Solderability and joint design: thermal solderability demands solder not cool below its melting point during joint formation.Successful wave soldering Applying liquid solder to the underside of printed circuit boards in order to bond the chips and discrete components that are placed on top of the board and whose metal leads (pins) extend through the board. requires that components to be joined by solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i. withstand the process without damage. This sounds obvious, but it is not always an easy requirement to satisfy considering that the components must have good wettability (or surface solderability) to form good solder joints, combined with adequate dwell times The time cargo remains in a terminal's in-transit storage area while awaiting shipment by clearance transportation. See also storage. in the liquid solder to form these good joints.
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The design of the joint must be such that the solder will stay liquid during the soldering soldering
Process that uses metal alloys with low melting points to join metallic surfaces without melting them. Tin-lead solders, once widely used in the electrical and plumbing industries, are now replaced by lead-free alloys. process and will not cool below its melting point melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and during joint formation. This is termed thermal solderability. Only when the demands for surface solderability and thermal solderability are met can solder fill the holes in a joint completely.
R.J. Klein Wassink provides several industrial standards that give directives for the measurement of (surface) solderability. (1) In these documents, test conditions such as test time, temperature and flux type are prescribed. Also, test criteria are provided.
In a thermal solderability test, we must deal with both the surface
and thermal aspects, but to achieve a clear understanding, it is useful to separate solderability aspects into these two categories. The thermal aspect in soldering becomes important if the heatsink effect during joint formation becomes so strong that good wetting within the process settings is obstructed ob·struct
tr.v. ob·struct·ed, ob·struct·ing, ob·structs
1. To block or fill (a passage) with obstacles or an obstacle. See Synonyms at block.
2. by solidifying solder.
Preventing thermal damage. During the soldering process, sufficient heat must be applied to the joint area to create a sound solder joint; conversely, part of that heat will be transported toward the component body. Most components cannot withstand excessively high temperatures; therefore, it is important to know some ways to prevent thermal damage during soldering.
The most effective method for wave or dip soldering Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board (those not protected with solder mask), creating a reliable , as well as selective soldering Selective soldering is the process of soldering only through-hole electronic components onto a printed circuit board that has surface mount components on the under-side. This is usually done because the surface mounted component is not glued into place, instead solder paste is used , is to use a standoff or spacer between the component body and joint area. This extra lead length acts as a heat resistor resistor, two-terminal electric circuit component that offers opposition to an electric current. Resistors are normally designed and operated so that, with varying levels of current, variations of their resistance values are negligible (see resistance). and thus inhibits temperature rise in the component body.
Specific soldering distance is the minimum space needed between a component body and the solder joint to reduce the heat sinking A material that absorbs heat. Typically made of aluminum, heat sinks are widely used in amplifiers and other electronic devices that build up heat. Small heat sinks are the most economical method for cooling microprocessors and other chips. effect of the component body and to guarantee good solderability for a specific component. This can be expressed as the minimum distance from the component body where good wetting can be achieved within two seconds of dwell time. The specific soldering distance is measured as termination length minus immersion depth. The test is done on a wetting balance. Before this test, the lead should be dip soldered over its entire length, to ensure that the surface solderability is excellent. (1)
PCB PCB: see polychlorinated biphenyl.
in full polychlorinated biphenyl
Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. solderability. The measure of the solderability of a circuit board is always a combination of surface and thermal solderability. For the PCB, there are not separate tests for both aspects. The solderability test of a PCB with plated through-holes is actually a destructive test: the PCB cannot be used afterward for mounting components because all the holes are filled with solder after testing. Often, only small coupons of a PCB are tested. In such cases it is important to use those parts for the test that will be most critical from a thermal point of view. If, after the test, even the most critical thermal layout will give completely filled holes, then the solderability fulfills both requirements.
Optimal gap between lead and hole. For good hole filling, the difference between lead and hole diameter should be at least 0.4 mm for component leads up to 0.8 mm diameter. The optimal gap is 0.7 mm. For larger lead diameters, the diameter should be at least 1.5 times the lead diameter. A hole diameter up to twice the lead diameter will normally not create problems for solder quality. In the case of PCBs thicker than 1.6 mm, or multilayer PCBs with more than one innerlayer connected to the barrel, larger holes may assist in better hole filling.
Pb-free? If Pb-free solder is used for soldering, test procedures and requirements should be modified to accommodate this solder. This means that higher test and process temperatures are involved due to the higher melting temperatures and requisite higher soldering process temperatures of Pb-free solders relative to SnPb solder.
It is good practice to have the test temperature at a setting 10[degrees] to 15[degrees]C lower than the solder temperature used in the solder process. This way, a good process window is guaranteed.
(1.) Reinard J. Klein Wassink, Soldering in Electronics, second edition, 1989.
Ursula Marquez is a process and research engineer at Vitronics Soltec's Technology Group (vitronics.com), email@example.com.