DVD-71C IPC-A-610E Common Inspection Errors

Transcription

1 DVD-71C IPC-A-610E Common Inspection Errors Below is a copy of the narration for DVD-71C. The contents for this script were developed by a review group of industry experts and were based on the best available knowledge at the time of development. The narration may be helpful for translation and technical reference. Copyright IPC Association Connecting Electronics Industries. All Rights Reserved. Introduction FLOYD (on camera) Hi I'm Floyd Bertagnolli. I m an IPC-A-610 Master Trainer. and President of STM, an authorized IPC Training Center. After years of working with companies in Electronics Assembly there appear to be certain common misinterpretations of the IPC -A-610 acceptance criteria. These misinterpretations can cost your company time, money, and even your reputation for quality. In this video we re going to explain some of these misperceptions, to make sure that your company doesn t fall prey to: The Common inspection errors of the IPC-A-610.l me in the morning. NARRATOR Without an industry standard like IPC-A-610, each company would be using an internal specification. Our industry would not have a common language and the consistent criteria required to properly evaluate electronic assemblies. Incorrect evaluation can result in loss of revenue. Over inspection or incomplete inspection is also expensive. Consider two different inspectors. One inspector is too lenient, and lets defective product go out the door. Later it fails and now the product must be returned for repair. The cost to return the product, the repairs made to fix the product, and the cost to re-ship the product back to the customer can be expensive, not to mention losing reputation -- and even customers. Another inspector is too strict. Over inspection is a common way companies spend money unnecessarily. The inspector asking for rework often doesn t see or think about the cost. By having an industry consensus workmanship standard, we can have a common language between assemblers, inspectors, and customers. Following the IPC-A-610 correctly and avoiding the most common inspection errors will save your company time and money. 1

2 The common misinterpretations that we ll be covering in this video include: inspector s responsibilities; identifying board sides; electrical clearance; wetting angles; vertical fill of through hole solder; through hole part clearance; gull wing leads; bottom termination components; measling; and billboarding. Inspector s Responsibilities Let s begin with inspector s responsibilities. A serious mistake can occur when an inspector evaluates an assembly with excellent solder joints -- and identifies them as Class 3 solder joints. The reality of this situation is that the customer determines the class of product your company is building. The inspector only applies the acceptance criteria. A solder joint that barely meets the minimum acceptance requirements for an mp3 player would definitely be undesirable for a life support system. As you know, electronic products are divided into three classes depending on the ultimate use; the life expectancy; and the operating environment of the electronic assembly. Let s review these classes of products. Class 1 refers to general electronic products which include consumer electronics such as televisions, stereos and video games. Class 2 includes high-end computers, telephone control systems and other commercial equipment that falls into the category of dedicated service electronic products. The Class 3 category is for high performance electronic products equipment with high reliability applications such as military, aerospace and life- support systems. The customer specifies the class your company will be building to and the inspector has the responsibility to determine the four levels of acceptance for the solder joint based upon the class the customer has chosen. These four levels are target, acceptable, process indicator and defect. The target solder joint is close to perfect the most desirable condition but not always achievable. An acceptable solder joint is not necessarily perfect, but provides a satisfactory mechanical and electrical connection to operate properly. A process indicator identifies a characteristic that does not negatively affect form, fit or function of a product. It is not a defect. The solder connection will work fine and be reliable. A process indicator tells you that there is an opportunity for improvement in the quality of your solder joints and other elements of the completed assembly. The process indicator usually points to a difficulty in the process that should be remedied. The process can include machine adjustments, product design, defective components and training needed. Finally, a defect does affect form, fit or function and will require rework, repair or even scrapping the assembly. The problem with applying class 1 acceptance criteria to class 3 products is obvious. However, applying class 3 criteria to class 1 products make the class 1 products much more expensive to manufacture. As you can imagine, it could be a time consuming and expensive mistake for an inspector to ask for rework of a solder joint that did not need to be reworked based on class of product. And that mistake can also add unwanted heat to the circuit board causing internal damage. 2

