Mixed Technology PCB Assembly: Combining SMT and THT on One Board

What “Mixed PCB Assembly” Actually Means

In the context of industrial electronics manufacturing, Mixed PCB Assembly, also sometimes referred to as mixed technology PCB assembly, hybrid PCB assembly, or mixed-tech assembly, incorporates both SMT and THT components. A single PCB manufactured using mixed-tech assembly is populated with both surface-mount and through-hole components on a single board, often utilizing a sequenced manufacturing flow rather than a single process.

It is important to note that mixed PCB assembly largely differs from pure SMT or THT workflows as individual SMT decisions can affect THT outcomes and vice versa. As such, having a balanced mindset in SMT and THT design is critical to ensure a successful wave step within the manufacturing line.

When You Need Mixed Assembly

Aside from knowing what mixed technology PCB assembly actually means, it is also just as important to know when you need mixed assembly. Some of the most common scenarios in which mixed assembly process becomes a necessity are as follows:

• Some of the connectors for the components require high mechanical strength, for instance, USB, RJ45, DIN, or board-to-board. For such components, combining SMT parts with through-hole components maximizes performance while maintaining insertion integrity.
• Large electrolytics, transformers, or screw terminals are present, which generally benefit from large through-hole lead areas to handle high-current density during operation.
• High-power dissipation components such as TO-220 regulators and large heatsinks are used, which efficiently conduct heat into copper plates via through-hole leads.
• Components like relays, HV devices, and panel mounts are expected to experience heavy and frequent mechanical stresses but can withstand shock and vibration when effective through-hole joints are established.
• Advanced circuit boards containing mixed-signal blocks with fine-pitch SMT ICs sometimes only have legacy analog parts sold in through-hole packages.
• If none of the previous cases apply, going pure SMT, sometimes via Pin-in-Paste for the remaining components utilizing THT form factors, can usually simplify the manufacturing process.

How Mixed Assembly Differs from Pure-SMT or Pure-THT

Although pure SMT and THT cater to specific applications, most production PCBs above hobby grade are mixed by default. As such, the question for most professional grade boards is not whether to use mixed assembly but how to sequence the workflow and prepare the right equipment for it.

The Mixed Assembly Process: Sequencing SMT and THT on One Board

The successful implementation of mixed assembly process relies heavily on the correct sequencing of workflows. In hybrid PCB assembly, the order matters. Every step exposes the previous step’s joints to thermal stress, thereby requiring the proper order to preserve yield. Currently, there are two industry-standard sequences for mixed assembly process.

The first one, hereafter referred to as sequence A, implements reflow and wave soldering chronologically. The second industry-standard sequence, hereafter referred to as sequence B, takes advantage of Pin-in-Paste to effectively subject the board in one reflow pass.

Sequence A: Reflow First, Then Wave Solder

1. Using a precision stencil, paste print on the SMT side. The quality of the stencil drives the yield. Issues such as bridging, deposit insufficiency, and tombstoning can arise from poor quality stencils.
2. Pick-and-place process commences to position the SMT components.
3. After successful SMT placement, reflow process is implemented to solder the parts.
4. The board is then flipped and red glue is applied on the bottom surface so that the SMD faces the wave. The red glue stencil deposits the adhesive dots.
5. After the glue is applied, the bottom-side SMD is placed and the board undergoes glue cure via thermal or UV application.
6. The THT components are then inserted from the top-side.
7. Depending on the equipment available or the application, wave or selective soldering may be done to solder the THT leads and the glued bottom-side SMD. In this step, selective soldering may be done when only a few THT joints exist.
8. Finally, thorough inspection is facilitated along with the required electrical, AOI, X-ray (BGAs) and in-circuit tests.

Sequence B: Pin-in-Paste (PIP) —Mixing in One Reflow Pass

An alternate mixed assembly sequence utilizes Pin-in-Paste that effectively involves only one reflow pass to solder both the SMD and THT components. The process starts by printing PIP paste into the through-hole barrels alongside the SMT pads on the same stencil. The THT parts are then inserted before the reflow process. In this case, both SMT and THT joints form in one reflow pass, eliminating the need for an isolated wave soldering step.

Since the wave solder step is removed in sequence B, the equipment cost is reduced and the flow is shortened by 1 to 2 steps compared to sequence A. However, the caveat with PIP is that it only works for low-to-medium pin THT count (connectors less than 20 pins with smaller caps), frequently failing on large, high-thermal-mass components that prevent the solder paste within the hole from reaching the required reflow temperature. Furthermore, PIP requires more complex stencil design, following enlarged apertures over the barrels to deposit enough paste to fill the hole. Since the extra paste volume is the tradeoff that makes PIP work, it renders some THT parts incompatible.

Choosing the Right Sequence for Your Board

Stencil Setup for Mixed PCB Assembly

As mixed boards cater for both SMT and THT components, it is common to utilize more than one stencil. When designing mixed PCB technologies, it is vital to consider upfront costs including stencil requirements so as to avoid late-stage surprises.

The Solder Paste Stencil (the foundation)

Everything starts with the solder paste stencil. As previously mentioned, a poor-quality stencil can impact the production yield. Standard fine-pitch SMT stencils typically feature a foil thickness of 0.10 – 0.15 mm, determined by the smallest component pitch. For mixed technology PCB assembly, the aperture rules remain consistent with pure-SMT based on IPC-7525C (2021): area ratio ≥0.66, aspect ratio ≥1.5.

