How HotBar Pulse Heat Soldering Works: Principle, Temperature and Pressure Control, and Choosing Thermodes and Machines, in 5 Stages from Part Design and Pre-Tinning to Profile Setting and Thermode Selection

HotBar soldering (also called pulse heat soldering) presses together two parts that have already been tinned and fluxed, heats them with an electrically heated thermode until the solder melts and flows, and then lets them cool and solidify under pressure; a thermocouple on the thermode feeds the temperature back to the power supply, and each bond is one heating and cooling cycle. It is commonly used to bond flex circuits to rigid boards, ceramics or glass, and also to solder ribbon cables, battery packs and small coaxial cables. Whether it goes well is largely decided before soldering: how the flex and board pads are designed, whether there are large heat-sinking copper areas beside the pads, and whether the pre-applied solder volume is right; on the machine, the remaining factors are temperature, time, pressure, whether the thermode sits flat, and thermode size. The same equipment, run with different temperature, time and pressure profiles, can also bond conductive adhesives, of which anisotropic conductive film (ACF) is one type. This article covers the principle of HotBar soldering, part design, pre-tinning, settings, and choosing thermodes and machines; troubleshooting solder bridging shorts and choosing ACF materials are outside its scope. It is divided into 5 stages:

• Stage ① Understand the principle and where it is used: how it heats, and the benefits and costs compared with connectors.
• Stage ② Part design: the three flex termination types, pad width, heat-sinking copper.
• Stage ③ Pre-tinning and flux: where to put the solder first, how much, and how to choose the flux.
• Stage ④ Set temperature, time and pressure, and inspect: profile segments, pressure, flatness, fixtures, checking the joint.
• Stage ⑤ Choose the thermode and machine: thermode shape, size and maintenance, machine components and ACF bonding.

Each stage below covers how to do it and what public sources say, with how JIAHON can help where it applies; sources, exceptions and compilation notes are in the Notes at the end. This describes the general case; individual cases still need their own assessment.

Figure 1. The five stages of HotBar soldering: principle, part design, pre-tinning, settings and inspection, and choosing the thermode and machine (illustration). ① Principle: the thermode heats when current passes; a thermocouple controls temperature; pressure is held until the solder solidifies → result: know whether this joint can be made this way. ② Part design: three flex terminations; flex pads narrower than board pads; avoid large copper areas → result: heat reaches every joint evenly. ③ Pre-tinning: start with 40% coverage; reflow the paste into a layer first; a small amount of low-solids flux → result: a stable solder volume. ④ Settings: preheat, reflow, cool; enough pressure, as too much can cause bridging; check flatness with pressure-sensitive paper → result: even joints. ⑤ Thermode: longer than both ends of the pad row; width leaves room for squeezed solder; cleaning and re-flattening → result: the right size and machine.

Stage ①: Understand the principle and where it is used

How to do it:

• How the joint forms: two parts, already tinned and fluxed, are heated until the solder melts and flows, then cooled under pressure, forming a permanent electrical and mechanical joint (Sources 1, 2).
• How it heats: the power supply sends current into the thermode, and the thermode’s own resistance makes it heat up; a thermocouple feeds the temperature back to the power supply (Source 1).
• Why it is called pulse: the thermode heats and cools once for each bond, unlike conventional soldering; pressure is held from heating through melting to cooling (Source 2).
• How hot the joint must get: the solder must reach 40 °C above its melting point and stay there for 2 to 3 seconds to wet and flow properly (Source 1).
• Where it is used: it is the main method for connecting flex to rigid boards, ceramics and glass, and can also solder surface-mount edge connectors, battery packs, ribbon cables and small coaxial cables; because it heats just the area around the joint, it also works with heat-sensitive parts (Source 3).
• Leaded parts are rarely bonded this way: lead coplanarity is a problem, and the part must also be glued in place before bonding (Source 3).
• Benefits compared with connectors: it saves the cost and space of a connector, and a direct connection may also be more reliable than a connector (Source 3).
• Costs: an extra process step; the joint may crack if the flex is pulled; rework is difficult, and removing a flex from a board almost always destroys the flex (Source 3).
• Below a certain pitch, switch to conductive adhesive: the magazine report says hot-bar soldering works well at pitches above 15 mil (about 0.38 mm), and below 15 mil, conductive adhesive film bonding is better (Source 3).

