The Lead Frame and the DIP

The industrial frame is stamped from strip at thousands per minute, or photochemically etched for fine pitches. Etching is what you can do, and you already have the process from the photolithography blueprint. THE MATERIAL. 0.25 millimetre brass or phosphor bronze shim. Real frames use copper alloys such as C194 for thermal conductivity, or an iron-nickel alloy where matching the thermal expansion of the moulding matters more than heat. Brass is a fine stand-in and it etches in the same ferric chloride. DESIGN THE FRAME at four times DIP scale so it is buildable by hand: leads on a 2.5 millimetre pitch instead of 2.54 tenths, a paddle 15 millimetres square, tie bars at the corners, and a dam bar crossing all the leads 8 millimetres out from the paddle. Draw it, print it on transparency, and etch it exactly as you etched copper board. FORM THE LEADS. The frame comes off the etch flat. A DIP's leads step DOWN from the moulding and then run vertically, so the frame must be bent twice per lead. Make a simple forming jig from two pieces of aluminium bar with the step machined or filed into them, and press the lead comb between them. Doing all the leads in one press is the point — bending them individually guarantees they will not be coplanar, and a DIP whose pins are not coplanar will not go into a socket. MOUNT A DUMMY DIE. A 5 millimetre square of silicon wafer, glass, or even ceramic tile, bonded to the paddle with a drop of epoxy. Nothing electrical is happening here; you are practising the mechanical sequence. MOULD IT. Real packages are transfer moulded: epoxy pellets softened and forced into a heated steel cavity at pressure, cured in a couple of minutes. You can cast instead. Make a two-part silicone mould around a wooden or printed master of the package body, place the frame with the die into it, and pour black casting epoxy. THE DAM BAR EARNS ITS NAME HERE. Cast without it and epoxy runs out between the leads and sets there as flash, and you will spend longer cleaning it off than the moulding took. Cast with it and the resin stops at the bar. That is worth doing wrong once deliberately. TRIM AND TEST. When cured, cut the tie bars and the dam bar with a sharp chisel or a fine saw against a backing block. Now — and only now — check with a multimeter that every lead is isolated from every other. Before trimming they will all read as shorted, which is the correct answer and a good demonstration of what the frame was doing. A MISSED CUT IS A REAL DEFECT. Leave one dam-bar segment uncut and two adjacent pins stay shorted inside a part that looks perfect. That failure mode is exactly why finished packages are electrically tested rather than visually inspected.