Mixed technology assembly isn’t simply placing two types of components on the same board. The manufacturing process requires careful sequencing because SMT and THT use fundamentally different soldering methods. SMT components attach to surface pads using reflow soldering, typically around 240-260°C. Through-hole parts need their leads inserted into drilled holes and soldered using wave soldering, selective soldering, or hand soldering techniques.
The real complexity emerges when you consider thermal exposure. Your SMT components will experience heat multiple times—first during reflow, then potentially again during THT soldering. This repeated thermal stress can damage sensitive components if you don’t plan the assembly sequence properly.
I’ve found mixed assembly makes sense in several scenarios:
High-power applications: Power supplies need large through-hole capacitors and transformers for their power-handling capacity, but the control circuitry benefits from compact SMT components. You can’t build a reliable 500W power supply using only SMT parts—the mechanical stress and heat dissipation requirements demand through-hole technology for key components.
Mechanical durability requirements: Automotive and industrial applications face vibration, shock, and temperature cycling. Through-hole connectors provide the mechanical strength to withstand these stresses, while SMT handles the signal processing and logic circuits. An engine control unit subjected to constant vibration needs through-hole connectors at its interface points.
Component availability constraints: Sometimes you simply can’t get a specific component in SMT packaging. Certain high-current relays, specialized connectors, and legacy components only exist in through-hole form. Mixed assembly lets you incorporate these parts without compromising your overall design density.
Understanding the characteristics of each technology helps you make better component placement decisions.
Getting mixed assembly right requires understanding the complete manufacturing sequence. Here’s how professional assembly houses handle these boards:
The process starts with solder paste printing for SMT components. A stainless steel stencil aligns with the bare PCB, and a squeegee forces solder paste through apertures onto the SMT pads. The paste contains tiny solder spheres suspended in flux. Paste thickness typically ranges from 0.004 to 0.006 inches, depending on component types.
For mixed assemblies, you need to mask or block through-holes during paste application. Otherwise, solder paste fills the holes, preventing proper through-hole lead insertion later. Some manufacturers use plugging paste or tape, while others design stencils that simply don’t cover the through-hole areas.
Pick-and-place machines position surface mount components onto the solder paste. Modern equipment achieves placement speeds of 30,000 components per hour with accuracy within ±0.001 inch. The machines use vacuum nozzles to pick components from reels, trays, or tubes, rotating them to the correct orientation before placement.
Component orientation matters significantly in mixed assemblies. Position SMT parts considering the upcoming through-hole soldering process. Keep heat-sensitive SMT components away from areas that will see high temperatures during wave or selective soldering.
The board enters a reflow oven where controlled heating melts the solder paste, creating permanent electrical and mechanical connections. A typical lead-free reflow profile includes:
The entire process takes 4-8 minutes depending on board thickness and thermal mass.
Automated Optical Inspection (AOI) systems check SMT placement quality before proceeding to through-hole assembly. Catching defects now saves time and cost compared to finding them after THT soldering. Common issues include tombstoning, bridging, and component misalignment.
Through-hole components can be inserted manually or using auto-insertion equipment. Manual insertion works for low volumes and prototypes, but high-volume production benefits from automated systems. Radial and axial component inserters handle common parts like capacitors and resistors, while specialized machines handle odd-form components.
Three primary methods exist for THT soldering in mixed assemblies:
Wave Soldering: The board passes over a molten solder wave that solders all through-hole leads simultaneously. This requires masking SMT components to prevent solder from bridging surface mount pads. Wave soldering works best when through-hole components are concentrated on one side.
Selective Soldering: A small solder wave or nozzle targets specific through-hole locations. This method prevents thermal damage to nearby SMT components and eliminates the need for extensive masking. It’s slower than wave soldering but offers better control in complex mixed assemblies.
Hand Soldering: Skilled technicians manually solder through-hole components using temperature-controlled soldering irons. This approach suits low-volume production, prototypes, and rework situations. It’s the most flexible method but doesn’t scale to high volumes efficiently.
