF test: S11 < −15 dB, ΔS21 < 0.2 dB.

Peak: 245–260°C

Void limit: <25% (Cl 2)

Max cycles: 2–3 per site

Understanding BGA Rework

BGA rework on RF assemblies demands tighter process control than standard SMT rework because the hidden solder joints directly affect impedance continuity, ground plane integrity, and signal path insertion loss. The rework station must deliver a precisely profiled thermal cycle (preheat, soak, reflow, cool) through a focused nozzle matched to the BGA outline, while bottom preheaters reduce thermal gradient across the board to prevent warpage and substrate delamination.

Post-rework verification is multi-stage: 2D/3D X-ray confirms void fraction and ball integrity, automated optical inspection (AOI) checks peripheral alignment, and RF functional testing validates that impedance and insertion loss remain within specification. IPC-7095D limits Class 3 assemblies to 3 rework cycles per site due to cumulative pad adhesion degradation and intermetallic compound (IMC) growth (~1–2 μm per reflow cycle).

Reflow Profile

SAC305 Lead-Free Profile:
Preheat: RT → 150°C at 1–3°C/s (60–90 s)
Soak: 150–200°C for 60–120 s
Reflow: peak 245–260°C, TAL > 220°C for 30–60 s
Cool: 2–4°C/s descent

Adjacent Component Protection:
ΔT_adj < 200°C at nearest neighbor
Bottom preheat: 100–150°C (reduces gradient)

Void-Induced Impedance Shift:
ΔZ/Z₀ ∝ (d/D)² × position factor
d = void diameter, D = ball diameter

Rework Process Steps

Step	Temperature	Duration	Tool	Critical Parameter
Remove (top heat)	245–260°C	30–60 s	Rework station + nozzle	ΔT_adj < 200°C
Site clean	Room temp	2–5 min	Braid + flux (ROL0/1)	Flat surface, no pad lift
Stencil paste	Room temp	—	Mini stencil (site-specific)	50–80% nominal volume
Place + align	Room temp	—	Vision alignment system	±50 μm placement
Reflow	245–260°C	60–90 s total	Rework station + bottom heat	Profile match to OEM
X-ray inspect	—	—	2D/3D X-ray (CT)	Void <25% (Cl 2) / <10% (Cl 3)

X-Ray Void Acceptance (IPC-7095D)

Class	Application	Max Void Area	RF Typical
Class 1	Consumer	<50%	—
Class 2	General industrial	<25%	<20%
Class 3	Mil/aero/medical	<10%	<10%
RF-critical	High-freq signal path	<15%	<15%

Common Questions

Frequently Asked Questions

What reflow profile for BGA rework?

SAC305: preheat to 150°C (1–3°C/s), soak 150–200°C for 60–120 s, peak 245–260°C with TAL 30–60 s above 220°C, cool at 2–4°C/s. Adjacent components: ΔT < 200°C. Bottom preheat 100–150°C reduces warpage. SnPb: peak 220–235°C, TAL 45–75 s.

X-ray after rework?

Mandatory for hidden joints. 2D X-ray: ball presence, bridging, gross voids. 3D CT: cross-sectional void volume, head-in-pillow, IMC thickness. IPC-7095D: void <25% (Cl 2), <10% (Cl 3). RF boards often tighten to <15% because voids create impedance discontinuities proportional to (d/D)².

RF-specific rework concerns?

Impedance continuity: void/geometry changes shift Z₀ by 2–10 Ω. Post-rework S11 < −15 dB, ΔS21 < 0.2 dB. Substrate damage: Rogers/PTFE more sensitive than FR-4. Max 2–3 cycles per site (IMC grows ~1–2 μm/reflow). Ground via integrity critical for decoupling at microwave frequencies.

Related Terms

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