Soldering And Staking OEpic TOSA/ROSA Flex Circuits To PWBs.

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1 APP Rev 03 This application note is intended to give a person involved in the manufacturing engineering of a typical XFP module an overview of the procedures and requirements for soldering an OEpic TOSA or ROSA to a printed wiring board (). There is a considerable amount of general information available from literature and solder equipment manufacturers on the subject of soldering flex circuits to s but this should help with the particulars of OEpic s components. Initial assembly assumptions: The has been populated with most of the regular surface mount components and at this point only secondary assembly operations are being performed with localized heating. This is because the main body of TOSA or ROSA must not be exposed to solder reflow temperatures. Only the terminal end of the flex board can be heated to reflow temperature. The land pattern on the has been designed according to OEpic document DWG , Rev 02. The land pads on the have been tinned with Sn60 solder and cleaned. The pads on the flex circuit are supplied with an SN60 tinned surface. OEpic is currently looking into the use of lead free solders and will offer a lead free version in the future. Soldering Instructions: The main objective is to bring the flex circuit pads in contact with the land pads and supply just enough localized heat to cause the solder to melt and join. There are several methods for supplying the localized heat such as Hot Bar, Thermode, and Hot Gas. In each case the is preheated from the backside on a heated stage to an intermediate temperature usually between o C. Then either the Hot Bar, Thermode, or Hot Gas supplies the extra heat needed to raise the local temperature to o C. A light coating of water soluble flux such as Kester 951 should be applied to the flex circuit and pads. For a Hot Bar system, the heat is supplied through a preheated flat rectangular bar that acts like a soldering iron. The bar contact area completely covers the area of the pads to be soldered. The must be fixtured to hold the flex board pads aligned and in contact with the land pads. When the bar is brought into contact with the flex circuit/ stack it causes the solder to melt and join. The Hot Bar temperature is set to a constant temperature of o C. The solder stays molten until the bar is removed. The challenge with this system is holding the flex board in contact with the when the Hot Bar is removed without disturbing the solder joint as it cools. April 2003

2 A Thermode system (figure 1) is similar to a Hot Bar system except that the bar, or Thermode, is itself a heating element that is initially cool when brought in contact with the flex circuit/ stack. Electric current is passed through the Thermode causing it to heat rapidly to o C and melt the solder. Then, while the Thermode is still in contact with the circuit, the current is removed and the Thermode cools to below the solder reflow temperature. After cooling the Thermode can be lifted off the circuit without concern for disturbing the solder joint, and therefore, simplifying the fixturing. THERMODE BUSS BARS THERMODE HEATER ELEMENT FLEX CIRCUIT BOARD HEATER BLOCK Figure 1 With Hot Gas, a heating element within a quartz tube causes a flow of nitrogen gas to be heated to o C which is directed to the solder joint area. The flex circuit must be fixtured well to hold it in contact with the. Then the Hot Gas is applied until the solder melts and joins. When the gas is turned off the joint cools and the fixturing can be removed. The problem with this type of system is keeping the heat localized. The Hot Gas can heat the to the point of damage if it is not shielded. OEpic recommends the Thermode type system for ease of fixturing and solder joint quality. Unitek Equipment manufactures a line of Thermode soldering equipment from simple manual systems to production scale systems Page 2 of 5

3 Unitek Equipment Corporate Office: 1820 S. Myrtle Ave. PO Box 5033 Monrovia, Ca Tel (626) Fax (626) Web Unitek sales representative in the San Francisco Bay Area: Mytech Associates, Inc B-1 Walsh Ave. Santa Clara, Ca Tel (408) Fax (408) The solder flux should be carefully cleaned from the flex circuit/ using o F deionized water. Great care must be used to avoid getting water into the ferrule bore of the receptacle which may contaminate the lens. A silicone rubber plug inserted into the receptacle bore is recommended. Epoxy Staking Instructions: When the flex circuit is formed to fit the into the housing, significant stress is applied to the solder joint which can cause the pads to peel off or the solder joint to separate. Therefore it is important to apply an epoxy stiffening fillet between the backside of the flex circuit and the edge of the. There are many epoxies that can be used but OEpic recommends the following: Emerson & Cuming Tel Web Stycast 2651 / Catalyst 15 Mix Ratio - 2 parts Stycast 2651 to 1 part Catalyst 15 by weight. Working life at 25 o C 2 hours. Cure at 85 o C for 1 hour Page 3 of 5

4 Using an syringe type epoxy dispenser with a 22 gage needle tip, apply a 0.75mm radius fillet of epoxy between the back of the flex circuit and the edge of the, (figure 2). Place in the curing oven fixtured such that the epoxy fillet is facing upward. During curing the epoxy will flow between the solder pads and add extra support. EPOXY FILLET FLEX CIRCUIT BOARD Figure 2 Page 4 of 5

5 At this point the flex board may be formed to fit the into the housing. Care should be taken to ensure that a 0.8mm minimum bend radius is not exceeded at any point in the forming process. Typically the natural shape the flex board assumes as the is rotated from the initial vertical position to the horizontal position for insertion into the housing will be the least stressful to the traces on the flex board, (figure 3). TYPICAL XFP MODULE ORIENTATION FLEX CIRCUIT MIN RADIUS 0.8mm Figure 3 OEpic Inc. reserves the right to make changes to the product(s) or information contained herein without notice. The information contained in this document is considered to be accurate as of the date of publication. No liability is assumed by OEpic for use of any information contained in this document, or for infringement of any patent rights or any other proprietary rights of third parties which may result from such use. Page 5 of 5