What public sources say: a pulsed-heat bonding equipment maker’s technical presentation (no publication date, copyright year 2024) says pulse heat soldering heats two pre-fluxed, pre-tinned parts until the solder melts and flows, then cools them under pressure to form a permanent electrical and mechanical joint; the thermode applies heat and pressure on a set time and temperature profile, with a thermocouple providing temperature feedback; the power supply sends alternating current into the thermode, and the thermode’s resistance generates heat; and the solder must reach 40 °C above its melting point for 2 to 3 seconds. The same maker’s part design guide (no publication date, copyright year 2020) says pulse heat soldering differs from conventional soldering in that the solder is melted by a thermode that heats and cools once for each bond, with pressure applied through the whole cycle of heating, melting and cooling. An assembly industry magazine report (2006, quoting staff from two hot-bar bonding equipment makers and the head of a flex design company) says hot-bar bonding is the main method for connecting flex to boards, ceramics and glass, and can also solder surface-mount edge connectors, battery packs, ribbon cables and small coaxial cables; it is a selective soldering method usable with heat-sensitive parts; in theory it can also solder leaded parts, but this is rarely done because lead coplanarity is a problem and the parts must be glued first; soldering flex directly saves the cost and space of a connector and may be more reliable than a connector, but adds a process step, joints may crack if the flex is pulled, and rework is difficult, with removal almost always destroying the flex; and hot-bar soldering suits pitches above 15 mil, while conductive adhesive film is better below 15 mil (Notes, Table 1).

Stage ②: Part design

How to do it:

• The three flex terminations: fully exposed (the polyimide is removed from both sides and the thermode presses directly on the traces), single-sided window (just the side facing the board is removed, and heat passes down through the upper polyimide), and double-sided window (“open windowed” in the source: removed on both sides, but with polyimide left at the sides and end for support) (Sources 1, 2).
• Trade-offs among the three: fully exposed and double-sided window allow a somewhat lower thermode temperature and give excess solder somewhere to flow; fully exposed traces are easily bent or damaged in handling; single-sided window is easy to handle but needs a higher thermode temperature, leaves excess solder nowhere to go and is not tolerant of solder volume on the board; double-sided window needs the thermode to land precisely within the window (Sources 1, 2).
• Limits of single-sided window: the upper polyimide layer is limited to 0.002 in (50 microns), and each 25 microns costs 60 to 80 °C; minimum pitch is 0.020 in (500 microns) to avoid shorts; pads must be at least 0.080 in (2 mm) long for a solder fillet to form (Source 1). Polyimide heated well above 260 °C chars on the surface and contaminates the thermode (Source 2).
• Flex pads narrower than board pads: when the solder melts and the parts are pressed down, solder is squeezed to the sides; slightly narrower flex pads give the solder room to flow, make bridging less likely and make alignment easier (Sources 2, 3).
• Board trace width at fine pitch: set board trace width to half the pitch, reducing the risk of shorts from misalignment (Sources 1, 2).
• Avoid heat-sinking copper: large copper areas beside the pads, widened traces and plated-through holes draw heat away and make temperature uneven along the row of joints; traces connecting to the pads should be equal in width and as thin as possible, acting as “thermal dams”; when thin traces are used as thermal dams, keep anything heat-sinking at least 0.080 in from the pads (Sources 1, 2).
• Multilayer boards: under the bonding area, run just thin signal traces, evenly distributed under the pads; if there is a shield layer, its effect must be the same across the whole row of joints (Sources 1, 2).
• Leave room for the thermode and fixture: the thermode must be able to extend past both ends of the pad area; do not place components on the back of the board directly under the bonding area, so they do not interfere with the support fixture below; provide locating holes in the flex and board for alignment (Sources 1, 3).
• Heat ceramic substrates slowly: common board materials withstand local heating, but ceramic substrates need a more controlled temperature rise to avoid cracking; if the two parts differ too much in heat-sinking capacity, the joint may also crack on cooling (Source 2).

