SMT007 Magazine

SMT-Aug2015

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88 SMT Magazine • August 2015 by Robert voigt DDM noVASTAr SMT QuICk TIPS Selecting a Reflow Oven, Part 1 Column A reflow oven is simply any device that can liquefy solder in a controlled way to produce an electrical conductive bond between the compo- nent and its host (the board). There are several different methods to do this, not all of which can literally be called ovens. For instance, the simplest form of heating is performed by con- duction, that is, by contact with a heated sur- face such as a hot plate. Other heating methods typically performed within a chamber (oven) but that rely on different heating methods in- clude: 1. Convection: Boards with their assembled components are passed through heated, circulating air from a conventional electrical heat source. 2. Infrared (IR): Boards are passed inside a chamber over a direct IR heat source. 3. Vapor Phase: Vapor is generated by heating a fluid with a specific boiling point (240°C) and transfers heat to the circuit board just above the melting point of the solder. Let's begin with a commonly asked ques- tion: What size oven do I need? The answer is also a question: How many zones can you af- ford? It seems like an odd way to start a discussion about reflow ovens, but it's not entirely unrea- sonable. As with any complex process, there will be tradeoffs between cost and capabilities, and more zones will always give you better flexibil- ity and more control over your profile—but at a cost. The decision has to be qualified primar- ily on your anticipated throughput; that is, how many boards you process in a day or a week. There are other considerations too, such as board size, component density, and appropriate thermal technology, but we'll talk about those (and more) after identifying the work-flow vol- ume. Following is a guideline (Table 1) for zones relating to typical volume needs. A typical soldering operation in today's world requires three main stages for tempera- ture profiling: preheat, soak (activate), and reflow, which perform these functions: 1) The preheat stage for a certain period of time to ac- tivation temperature; 2) the soak stage for a dif- ferent period of time to activate the solder; and 3) the reflow stage where temperatures peak for yet a different time frame. After this, the board is typically cooled and removed. Depending on the material, e.g., leaded, lead-free or specialty mate - rials such as epoxies, the heating profile for each stage will vary according to the manufacturer's specifications to achieve optimal bonding. Board Throughput Type/Zones Typical Cost Range 1–5 boards/day Single or dual hot plate; no zones $1,000–$2,000 12–15 boards/day batch oven, single zone $2,500–$6,000 100 boards/day** conveyorized 3-zone oven $10,000–$15,000 over 100 boards/day no. of zones dependent on speed & target volume up to $200,000 ** in a typical low- to mid-volume production world, a small conveyorized horizontal convection oven meets most basic requirements. Table 1.

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