Table:
|  |  | Fiscal Year Ended |  |
| --- | --- | --- | --- |
|  | December 28, 2019 | December 29, 2018 | December 30, 2017 |
| Balance at beginning of year | $185 | $200 | $299 |
| Charges (reversals) to costs and expenses | 37 | (15) | (99) |
| Balance at end of year | $222 | $185 | $200 |
Paragraphs:
Accounts Receivable and Allowance for Doubtful Accounts
The majority of our accounts receivable are derived from sales to large multinational semiconductor manufacturers throughout the world, are recorded at their invoiced amount and do not bear interest.
In order to monitor potential credit losses, we perform ongoing credit evaluations of our customers' financial condition. An allowance for doubtful accounts is maintained based upon our assessment of the expected collectability of all accounts receivable. The allowance for doubtful accounts is reviewed and assessed for adequacy on a quarterly basis.
We take into consideration (1) any circumstances of which we are aware of a customer's inability to meet its financial obligations and (2) our judgments as to prevailing economic conditions in the industry and their impact on our customers. If circumstances change, and the financial condition of our customers is adversely affected and they are unable to meet their financial obligations, we may need to take additional allowances, which would result in an increase in our operating expense.
Activity related to our allowance for doubtful accounts receivable was as follows (in thousands):
Question:
What is the main source of accounts receivable?
Answer: derived from sales to large multinational semiconductor manufacturers throughout the world