Figure 2. The three flex terminations and their trade-offs: fully exposed, single-sided window and double-sided window (illustration, based on Sources 1 and 2). Fully exposed: the thermode presses directly on the traces, and temperature can be a little lower; excess solder has somewhere to flow; take care, as traces are easily bent in handling. Single-sided window: heat passes through the polyimide; easy to handle but needs higher temperature; excess solder has nowhere to go, so it is not tolerant of solder volume. Double-sided window: supported at the sides and end, so it handles better than fully exposed; excess solder has somewhere to flow, and temperature can be a little lower; take care, as the thermode must land precisely within the window. Flex pads narrower than board pads give squeezed solder somewhere to go.

What public sources say: the technical presentation above says flex commonly has copper traces covered by polyimide; fully exposed allows a lower thermode temperature, but traces are easily bent in handling; single-sided window is easy to handle but needs a higher thermode temperature; double-sided window allows a lower thermode temperature, but alignment is critical; for single-sided window, the upper polyimide is limited to 0.002 in (50 microns), each 25 microns causes a 60 to 80 °C loss, minimum pitch is 0.020 in, and pads are at least 0.080 in long; board trace width is 50% of pitch, board pads should be wider than flex pads, locating holes should be provided, the thermode should extend at least 0.040 in (1 mm) past each end of the pad area, and no components should be on the back directly under the bonding area; differences in heat sinking between pads cause uneven heating, equal-width thin traces act as thermal dams, nothing heat-sinking should be within 0.080 in of the pads, and multilayer boards should run just thin traces under the bonding area. The part design guide above says flex pads are narrower than board pads, so squeezed solder has room to flow, which is more tolerant of solder volume and eases alignment; board trace width at fine pitch is 50% of pitch; the single-sided window design is not tolerant of excess solder; polyimide heated well above 260 °C chars and contaminates the thermode; ceramic substrates need more controlled heating, and if the two parts differ too much in heat sinking, the joint may crack on cooling; and variation in heat sinking along the length of the joint is a design problem that often has to be overcome. The magazine report above says room must be left on both sides of and under the joint for the thermode; metallization or gold traces near the pads draw heat away; and flex pads are ideally narrower than board pads (Notes, Table 2).

How JIAHON can help:

• JIAHON’s engineering department can help review whether a PCB design is easy to manufacture, including panelization.

Stage ③: Pre-tinning and flux

How to do it:

• Apply the solder beforehand: in the process the magazine report describes, the board pads are stencil-printed with paste and reflowed during the surface-mount process, and at the HotBar step just flux is applied and the flex pressed on (Source 3); both parts are usually pre-tinned; without pre-tinning, a single-sided window flex on gold- or tin-plated pads may still wet, and the plating on the two parts is often enough to make a reliable joint (Source 2).
• Start solder volume trials at 40% coverage: one guide suggests starting with a 0.006 in stencil with apertures reduced to about 40% of the pad area, then adjusting from actual results (Source 2); another presentation gives 0.005 in (0.127 mm) thick and about 40% coverage (Source 1); the magazine report also says printed solder should cover about 40% of the pad (Source 3).
• Solder height after reflow: an equipment maker’s staff member says the average solder height on the pad after reflow is about 10 microns at fine pitch and about 50 microns at coarse pitch; the finer the pitch, the more important it is to get solder volume and thermode size right (Source 3).
• Reflow printed paste first: using paste directly for hot-bar bonding is not recommended, as much of the solder powder will not melt; printed paste should first go through a reflow oven to melt into a layer (Sources 1, 2).
• Tinning methods: printed paste plus reflow gives too much solder if volume is not controlled; dip tinning with hot-air leveling needs a very clean air knife; plating is accurate but gives little solder and costs more; wave tinning may give uneven volume (Source 1). Fine pitch usually uses hot-air leveling, which spreads the solder evenly with a flat surface, giving better heat transfer and easier alignment (Source 2).
• Pad size and flex type decide solder volume: pad size and pitch determine the maximum and minimum printable volume; double-sided window and fully exposed flex can take slightly more solder, and single-sided window a little less (Source 2).
• Use low-solids flux: easily soldered parts need just a small amount of non-activated flux, commonly no-clean; the lower the solids, the less the thermode gets dirty; let the solvent dry before bonding (Source 2). No-clean flux should be bonded within 5 minutes of application (Source 1).
• Keep parts clean: parts should have no dust and no oxidation (Source 2).

