Surface Mount and the Gull Wing

Surface mount is assembled by printing solder paste, placing parts on it, and heating the whole board through a profile. Reproduce that, then attack the result. PRINT THE PASTE. A stencil is a thin steel or polyimide sheet with apertures over each pad. You can cut one from 100 micrometre polyimide with a craft knife for a coarse footprint, or buy one, or simply dispense paste from a syringe onto each pad. Aim for a consistent volume — the joint's strength and its stand-off both depend on it, and inconsistent paste is the commonest cause of a bad hand-reflowed board. PLACE THE PARTS with tweezers. Precision is less critical than people expect, because of a genuinely useful effect: molten solder's surface tension pulls a slightly misplaced part into alignment with its pads. A part placed within a third of a pad width will usually self-centre. That self-alignment is one of the quiet reasons surface mount scaled to machine assembly at all. REFLOW IT. A hotplate works for a small board, or a modified toaster oven, or a proper reflow plate. The profile matters and has four phases: preheat to about 150 degrees to bring everything up evenly, soak at 150 to 180 for a minute or two to activate the flux and let the board equalise, ramp to peak — around 240 for lead-free, 215 for leaded — for 30 to 60 seconds above the melting point, then cool. WATCH IT REFLOW. There is a distinct moment where the paste goes from dull grey to bright and liquid, and parts visibly twitch into alignment. That moment is the process; everything before it is preparation and everything after is cooling. INSPECT THE FILLETS. A good gull-wing joint has a concave fillet that wets up the side of the lead. Too little paste gives a thin joint with no fillet; too much gives a bulging joint and risks bridging to the neighbour. Photograph them under the microscope — and note that you CAN photograph them, which you could not do with a J-lead or a BGA. NOW THE MEASUREMENT THAT MATTERS. Wire the board so you can measure continuity through a chain of joints — a daisy chain through every pin of a part, which is exactly what test coupons do in industry. Thermal cycle it. Between a domestic freezer and an oven at 100 degrees, twenty minutes each way, and measure the chain resistance every ten cycles. Or use a hotplate and a fan for faster, smaller cycles. The resistance rises before anything opens. A solder joint under cyclic strain accumulates fatigue cracks, and a partially cracked joint is a higher-resistance joint long before it is an open one. Plot resistance against cycle count and you get a curve that is flat, then rises, then goes open — the same shape as flash endurance and the same shape as die-attach delamination, from a completely different mechanism. That is why through-hole survives in high-vibration and high-cycling applications: a through-hole lead is mechanically anchored by the board itself, and its solder joint is not the only thing holding it on.