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DVD-97CC TranscriptAn inner layer connection is one where the connection to the plated through hole is made insideor below the surface of the board. Eyelet insertion on a plated through hole with internalelectrical connections is a complex procedure and is not described in this tape. We will onlycover the eyelet insertion procedure for plated through holes with external surface conductors.Let’s began the repair process with an analysis of the damage. When analyzing plated throughhole damage on multi layer boards, first check for inner layer electrical connections. The mostreliable method for discovering if there is an inner layer connection is to review the engineeringschematics (sometimes called the artwork) for that particular multi-layer board.Next, look for board damage and repair it if necessary. If the land is partially lifted, check to seeif the plating around the inside of the land is cracked, chipped or separated from the land. If theplating has been damaged, proceed with an eyelet replacement. Now, let’s begin the actual repairwork.We’ll start by cleaning the repair area. Then remove any contamination or solder masking ifnecessary and clean the area once again. Next we’ll select a replacement eyelet. Most eyeletscome from the manufacturer with a flat surface on one end, called a flange. We will call thisprefabricated flange the “factory flange. A technician makes the second “non-factory” flange byflaring and flattening the material that sticks through the other side of the hole after the eyelet hasbeen inserted.The selection of the replacement eyelet requires some simple math. To do the math you have tomake several measurements. We’ll explain what these measurements are right now. Allmeasurements can be made in the metric system of measure (centimeters, millimeters etc.) or inthe English system of measure (inches, feet etc.) For the purposes of this program we will use theEnglish system.The first measurement that must be made is the overall thickness of the board. Board thickness ismeasured in thousandths of an inch or “mils.” If a board is measured at 62 thousandths of an inchthick, you may hear the term, “the board is 62 mils thick.”Next, we’ll look at the Inside Diameter or ID of the replacement eyelet. The eyelet diametershould always be between three thousandths and twenty thousandths of an inch greater than thediameter of the component lead that will go through the eyelet. In other words the inside diameterof the eyelet must be bigger than the component lead but not too much bigger.Another critical measure is the Length Under the Flange, also called the LUF. This is the totallength of the eyelet minus the thickness of the factory flange. This length should be between 25and 35 thousandths longer than the thickness of the board. The protruding portion will beflattened or swaged to form the flange on that side of the board. In other words the inserted eyeletneeds to stick out of the board enough to create a flange.FD is Flange Diameter, or the width of the factory flange. The eyelet’s factory flange diametershould be slightly smaller than the land it sits on to provide for an acceptable solder fillet.Acceptable solder fillets are described in IPC’s publication A-610, Acceptability Of ElectronicAssemblies. This specific information can be found in the section on soldering.2
DVD-97CC TranscriptThe next measurement is called the Outside Diameter of the eyelet, or OD. The hole that theeyelet goes into can be between 1 and 5 thousandths larger than the eyelet’s outside diameter. Thehole should be big enough to allow the eyelet to be inserted without force, but small enough sothat the eyelet fits snugly: Not too tight, not too loose.Let’s review an example to show how a typical eyelet is selected. The first step is to measure thethickness of the board using a dial caliper or micrometer. Place the board between the jaws of thecaliper or the measuring faces of the micrometer and move the ends together until they lightlytouch either side of the board. Then check the reading. For the sake of our example, let’s say thatwe have measured a board, and it’s 62 thousandths, or 62 mils thick.Next, lets find the flange diameter measurement. To get this measurement locate one of the otherundamaged holes that the through hole component used. This is sometimes called a “sister” or“companion” hole. We’ll measure the diameter of the land above the companion hole. Thepreferred diameter of the factory flange on your replacement eyelet should be slightly smallerthan the companion hole’s land. It can’t be any smaller than the hole in the board or the eyeletwould fall out. It also can’t be so large that it interferes with the minimum spacing requirementsof nearby circuitry. For our example, the companion hole’s land measured 95 thousandths indiameter.Now we need to find the inside diameter of the replacement eyelet. As mentioned earlier, acorrect amount of space between the component lead and the eyelet wall is within the range of 3to 20 thousandths. The eyelet ID has to be large enough to leave space between the componentlead and the inside wall of the eyelet. This space will allow solder to wick up through the eyeletall the way to the top of the hole. However, if the space between the side of the lead and theinside wall of the eyelet is excessive, the molten solder won’t stay up in the barrel. It may justflow back out, or cause pits, voids or insufficient solder coverage.The typical method for a technician to measure the ID of the eyelet is to use a set of pin gauges tomeasure the inside diameter of a companion hole’s barrel. We know that the inside diameter ofour eyelet must be very close to, or the same size as, the inside diameter of the companion hole.Remember that the leads of the replacement component that will go in both holes are the samesize. So for the sake of our example lets say that the ID of the companion hole is fortythousandths.The next measurement we need to find is the OD or “outside diameter” of the eyelet. To find thismeasurement you must know the diameter of the hole that the replacement eyelet will fit into.The diameter of the hole will be the same size as the diameter of the drill bit or the ball mill youuse to ream out the hole. Remember that in order for the component lead to fit into the repairedhole, we must grind all of the damaged plating out of the hole. So we’ll select a ball mill or drillbit that is just slightly larger than the measurement of the companion hole’s diameter.3
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