What public sources say: the magazine report above says flux is applied to the board and the flex is placed on it, with solder already printed on the pads and reflowed during the surface-mount process; hot-bar bonding can use any flux and solder, including lead-free, and what matters more is the solder volume on the pad; after printing, solder should cover about 40% of the pad, and after reflow the average solder height is about 10 microns at fine pitch and about 50 microns at coarse pitch. The part design guide above says both parts are usually pre-tinned; without pre-tinning, a single-sided window flex on gold- or tin-plated pads may still wet, and the plating on both parts is often enough for a reliable joint; most designs need solder printed separately and reflowed first; repeatability of solder volume is key to process control, the starting point is a 0.006 in stencil with apertures reduced to 40% coverage, and trials often take some time; pad size and pitch determine the upper and lower limits of solder volume, and double-sided window and fully exposed flex can take slightly more solder; fine pitch is usually hot-air leveled first; and just a small amount of non-activated flux is needed, commonly no-clean, with lower solids keeping the thermode cleaner and the solvent dried first. The technical presentation above says printed paste plus reflow gives too much solder if volume is not controlled, dip tinning with hot-air leveling needs a very clean air knife, plating is accurate but gives little solder and is expensive, applying paste directly is not recommended (much of the powder will not melt), and wave tinning may give uneven volume; it recommends 0.127 mm thick paste at about 40% coverage; and no-clean flux should be bonded within 5 minutes of application (Notes, Table 3).

Stage ④: Set temperature, time and pressure, and inspect

How to do it:

• Variables to control: temperature, time, pressure, contact, alignment, materials and heat sinking (Source 1); an automated system for automotive electronics lists temperature, bonding force, distance between parts, flatness and flux coverage as its key parameters (Source 5).
• Profile segments: the standby segment keeps the thermode warm; the base segment starts each part from the same temperature (below the flux’s activation temperature); the preheat segment lets the flux act first and is used just for substrates with very high heat sinking or that crack easily, such as ceramic; the reflow segment brings the solder to 40 °C above its melting point for 2 to 3 seconds; and the cooling segment lifts the head as soon as the solder solidifies (Source 1).
• Set the thermode well above the solder melting point: the heat must pass through the polyimide and is drawn away by the copper underneath: one guide gives 280 to 330 °C when the thermode presses directly on the joint, and 330 to 400 °C for single-sided window (Source 2); another gives 350 to 400 °C for single-sided window (Source 1); the magazine report’s examples are about 300 °C for fully exposed, about 400 °C through polyimide and about 500 °C for metal substrates, while the joint itself reaches just a normal reflow temperature (Source 3).
• Use the lowest temperature and shortest time that work: as long as the joint is good, the less heat the parts take, the less likely they are to be damaged (Sources 1, 2).
• Order of magnitude for time: the thermode reaches peak in about 1.5 to 2 seconds; it stays at peak for about 3 to 8 seconds for the flux to act and the solder to melt; cooling takes about 2 seconds and can be shortened with air (Source 3).
• Lift-off temperature: set cooling to lift the head below the solder’s solidification point; the actual solder temperature is lower than the measured thermode temperature, and in most cases a setting of 180 °C gives no dry joints (Sources 1, 2).
• Pressure: pressure must be sufficient for heat to transfer: one presentation gives about 12 kg/cm² on the thermode face as the lower and upper limit (Source 1); one guide says most joints need under 20 lb (9 kg) (Source 2); equipment maker staff in the magazine report say it is usually very light, under 5 lb, and must be increased in some cases, for example when the board is not flat (Source 3). Too much pressure pushes solder out of the joint and may cause bridging (Source 3). Measure pressure with a mechanical or electronic force gauge (Source 1).
• The thermode must sit flat: the thermode must be coplanar with every joint; uneven contact gives uneven heat transfer and cold joints; check with pressure-sensitive paper or a burn test (Source 1); in the automated system, each head’s position and planarity can be adjusted, and adjusting planarity also changes the thermocouple position (Source 5).
• Alignment: a flex misaligned to the board may short, and a misaligned thermode heats unevenly (Source 1); the fixture must be very accurate, and locating pins, vacuum to hold the flex, or a camera to assist alignment can be used (Sources 2, 3).
• Don’t let the fixture become a heat sink: fixtures of aluminum, brass or copper draw heat away, so tool steel is generally used, with a heat-resistant plastic insert (such as PEEK) under the bonding area; phenolic resin also works but wears quickly (Sources 1, 2). The fixture another report describes is a heat-resistant, ESD-safe plastic nest on an aluminum base (Source 3).
• Inspect the joint: with pressure held until cooling, the chance of a dry joint is small; the joint should show a thermode imprint even in width and length, with signs of melted solder; when the flex is peeled off, the joint area should look grainy; pads must not be burned or lifted from the board or flex; the polyimide surface of a single-sided window may show imprints or discoloration but must not be burned or delaminated (Sources 1, 2).
• Keep the data: the power supply (controller) can set envelopes and upper and lower limits to check that each bond is consistent (Source 1); setting the limits requires recording and analyzing data from many products, and bonding force must be measured and calibrated (Source 5).