For hybrid PCB expected to undergo wave after reflow, electropolished apertures and nano-coated foils are recommended to reduce paste smears across long runs. For mixed boards that combine 0.4 mm-pitch BGAs with TO-220 thermal pads, step stencils can be used to vary thickness by zone.

The Red Glue Stencil (when bottom-side SMD meets the wave)

For applications integrating bottom-side SMT and through hole on same board, the surface-mounted components that pass over the wave should be glued beforehand to avoid being washed off in the molten solder. In this case, a separate red-glue stencil deposits epoxy dots between/beside SMD pads under each bottom-side part. The thickness of the foil for this process should be around 0.15 – 0.30 mm so the dot clears the component standoff.

To ensure perfect alignment, the paste and glue stencils must reference the same PCB origin, ideally from one Gerber upload. JLCPCB supports both stencil types from one upload, preventing registration drift.

Design Rules for a Mixed PCB Assembly Board

Like other engineering products and prototypes, problems in manufacturing often emanate from poor design. Mixed board failures are no exceptions. Whether you are designing as an amateur or a seasoned mixed board designer, the subsequent guidelines can help in mitigating board failures during production.

Component Placement Rules

As mixed PCBs integrate SMT and through hole on same board, the placement of the components can also be a deciding factor for failure. To ensure both SMT and THT leads are soldered properly during the wave step, there should be a minimum clearance of 2.5 mm between SMT and THT. This gap ensures the wave solder can approach at an appropriate application angle. In addition to this, heat-sensitive SMT parts like connectors and plastic bodies must be placed away, if possible, from wave-soldered zones.

During wave soldering, the correct orientation of bottom-side SMD depends on the component type to prevent wave shadowing and incomplete soldering. For two-terminal chip components, the long-axis should be placed perpendicular to the wave path to ensure even soldering. Conversely, multi-pin ICs should be placed parallel to the wave direction to prevent solder bridging. Finally, it is good practice to group THT parts by lead direction, as rows of THT parts arranged in parallel are generally cheaper to auto-insert than scattered components.

Thermal Profile Considerations

Regardless of the sequence that you are going to use for your mixed board, each component must be able to withstand the thermal stress during reflow. For sequence A, which includes a standard reflow and wave soldering, the bottom-side SMD parts experience a second thermal cycle. Although the thermal peak profiles of the lead-free reflow (235 to 250 °C) and the wave bath (250 to 260 °C) are fairly close, the boards still perceive two heating cycles. Because of this, it is necessary to verify the thermal tolerance of the individual components for a second cycle in their respective datasheets.

In the same way, for sequence B employing Pin-in-Paste, careful verification of each THT component is necessary to assess reflow compatibility. Normally, the plastic parts of THT components do not experience high thermal stress during wave soldering. In Pin-in-Paste, however, since the THT parts are reflowed alongside the SMT parts simultaneously, they are subjected to the higher thermal profiles.

DFM Rules That Save You a Spin

In the electronics manufacturing industry, redoing a board, also referred to as a spin, costs time, money, and effort. As much as possible, the circuit board must be well-designed for manufacturing to save unnecessary spins arising from design failures or defects.

In sequence A, before sending bottom-side components through a wave bath, they need to be glued down first. This adhesive or glue layer, however, must be marked explicitly and separately in the Gerber, as many CAM systems generate the glue stencil directly from this specification. Failing to do so might cause the factory's automated CAM software to omit the glue stencil. On the other hand, for mixed PCBs expected to use sequence B or Pin-in-Paste, the Gerber must specify whether the barrels are paste-filled or left for wave soldering, as different stencils will be generated accordingly.

For boards mixing 0.4 mm BGAs with large connectors, requesting a design for manufacturing (DFM) review prevents failures early on from conflicting paste volume requirements. When mixing fine-pitch components with massive connectors, a custom step stencil is a must to ensure the board works as intended. Furthermore, for all parts above 0.5 mm, the standoff height must be indicated in the design process to guide the factory’s choice of glue-stencil thickness and step stencil specifications.

Common Challenges in Mixed PCB Assembly

FAQ about Mixed PCB Assembly

Conclusion: Treating Mixed PCB Assembly as a System, Not a Step

In conclusion, a mixed PCB assembly isn’t merely an SMT board with a few THT parts bolted in. It is a sequenced flow where every step affects the next. Implementing the right sequence (A vs PIP), stencil setup (paste + optional glue + step), design-for-manufacturing rules, and solutions to address common challenges altogether determine production yield. Missing or failing any of these can lead to mixed board failures.

To build a mixed PCB assembly properly and ensure perfect alignment, using a reliable stencil is a must. JLCPCB offers a wide variety of stencil services and products, developed for extensive PCB prototyping and trusted by millions of engineers worldwide. They produce paste and red-glue stencils on the same laser lines with ±0.003 mm precision, nano-coated/electropolished stencils, and step stencils. Their 12-hour build, unified Gerber upload, one-piece minimum order quantity, and $3 starting price provide everything you need to get started and make further iterations practical.

DFM mixed boards are now the default in the electronics industry. They can be a headache to produce without following the guidelines and the right sequence. Get the sequencing right, equip the line for both processes, and a mixed PCB assembly stops being where revisions happen and starts being where products ship.