3 Identifying Board Sides The customer specifies the class your company will be building to and the inspector has the responsibility to determine the four levels of acceptance for the solder joint based upon the class the customer has chosen. These four levels are target, acceptable, process indicator and defect. The IPC-A-610 has expanded the terminology. For example, board sides for through hole assemblies are called the solder source side and solder destination side -- but have no meaning for surface mount assemblies. Solder source refers to where the solder is applied and solder destination refers to the side of the board where the through hole components can be seen. For surface mount assemblies, we identify the primary and secondary sides of the board. The primary side is usually the side of the board that contains the most complex, or the highest number of components. The secondary side of a surface mount assembly has the less complex or lowest number of components. When referring to the two sides of the circuit board assembly, it s important to understand and use the correct terminology so there are no misunderstandings. That s because many A-610 inspection criteria are specific to one side of the board or the other. If there s confusion over the side of the board, an inspector can easily turn to the wrong page and get the wrong answer. For example, the criteria regarding land coverage for a through hole solder joint relates specifically to the solder source side of the circuit board. Examining the A-610, we see that lands on the solder source side need some percentage of wetting to be acceptable and that the land area on the solder destination side does not need to be wetted. If we switched and applied the solder destination criteria to the solder source side, then we would be allowing defects through. Electrical Clearance Now, let s take a look at a common difficulty that occurs when inspecting for electrical clearance. Minimum electrical clearance is defined in the applicable design standard or on the approved documentation for the assembly. If there is a question, Table 6.1 in Appendix A of the A-610 contains the specific information that can be used to determine whether the electrical clearance of conductors is acceptable or in violation. Some inspectors refer to an unacceptable electrical clearance as a VMEC which stands for violating minimum electrical clearance. Understanding minimum electrical clearance can be tricky because it s based on how much voltage is generated between conductors. Be sure to refer to Appendix A for specific measurements. For example, any two uncoated, or electrically non-common through hole leads protruding from the solder source side may be clinched too close to one another. A VMEC condition may result in a short circuit, arc or signal crosstalk. Category A6 in table 6.1 lists the minimum electrical clearances between conductors based upon the peak voltages specified in the left hand column. Those peak voltages are usually called out in the customer documentation. If you have any questions about peak voltages or on how to use this table, ask your supervisor or trainer. 3

4 Wetting Angles Now let s discuss wetting angles. How do you recognize an acceptable solder joint? There are acceptance guidelines, but there are also exceptions. Wetting is the most important characteristic of a solder connection. Wetting is defined as the formation of a relatively uniform, smooth, unbroken and adherent film of solder metallurgically bonded to a basis metal. Misinterpreting the requirements of an acceptable solder joint can affect a company s bottom line. Excessive rework not only causes delays, but also costs our industry millions of dollars. Just because a solder joint doesn t look pretty isn t justification for recommending rework. Various degrees of wetting are characterized by the wetting angle, or angle of contact between the solder and the basis metal. A smaller contact angle between the two surfaces is a general indication of better wetting and a stronger bond. Larger contact angles can be an indication of reduced strength, and may indicate the use of excess solder which is often not a defect. Also, keep in mind that lead free solder may typically have a greater contact angle than tin lead solder. On any type of solder joint, the target wetting angle is 90 degrees or less. A contact angle that exceeds 90 degrees usually indicates poor wetting, or excessive solder. If a solder joint has a convex appearance as a result of excessive solder, it is still acceptable if it covers all of the metal land, or pad and doesn t curl underneath itself leaving exposed land area visible. Convex fillets caused by excess solder extending over the land are an acceptable condition if the solder extends to the full length of the solderable land surface. Figure 5-1 of the IPC-A-610 illustrates four wetting angles. The text says that the wetting angle of a solder joint shall not exceed 90 degrees. As you can see, example A shows a wetting angle of less than 90 degrees and we love that because that s the target. Example B is also fine since it s exactly 90 degrees. Examples C and D are also acceptable but are an exception to the rule. That s because they are both greater than 90 degrees. Connections exceeding 90 degrees can be acceptable when the solder contour extends over the edge of the solderable termination or solder resist due to the quantity of solder. Asking for rework on these acceptable connections is a waste of time and money. On the other hand, that exception to the rule would not include the example shown in Figure In this case, we have a non-wetting condition on most of the land that makes this solder joint a defect even though it contains similar wetting angle characteristics as shown in examples B and D of Figure

5 Vertical Fill of Through Hole Solder Next, let s turn our attention to through hole solder specifically the criteria for vertical fill of solder inside the plated through hole. To evaluate this condition we must look at the solder joint from both the component and the solder sides. We've seen that the ideal or target connection has a slightly concave, cone-shaped fillet that rises from the outer edge of the land to the component lead. But the solder joints you see every day may not always live up to this ideal. One common variation of the fillet's shape is a slight dip into the through-hole before the fillet rises onto the lead. This condition is especially common on the solder destination side of wave soldered boards since gravity can cause the solder to sag slightly into the hole as it solidifies. As long as the solder joint meets all the other requirements for acceptance, some amount of solder depression is allowed. A maximum of 25% solder depression, on either the solder destination or solder source side, is permitted. This means that a minimum of three quarters of the barrel must be filled with solder. Some inspectors have a tendency to ask for rework whenever there is less than 100% vertical fill. Ignoring vertical fill or not understanding that 100% is not needed can be costly. It s important to apply the correct amount of vertical fill to the class of product that you are building. Section of the A-610 is the place to go. As you can see, Figures 7-88 through 7-91 provide the criteria. Notice that for class 2, 50% vertical fill is acceptable when there is a thermal heat sink plane surrounding the plated through hole. Through Hole Part Clearance Through hole part clearance is another inspection criteria that causes confusion. Again, over inspection, or the lack of it is expensive. Part clearance is defined as the space between the bottom of a through hole component and the top of the board. But what is the true minimum and maximum clearance? The answer is not in the Acceptable criteria, but often in the Process Indicator requirements. For example, let s look at axial lead, vertically inserted components. Even though Table 7-2 states there is a minimum amount allowed for component to land clearance, you will notice it is a process indicator when the clearance is less than the minimum specified as acceptable in Table 7-2. It s also a process indicator when the clearance is greater than the maximum -- specified in Table 7-2. In this situation, these process indicators are completely shippable even when there s no clearance at all. Once again, misunderstanding or misinterpreting the criteria is a possible source of unnecessary rework. That s why it s so important to carefully read all of the acceptance criteria in the A