Figure 3. HotBar temperature profile and pressure: heating, reflow and cooling, with pressure held from start until the solder solidifies (illustration, based on Sources 1, 2 and 3; not a measured profile). Vertical axis: thermode temperature; horizontal axis: time. Segments: preheat, ramp, reflow, cool, lift-off near the solder’s solidification point. The dark blue bar shows pressure held from the start until the solder solidifies. Preheat is used just for high-heat-sinking or ceramic substrates. The thermode is set much higher than the joint, because heat must pass through the flex and is drawn away by copper. Use the lowest temperature and shortest time that produce a good joint. Too much pressure squeezes solder out and may cause bridging.

What public sources say: the technical presentation above says the process variables are temperature, time, pressure, contact, alignment, materials and heat sinking; the profile has standby, base, ramp, preheat, reflow and cooling segments; the base temperature should be below the flux activation temperature; preheat is used for substrates with high heat sinking or that crack easily, such as ceramic; the reflow segment should bring the solder to 40 °C above its melting point for 2 to 3 seconds; although SAC305 melts at 218 °C, the thermode must be set higher, typically 350 to 400 °C for single-sided window; it is better to use the lowest possible time and temperature; solder temperature is usually lower than the measured thermode temperature, and cooling lift-off can be set to 180 °C in most cases; the thermode face takes about 12 kg/cm² as the lower and upper limit, measured with a force gauge; the thermode must be coplanar with the joints or cold joints will result, checked with pressure-sensitive paper or a burn test; misalignment may cause shorts; aluminum, brass and copper fixtures draw heat away, so tool steel is generally used, with a PEEK insert under the bonding area; the power supply can set envelopes and upper and lower limits; and joints should show an even imprint, look grainy when peeled, and not be burned or lifted. The part design guide above says the thermode is set to 280 to 330 °C for direct contact and 330 to 400 °C for single-sided window; most joints need under 20 lb (9 kg), and force must be precisely controlled and can be calibrated; the fixture must be completely flat, with grinding better than milling; and fine pitch can use an X-Y stage and camera for alignment. The magazine report above says the thermode reaches peak in 1.5 to 2 seconds, about 300 °C for fully exposed, about 400 °C through polyimide and about 500 °C for metal substrates, with about 3 to 8 seconds at peak and about 2 seconds of cooling; the pressure range is 2 to 160 lb, but usually under 5 lb; the three key parameters are time, temperature and pressure, and too much pressure squeezes solder out and may cause bridging. A university research team’s journal paper (2026, studying an automated hot-bar soldering system for automotive electronics) says the key parameters are temperature, bonding force, part distance, flatness and flux coverage; setting limits requires recording and analyzing data from many products; bonding force must be measured and calibrated; and each head’s position and planarity are adjustable (Notes, Table 4).