Getting your mixed assembly design right the first time requires attention to manufacturability from the start. I’ve reviewed hundreds of designs over the years, and these issues consistently cause problems:
Adequate spacing prevents assembly defects and allows for rework if needed. Follow these minimum spacing guidelines:
These represent practical minimums. When possible, exceed these values for better yields and easier rework.
Mixed assemblies often combine high-power through-hole components with temperature-sensitive SMT parts. Managing heat flow requires deliberate design decisions:
Use thermal vias: Place 0.3-0.5mm diameter vias under power components to conduct heat to internal copper layers. A grid of 8-16 vias can reduce junction temperature by 15-20°C.
Copper pour zones: Dedicate internal or back-side copper layers as thermal planes. Connect them to component thermal pads through via arrays.
Component location: Separate heat-generating through-hole parts from temperature-sensitive SMT components. Position high-power devices near board edges where heat dissipates more easily.
Selective solder masking: In wave soldering, mask temperature-sensitive SMT components to shield them from direct solder wave contact.
The order you specify for component installation directly impacts manufacturing efficiency and defect rates. Here’s my recommended sequence for most mixed assemblies:
For double-sided mixed assemblies, the sequence becomes more complex. Typically, you’d process the side with fewer/smaller components first, using adhesive to hold them during the second-side reflow operation.
Understanding potential defects helps you design boards that minimize manufacturing issues.
Tombstoning: One end of a small chip component lifts during reflow. This happens when solder paste volume differs between pads or heating isn’t uniform. Prevention involves balanced pad designs and controlled reflow profiles.
Solder Bridging: Excess solder connects adjacent SMT pads. Causes include too much solder paste, insufficient pad spacing, or contamination. Use proper stencil aperture design and maintain minimum 0.2mm pad spacing.
Component Shift: Parts move from their intended position during reflow. Vibration during transport to the oven, paste adhesion problems, or board warpage can cause this. Proper paste tack time and stable board support prevent shifting.
Insufficient Solder Fill: The plated through-hole barrel doesn’t fill completely with solder. IPC Class 2 requires 50% fill minimum, while Class 3 demands 75%. Inadequate preheat, incorrect flux chemistry, or contaminated holes cause this problem.
Cold Solder Joints: Joints appear dull and granular instead of smooth and shiny. Insufficient heat or contaminated surfaces create cold joints that may fail under thermal cycling. Proper preheat and clean PCBs prevent this issue.
Solder Bridging Between Leads: Adjacent through-hole leads short together. Excessive solder, poor board support angle during wave soldering, or slow conveyor speed cause bridging. Optimize wave parameters and board angle (typically 5-7 degrees).
Professional mixed assembly operations follow IPC standards to ensure consistent quality:
IPC-A-610: Acceptability of Electronic Assemblies. This standard defines what constitutes acceptable solder joints, component placement, and overall assembly quality. Three classes exist:
Higher classes require stricter criteria. For instance, Class 2 allows 50% solder fill in through-holes, but Class 3 demands 75% minimum.
Inspection Methods:
Mixed assembly typically costs more than pure SMT due to additional process steps. Here’s how to control costs:
Minimize through-hole component count: Every through-hole part adds labor and cycle time. Evaluate whether true through-hole mounting is necessary or if SMT alternatives exist.
Standardize on common components: Using standard resistor and capacitor values reduces inventory costs and simplifies procurement. A 1% resistor tolerance often suffices instead of 0.1%, and the standard part costs significantly less.
Design for automated insertion: Through-hole components that work with auto-insertion equipment cost less to place than odd-form parts requiring manual work. Axial and radial lead components insert easily; connectors with non-standard pin spacing require manual labor.
Consider design consolidation: Sometimes splitting functionality across two boards costs less than a complex single-board mixed assembly. Evaluate total system cost, not just board cost.
Volume commitment: Assembly houses offer better pricing for committed volumes.