6 Gull Wing Leads At this point, we ll take a look at some common misunderstandings associated with surface mount gull wing leads. Misidentifying gull wing leads can cause the inspector to apply the wrong rules to the component they are inspecting. The general inspection criteria is dependent on whether the gull wing lead has a long foot or a short foot. For example, If there is a long foot length meaning that L is equal to or greater than three lead, or toe widths the side joint length should be a minimum of three lead widths, or 75% of the foot length, whichever is longer. If there is a short foot length meaning that L is less than three lead, or toe widths, the minimum side joint length is 100% of the foot length. There are also different rules for the low profile leads found on SOICs and SOTs; and the high profile leads found on QFPs. In addition, heel fillets for gull wing leads provide the most important electrical and mechanical connection. Therefore, care must be taken to make sure that the heel fillet is the proper height. The fillet height may extend to the top bend of the lead, or knee, but should not touch the component body or end seal as a maximum fillet height. As an exception to this rule, solder may touch the body of plastic SOIC or SOT components. If you overlook this exception, you may ask for unnecessary rework thereby creating extra operating costs for your company. Bottom Termination Components Now that we ve discussed gull wing leads, let s examine another category of surface mount components called bottom termination components, or BTCs. Misunderstanding these new parts, or the new grouping of many older parts can be a cause for misapplying the 610 standards. Examples of BTCs include Dual Flat No-Leads, or DFNs; Quad Flat No-Leads, or QFNs; and Leadless Chip Carriers, or LCCs. There are other types of leadless components that don t fall into the BTC category. For example, array style components, such as ball grid arrays, or BGAs, have their own acceptance requirements. It s important to apply the dimensional criteria specified in Table 8-15 for Bottom Termination Components. Figure illustrates the heel and toe dimensions for BTCs. Dimension C defines the minimum end joint width. Notice that the minimum end joint width is 50 percent of the termination width for class 1 and 75 percent of the termination width for classes 2 and 3. Measling Now, let s take a look at how measling can be misunderstood. It s not uncommon for an inspector to examine measling on the circuit board and to identify it as a defect. After all, measles can be pretty ugly. Measling occurs in laminated base material when the glass fibers are separated from the resin at the weave intersection. This condition appears as discrete white spots or crosses below the surface of the base material, and is usually related to thermally induced stress. 6

7 What s important to be aware of is there is no defect condition for measling no matter how unattractive it is. Even when measled areas in laminate substrates exceed 50 percent of the physical spacing between internal conductors, it is still a process indicator for class 3 and the product can be shipped. There are other laminate conditions such as blistering and delamination that do have defective criteria. You ll need to be able to distinguish between the various laminate conditions you may encounter. Billboarding Our last common inspection error has to do with billboarding. Billboarding occurs when a surface mount chip component is mounted on its side rather than on the bottom of the component. Billboarding can be confused with tombstoning which is always a defect. Treating billboarding as an instant defect can be an unnecessary rework expense. Billboarding is not an automatic acceptance condition either. If you carefully follow the criteria for acceptability in paragraph , then billboarding is allowed and is a totally acceptable electrical and mechanical connection. Summary This program has presented the most common inspection errors when using the IPC-A-610 for acceptance criteria. We discussed the inspector s responsibilities; properly identifying board sides; electrical clearance; wetting angles; vertical fill of through hole solder; through hole part clearance; gull wing leads; bottom termination components; measling; and billboarding. The more familiar you become with the terminology and the acceptance criteria of the IPC-A-610 the more successful you ll be in accurately determining whether a circuit board assembly should be shipped to the customer or whether the assembly requires rework. That determination can make a big difference in the quality of the product you produce and the profitability of your company. Those ingredients make it a win-win situation for everyone involved. 7