How JIAHON can help:

• To check the joint’s imprint and signs of melted solder after bonding, and whether pads are burned or lifted, you can photograph and record with a 7-inch digital microscope (the product page says it has 1 to 1,200× continuous zoom, an adjustable lens angle to avoid glare, and photo and video capture for quality records).

Stage ⑤: Choose the thermode and machine

How to do it:

• Thermode shapes: there are pointed, blade, folded and other types (Source 1); there are also three-dimensional (3D) thermodes, which route the current around the working face so there is no potential difference across the traces (Source 2).
• Length: it should fully cover the row of traces, with at least one pad pitch extra at each end (Source 2); another presentation says at least 0.040 in (1 mm) past each end (Source 1); the magazine report says one pad width extra at each end, with length usually no more than 4 in (Source 3).
• Width: one guide gives a minimum of 0.059 in and a standard of 0.079 in, with better life and performance; 0.047 in can be used when solder volume is poorly controlled or space is short, but life and performance will suffer (Source 2). The magazine report says usually 0.05 to 0.08 in wide; it need not be as wide as the pad, and covering about one-third of each pad is enough; when joints are poor, changing the thermode width sometimes solves it (Source 3).
• Width should leave the solder somewhere to go: thermode width should deliver heat in as short a time as possible and leave room for squeezed molten solder, to avoid bridging (Source 2); thermode thickness should be less than pad length so a solder fillet can form (Source 1).
• Board pads about three times as wide: in one guide’s design example, the board pad is about three times the thermode width, giving excess solder somewhere to go and easing inspection (Source 2).
• Materials and construction: wire EDM and special materials produce thermodes with even temperature along their length that stay flat when heated, that solder does not stick to and that resist oxidation (Source 2); the equipment in the magazine report uses titanium thermodes (Source 3).
• Maintain the thermode: regularly clean off flux and polyimide burned onto the surface with isopropyl alcohol or acetone; it can be re-flattened with 600-grit silicon carbide paper at low pressure; cleaning or flattening wrongly damages the thermocouple or leaves the surface uneven; a release film between the thermode and flex reduces contamination, and automatic film feeders are available (Source 1).
• Four parts of the machine: the power supply (controls temperature and time), the bonding head (controls pressure), the thermode (generates heat), and the fixture (positioning and alignment) (Source 4).
• Semi-automatic and automatic: the equipment in the magazine report resembles a benchtop resistance welder, fed by a linear slide or a two-fixture rotary table (Source 3); a five-station rotary automated system has high repeatability, but limited application flexibility, high initial cost and parameters that need frequent calibration (Source 5).
• The same equipment can bond conductive adhesives: one article says conductive adhesive bonding (called heat sealing in the article, covering both ACF and heat seal connectors) can use the same equipment, but with different temperature, time and pressure profiles (Source 4); the magazine report also says hot-bar systems can be set up to bond with adhesive film, where soldering is ramp, hold and cool, while adhesive bonding holds temperature flat without ramp or cooling segments (Source 3).
• How ACF conducts: the conductive particles in the adhesive are usually 3 to 12 microns, and conduct just when pressed between the upper and lower traces, so it conducts in the thickness direction alone; ACF is generally used at fine pitch (6 mil, 0.15 mm); ACF is first tacked at 80 to 100 °C for 3 to 5 seconds, aligned, and then heated and pressed, often with a silicone interface layer or pad between the thermode and flex to keep pressure even across the joint (Source 4).

