Transcript - IPC Training Home Page

DVD-41C TranscriptThis reduction of unnecessary rework has several advantages... first, it saves time... but it can alsohave a beneficial effect on the overall reliability of the assembly. To understand how this works,we’ll need to look inside a plated-through hole - to understand what happens to the structure ofthe materials whenever heat is applied.Let’s begin by reviewing how the plated-through holes were originally created. After the holesare drilled in the circuit board, the walls of the holes are plated with copper to create an electricalconnection between both sides of the board - and any internal layers of circuitry that may exist.The copper is often overplated with tin or tin lead during the electroplating operation.Eventually the component leads will be inserted into these plated-through holes - and soldered -either by hand - in a wave soldering machine - or in a solder reflow process. The heat of thesoldering operation is pretty intense... in fact, it’s hot enough to cause the laminate material toexpand in thickness.Every material reacts to heat differently... and will also expand at different rates. There’s actuallya numerical index that exists called the Coefficient of Thermal Expansion - which measures howmuch a specific material will expand when it’s heated. Notice the difference between thesenumbers for a typical epoxy laminate material and the copper plating on the walls of a platedthrough-hole.What this means is that epoxy laminate will expand much faster than the copper - during solderrework. When the laminate grows - the copper can actually crack - if it’s not soft or ductileenough to absorb this expansion. Most rework operations will require two heating cycles - one toremove and another to replace the solder. That’s two extra chances to create internal cracks in theplated through-holes.On multilayer boards that have internal connections to the innerlayer circuitry, even a small crackcan cause an electrical open or intermittent failure. This type of internal damage will not even bevisible to the rework technician. Also keep in mind that the laminate will also expand and shrink- to a lesser degree - whenever an electronic product is turned on and off. This can aggravate asmall crack that might not show up during electrical test - into an eventual failure out in the field.And that translates - in any language - into unhappy customers.There’s another reason why we don’t want to rework unless it’s absolutely necessary. Everytime heat is applied to the connection, it will increase the growth or thickness of the intermetalliclayer. The intermetallic layer is a thin copper-tin compound that is created when the solder bondsto the copper land and the component lead or termination.While the intermetallic layer is the critical and necessary part of the connection forming the bond– excessive intermetallic thickness will cause solder joint strength to be reduced. This is becausethe intermetallic portion of the solder joint is more brittle than the rest of the joint. Therefore, thethicker the intermetallic becomes, the more prone it is to a physical failure - such as cracking.This defect is also invisible to the eye. You won’t even know the solder joint is degraded untilit's too late – meaning a failure occurs.The point of all this is to explain why it’s very important to avoid unnecessary rework. Theattempt to obtain cosmetic perfection in the past may have actually decreased the quality andreliability of some interconnections. The acceptance of a solder connection should always bebased on the requirements of your company’s workmanship standards -- and not on anyindividual’s personal expectations. If you would like a detailed explanation of the acceptance2