Mixed-Technology Assembly: Process Sequence and Control
A mixed-technology assembly carries surface mount parts and through-hole parts on the same board, and the sequence in which they are attached is a design decision rather than a matter of shop preference. The sequence determines how many thermal excursions the assembly sees, which parts are exposed to which process, and whether the through-hole joints are made with solder that has to be printed or with solder that comes from a wave or a fountain. Getting the sequence right at the design stage costs nothing; changing it after the tooling exists costs a great deal.
The Four Basic Process Sequences
The first sequence is a full reflow of both sides followed by a wave for the through-hole parts. This is the classic mixed-technology route: the surface mount parts are attached first, with adhesive on the underside, and the through-hole parts are inserted afterwards and soldered in a wave. The advantage is a single thermal cycle for each side and a wave that makes every through-hole joint at once. The disadvantage is that the wave treats the whole board as a wave does, and any bottom-side fine-pitch parts are exposed to it.
The second sequence attaches the through-hole parts first, usually by hand or by selective soldering, and reflows the surface mount parts afterwards. This suits a board with a small number of through-hole parts that can be hand soldered before the reflow, and it avoids exposing the bottom side to a wave. The third is a fully selective process, where every through-hole joint is made individually. The fourth prints paste into the through-hole holes and reflows both types together, which is the paste-in-pin route.
Choosing a Sequence From the Component Set
The component set decides the sequence more than the layout does. Where the through-hole parts are connectors, transformers or large capacitors that cannot survive a second reflow, the wave or selective route is required, because the parts have to be inserted after the surface mount reflow. Where the through-hole parts are small and few, and the board has fine-pitch devices on the underside, hand or selective soldering followed by reflow is often the cleaner arrangement.
The number of thermal excursions is a useful way to compare the options. A two-sided reflow plus a wave gives the top side one excursion and the bottom side two, plus the wave contact. A selective route can give some joints three or four. Each excursion consumes some of the component’s thermal budget and some of the flux activity, so the sequence with the fewest excursions that still respects the component ratings is usually the right one.
Paste-in-Pin and Printed Through-Hole Solder
Printing paste into a through-hole relies on the same stencil that prints the surface mount pads, with oversized apertures positioned over the holes. The paste bridges the hole and fills it during reflow, forming a fillet on both sides if the volume is right. The method works well for a modest number of holes with adequate spacing, and it removes the need for a wave or a selective machine entirely.
The limits are volume and geometry. A hole of 1.0 mm or larger needs more paste than a single print can reliably deliver, so the aperture becomes so large that it interferes with neighbouring apertures and the paste does not release cleanly. The gap between the lead and the hole wall also matters: too large and the paste drains through before reflow; too small and the volatiles cannot escape, producing voids and blowholes. The standard practice is to keep the hole diameter within about 0.2 to 0.3 mm larger than the lead, and to reduce the board thickness under the aperture with a step or a counterbore where the volume is insufficient. The layout for soldering yield notes cover these dimensional relationships.
Adhesive and Bottom-Side Retention
Where a wave is used, the bottom-side surface mount parts have to be held in place through it. Adhesive is applied between the pads and cured before the wave, and its volume and cure state are process parameters rather than details. A part that is under-adhered will shift or fall during the wave, and a part that is over-adhered may have its terminations lifted off the pads so that the reflow joint never forms properly.
The adhesive also has to be compatible with the flux used in the wave. Some chemistries soften the cured adhesive during the dwell, and the part then moves. Testing a cured sample in the flux for the dwell time is a quick way to check compatibility, and it should be done whenever either the adhesive or the flux is changed. The flux itself should be chosen for the alloy and the board, and its residue has to be either cleanable or acceptable, which is the subject of the no-clean flux guidance.
Thermal Load on the Second Pass
The second reflow pass exposes the joints made in the first pass to another excursion. The joints do not re-melt if the alloy is the same and the peak is lower than the first peak, but the intermetallic layer does continue to grow, and a joint that was marginal after the first pass will be more so after the second. Components that were at their thermal limit on the first pass are now one excursion closer to the limit.
Where the assembly has two reflow passes plus a wave, the cumulative exposure should be assessed against the component ratings rather than each process separately. The assessment should use measured board temperatures, not setpoints, because the thermal load on a component depends on where it sits on the board. A component near a large copper area can see a shorter and cooler excursion than one in an open area, and the difference is large enough to change the decision.
Fixtures, Pallets and Selective Options
A wave soldering pass on a mixed-technology board usually needs a pallet to protect the surface mount parts and to define which areas see the wave. The pallet also carries the parts that have to be masked, and its openings determine the contact length for each through-hole row. A pallet designed for one board cannot be adapted to another, so it is a tooling item on the critical path of a new product introduction.
Selective soldering removes the pallet from the equation and gives each joint its own thermal history, at the cost of cycle time. It is the natural route for a board with few through-hole joints, for a connector that cannot take a wave, or for a board where the bottom-side surface mount density makes a pallet impractical. Where both a wave and a selective machine exist, the choice per board should be made on the joint count and on the sensitivity of the parts. The spray fluxing arrangement is common to both routes and its control affects both.
Inspection and Rework Implications
The sequence sets what can be inspected and what cannot. A joint made in the first pass and covered by a part in the second cannot be seen afterwards, so it has to be inspected at the intermediate stage, which means an extra inspection operation and an extra handling step. Where the design allows the sensitive joints to be placed so that they remain visible, the inspection cost falls and the escape risk falls with it.
Rework is likewise sequence-dependent. A through-hole joint made after the surface mount reflow can be reworked without disturbing the surface mount parts, provided the heat is localised. A joint made before a later reflow is difficult to rework afterwards because the surrounding parts have already been attached. The assembly drawing should state which joints are considered reworkable and which process step they belong to, so that the operator does not have to infer it. The inspection standard for the product should list the acceptance criteria for each joint family separately, because a wave joint and a hand joint have different appearance requirements.
Documentation and Change Control
The process sequence belongs in the work instruction, with the sequence number of each operation, the thermal exposure of each pass and the parts that are installed at each stage. A change to the sequence is a change to the product, and it should be assessed against the component ratings, the tooling and the inspection plan before it is released. The most common error is a change made for scheduling convenience, which is then not reflected in the drawing and produces a board that no one can explain.
Where the sequence is changed, the first article should be a full verification rather than a visual check, because the effect of a reordered process is a thermal and metallurgical one that shows up in cross-section rather than on the surface. Recording the sequence in the batch documentation makes the product traceable to the process that produced it, which is what the customer’s audit will ask for. The sequence, once fixed, should be treated with the same change control as the bill of materials.
FAQ
Can a wave and a selective machine be used on the same board? Yes, and it is common: the wave makes the dense rows and the selective machine handles the parts the wave cannot reach safely. The two processes produce different joint appearances, so the acceptance criteria should distinguish them rather than applying one visual standard to both.
Is paste-in-pin always cheaper than adding a wave? Only where the hole count is small and the geometry suits it. For a board with hundreds of through-hole joints, the printed volume becomes impractical and a wave is both faster and more reliable. The decision should be made on the count and the geometry, not on the equipment list.
How many reflow passes can a component take? The rating is given by the component maker, usually as a number of excursions above a stated temperature. Where the assembly needs more, the component must be installed in the last operation so that it sees the fewest excursions, and that requirement should be on the assembly drawing.