What public sources say: the technical presentation above says thermodes come in pointed, blade, folded and other types; the thermode must be sized for the application, extending 0.040 in (1 mm) past each end of the pad area, with thickness less than pad length so a solder fillet forms, and a working face large enough to transfer heat; flux and polyimide burned onto the surface should be cleaned off regularly with isopropyl alcohol or acetone, and the face can be re-flattened with 600-grit silicon carbide paper at low pressure; improper cleaning or flattening damages the thermocouple and leaves the face uneven; and release film reduces contamination, with automatic film feeding available. The part design guide above says thermode length should fully cover the traces with one pad pitch extra at each end; width minimum 0.059 in, standard 0.079 in, and 0.047 in where solder volume is poorly controlled or space is limited, with poorer life and performance; width should leave room for molten solder to be squeezed aside; the board pad is about three times the thermode width; wire EDM and special materials produce thermodes with even temperature along their length, flat when heated, that solder does not stick to and that resist oxidation; and 3D thermodes have no potential difference across the traces. The magazine report above says the equipment resembles a benchtop resistance welder, with a titanium thermode mounted on an air cylinder and feeding by linear slide or rotary table; thermodes are custom-made for the part and easy to change, usually 0.05 to 0.08 in wide and no more than 4 in long, with one pad width extra at each end, and need cover just about one-third of each pad; and hot-bar systems can be set up to bond with adhesive film, with adhesive bonding holding a flat temperature without ramp or cooling segments. An article published in an electronics assembly magazine (2000, written by a product manager at an equipment company) says conductive adhesive bonding is called heat sealing in the article, covering ACF and heat seal connectors; the system consists of four parts: power supply, bonding head, thermode and fixture, and the same equipment can do heat sealing but with different temperature, time and pressure profiles; the thermode cross-section should be small so it heats and cools quickly; conductive particles are usually 3 to 12 microns and conduct in the thickness direction alone, with ACF generally used at fine pitch (0.15 mm); and ACF is tacked at 80 to 100 °C for 3 to 5 seconds, then aligned and bonded, often with a silicone interface layer or pad. The 2026 paper above says the automated system has high repeatability, but limited application flexibility, high initial cost and parameters that need frequent calibration (Notes, Table 5).

Notes: sources and exceptions

The following is for readers who want to check; you can follow the steps above without it. Where the equipment, flex or solder makers’ data, your product specifications or your own validation differ from this article, follow them.

Sources for each stage

Table 1 | Principle and uses

Sources: see each row; the pitch boundary is as the magazine report gives it, and the report does not say which interviewee said it.

Table 2 | Part design

Sources: see each row; for the temperature loss through polyimide, Source 2 gives 50 to 80 °C.

Table 3 | Pre-tinning and flux

Sources: see each row; Source 2 states that its solder volume table figures are a reference and must be tested in practice.

Table 4 | Settings and inspection

Sources: see each row; the three sources give temperature and pressure differently, as examples for different equipment and parts.

Table 5 | Thermodes and machines

Sources: see each row; Source 4 dates from 2000, and ACF materials have changed since.

Exceptions and limits

What was seen: this article draws on 5 public sources; 3 of them are hosted on the same hot-bar bonding equipment maker’s website (Sources 1 and 2 are that maker’s materials, and Source 4 was written by a product manager at an equipment company), and Source 5 also studies that maker’s equipment; of the three people Source 3 quotes, two are equipment maker staff; no standard text on HotBar soldering was read.

Conditions behind the figures: thermode temperature, pressure, time and size are given differently in the three sources, and are each source’s examples for its own equipment and parts, not general values; test them with your own parts before production.

Source dates: Sources 1 and 2 carry no publication date, just a copyright year (2024, 2020); Source 3 dates from 2006 and Source 4 from 2000; Source 5 studies one specific automated system.

Scope: troubleshooting solder bridging shorts, and choosing ACF materials and validating their reliability, are outside this article’s scope.

This article’s own compilation: Figures 1 to 3 (Figure 2 based on Sources 1 and 2, Figure 3 on Sources 1, 2 and 3) are this article’s compilation; every other sentence has a matching statement in the sources or on JIAHON’s pages.

Scope of JIAHON support: “How JIAHON can help” gives just the scope of JIAHON engineering support and sentences quoted from pages already published on the site; product specifications follow the product pages, and link text does not include product page models or brands.

This article is a summary of the general case, not a restatement of any standard, not an endorsement of any hot-bar bonding equipment maker, flex maker or solder maker, and not a promise about your product or process. Follow the equipment, flex and solder makers’ data, your product specifications and your own validation; this describes the general case, and individual cases still need their own assessment.

Related reading

• “How to Troubleshoot HotBar Solder Bridging” (/en/hotbar-solder-bridging-troubleshooting/, EN593)

Request a quote

Request a quote →